Skip to main content

Fats

FATS

FATS. Fat is a generic term for triacylglycerols, which are a class of structurally similar chemical compounds that contain three fatty acid molecules that are linked or chemically esterified to one glycerol molecule (Figure 1). Mammals store triacylglycerol as lipid droplets in specialized cells referred to as adipocytes, which compose the white or yellow tissue known as adipose or neutral fat tissue. White fat has several functions in mammals. It is a reservoir for storing excess energy obtained from the diet. Fatty acids are a dense storage form of chemical energy in mammals. Adipose tissue also has an important role in padding and thereby protecting various organs throughout the body from temperature extremes and physical impact or trauma. Triacylglycerol, when converted to phospholipid, is a primary constituent of cell membranes and therefore is critical for all forms of life. Mammals also contain brown fat in various locations throughout the body including the neck, thorax, and abdomen. Brown fat functions to generate body heat and therefore it is an important tissue for energy expenditure, otherwise referred to as "burning calories." Brown fat can generate heat because it contains mitochondria with a unique and specialized function. In most other cells, mitochondria are the energy-producing compartments in the cell that generate adenosine triphosphate (ATP), which is a chemical form of cellular energy that is required for numerous cellular chemical reactions. In brown fat, the energy generated by mitochondria is used to generate heat. Brown fat cells contain an "uncoupling" protein that diverts energy away from ATP synthesis and toward heat production. Energy utilization by brown fat is tightly regulated by signals it receives from the sympathetic nervous system. Animals that are adapted to cold temperatures display increased heat production from brown fat, and brown fat is proportionally more abundant in infants than in adults.

Classes of Fatty Acids

Fatty acids are a diverse family of structurally similar carbon chains that contain a single carboxylic acid group (see Figure 1). Fatty acids differ from one another by their carbon chain length, which is usually an even number of carbons that can exceed twenty carbon atoms. Fatty acids are often categorized as short-chain, medium-chain, or long-chain fatty acids because each of these groups displays distinct physical properties. Short-chain fatty acids contain up to seven carbon molecules and are liquids even at cold temperatures. Medium-chain fatty acids, which contain between eight and twelve carbons, are liquids at room temperature but solidify when refrigerated. Long-chain fatty acids contain greater than twelve carbons and are solids at room temperature, but liquefy at elevated temperatures. Long-chain fatty acids are the most abundant fatty acids in plant and animal foods. Short-chain fatty acids are found in whole cow's milk, and medium-chain fatty acids are abundant in coconut milk.

Fatty acids also differ by the number and location of carboncarbon double bonds, otherwise called the degree of saturation. Saturated fatty acids do not contain any carboncarbon double bonds because all carbon molecules are "saturated" with hydrogen molecules. The most abundant saturated dietary fatty acids are palmitic and stearic acids, which are long-chain fatty acids found in foods derived from animals and are abundant in meat and dairy products (Table 1; see Figure 1). Monounsaturated fatty acids contain a single carboncarbon double bond (see Figure 1). Oleic acid is a monounsaturated fatty acid and a common dietary component found in canola and olive oil. Polyunsaturated fatty acids contain up to six carboncarbon double bonds that are always separated by a methylene group (wCH2w) (Figure 1). Polyunsaturated fatty acids that contain a series of double bonds that begins between the third and fourth carbon from the methyl or omega end of the molecule (see nomenclature system below) are referred to as omega-3 fatty acids. Linolenic, eicosapentaenoic (EPA), and docosahexaenoic (DHA) are omega-3 fatty acids and flaxseed oil, walnut oil, and fatty fish are good sources of omega-3 fatty acids. Omega-6 fatty acids are another class of polyunsaturated fatty acids that includes linoleic acid and arachidonic acid. They contain a series of carboncarbon double bonds that begin between the sixth and seventh carbon from the omega end of the fatty acid. Linoleic acid is the most common omega-6 fatty acid in Western-style diets and is found in corn, safflower, and soy oils.

The fatty acid composition of triglycerols found in mammals is usually complex and is influenced by the fatty acid consumed in the diet and by the tissue where it resides. The most common fatty acids in humans are 16, 18, and 20 carbons in length, but longer-chain fatty acids are found in the central nervous system. Most diets contain mixtures of all types of fatty acids, but saturated and monounsaturated fatty acids constitute the vast majority of fatty acids that are consumed in a typical Western diet. A single triacylglycerol molecule rarely contains three identical fatty acids.

Essential Fatty Acids

Rodents placed on a fat-restricted diet are growth impaired, infertile, and develop lesions in the skin and kidney. These pathologies are not observed if the diet is supplemented with linolenic (omega-3) and linoleic acid (omega-6). The results of these studies indicated that mammals cannot synthesize these fatty acids and therefore that these fatty acids are essential components of a healthy diet. Human deficiencies of these essential fatty acids are rare but can occur in infants and children or as a result of intestinal absorption disorders. Human essential fatty acid deficiency compromises liver function, results in unhealthy skin, and impairs growth and development in infants including impaired cognitive function, visual acuity, and hearing.

Essential fatty acids are necessary to maintain the architecture of cell membranes and the integrity of the skin. They are also precursors for the synthesis of eicosanoids ("eicosa" meaning twenty carbons in length), which are bioactive, hormone-like compounds derived from linoleic and linolenic acid. The eicosanoids include prostaglandins, which elicit numerous and varied biological responses including induction of labor, regulation of the female reproductive cycle, and modification of pituitary function. Thromboxane is an eicosanoid that functions in platelet aggregation and blood clotting; leukotrienes function in the inflammation and allergic responses. The omega-3 fatty acid alpha-linolenic acid is also a precursor for eicosapentaenoic acid (EPA) and docohexaenoic acid (DHA) synthesis. Both DHA and arachidonic acid are important for nervous system and retina development. DHA may be an essential dietary fatty acid for preterm infants because studies indicate that it is not synthesized in sufficient quantities to meet the infant's needs.

There is no Recommended Dietary Allowance (RDA) for essential fatty acids. The minimal adequate adult intake of omega-6 fatty acids is estimated to be 2 to 4 g/day of linoleic acid. Americans normally consume about 10 to 15 g/day. The minimal adequate adult intake of omega-3 fatty acids is estimated to be 0.2 to 0.4 g/day, but intakes as high as 3 g/day may have added benefit. Omega-3 fatty acid intakes should be increased during pregnancy and lactation. The World Health Organization recommends an omega-6/omega-3 ratio of 4:1 to 10:1.

Fatty Acids Derived from Food Processing

Synthetic or unnatural types of fatty acids are also common components of Western diets and result from food processing. Fats are processed to increase their shelf life and to alter their physical properties. Monounsaturated and unsaturated fatty acids are chemically inert, whereas polyunsaturated fats are susceptible to oxidation. Polyunsaturated fatty acids degrade by oxidation and become rancid, thereby spoiling foods that contain these compounds. Therefore, products containing polyunsaturated fatty acids tend to have a reduced shelf life, but can be stabilized by converting the polyunsaturated fatty acids contained within these products to more stable monounsaturated and saturated fatty acids through the process of chemical hydrogenation. This processes converts carboncarbon double bonds to single bonds (Reaction 1):

(Reaction 1; Chemical Hydrogenation)

This process not only stabilizes food, but also changes its physical properties. For example, margarine is produced by the chemical hydrogenation of vegetable oils. This process produces a product that is more stable and solid than vegetable oil and mimics the consistency of natural butter. However, chemical hydrogenation of polyunsaturated fatty acids also results in the formation of "unnatural" trans -fatty acids, which are normally found only in trace quantities in foods from natural sources. Trans -fatty acids do not differ from natural fatty acids in their carbon chain length or degree of saturation, but differ in the orientation or stereochemistry of the carboncarbon double bonds. Carboncarbon double bonds can exist in both a cis (the hydrogen atoms that are attached to the carbon atoms that flank the double bond reside on a common plane) or trans (hydrogen atoms reside on different planes) conformation; this is a fundamental principle of organic stereochemistry. The double bonds present in fatty acids from natural, unprocessed food sources usually exist in the cis conformation (see Figure 1). Trans -fatty acids are abundant in foods that undergo chemical hydrogenation and their consumption may increase risk for disease.

Nomenclature of Fatty Acids

All fatty acids can be identified by their "trivial" names, such as oleic or linoleic acid, but these names do not contain information that is necessary to infer their structure or physical properties, that is, the length of their carbon chains or the number and location of carboncarbon double bonds. Therefore, a nomenclature system has been devised that describes the precise chemical structure of the molecule (see Table 1). The carbon atom that constitutes the carboxylic acid of the fatty acid is referred to as the alpha carbon and is designated as carbon number one; the methyl carbon that constitutes the other end of the molecule is referred to as the omega carbon. Fatty acids are named by the number of carbons in the chain and the number and location of carboncarbon double bonds. For example, oleic acid is referred to as cis -9-octadecenoic acid, or 18:1(9); the 18 refers to the number of carbons in the fatty acid carbon chain, the 1 refers to the number of carboncarbon double bonds, and the 9 in parentheses refers to the position of the double bond counting from the carboxylate carbon that is in the cis conformation.

Fatty acids as membrane components and emulsifiers. Fatty acids and triglycerols are lipid soluble and therefore

Classes of fatty acids
Trivial name Systematic name Numerical symbol
Saturated fatty acids
Lauric acid Dodecanoic 12:0
Myristic acid Tetradecanoic 14:0
Palmitic acid Hexadecanoic 16:0
Stearic acid Octadecanoic 18:0
Monounsaturated fatty acids
Palmitoleic acid cis-9-hexadecenoic 16:1(9)
Oleic acid cis-9-octadecenoic 18:1(9)
Polyunsaturated fatty acids (omega-6)
Linoleic acid cis, cis-9, 12-octadecadienoic 18:2 (9,12)
Arachidonic acid All cis-5,8,11, 14-eicosatetraenoic 20:4 (5,8,11,14)
Polyunsaturated fatty acids (omega-3)
Linolenic acid All cis-9-12-15-octadecatrienoic 18:3 (9,12,15)
EPA All cis-5,8,11,14, 17-Eicosapentaenoic 20:5 (5,8,11,14,17)
DHA All cis-4,7,10,13,16, 19-Docosahexaenoic 22:6 (4,7,10,13,16,19)

are hydrophobic molecules that do not dissolve readily in water (as evidenced by the appearance of distinct oil and water layers in many oil-based salad dressings). In aqueous environments, fatty acids aggregate and form ordered structures. All life forms have taken advantage of the hydrophobic properties of fatty acids to make cell membranes, which are semipermeable barriers that separate cells from their environment. Membranes delineate the boundaries of the cell, enable cells to retain water, and form specialized internal structures called subcellular organelles that include mitochondria, Golgi apparatus, and lysosomes. Cell membranes are lipid bilayers that are primarily composed of lipid and membrane-bound proteins. Fatty acids present in cell membranes are components of phospholipids, and phosphoglycerides are the most abundant phospholipids in membranes. Phosphoglycerides are similar in structure to triglycerols. They contain two fatty acid molecules and one phosphate molecule esterified to a glycerol molecule. The phosphate molecule has a hydrophilic amino acid or sugar molecule attached to it. Phospholipids are amphipathic molecules because one end of the molecule contains a water-soluble phosphate molecule, and the other end contains a lipid-soluble carbon chain of the fatty acids. Therefore, phospholipids are ideal components of cell membranes because the phosphate end can dissolve in water while the fatty acid end interacts with other lipid molecules to form a barrier that restricts the efflux of water.

The amphipathic properties of phospholipids make them effective emulsifiers, which are chemicals that interact with both water and oils and prevent them from separating and forming two layers. Lecithin is a phospholipid that is synthesized by mammals and is found in high concentrations in eggs. It is also an effective emulsifier and a common food additive in margarine, salad dressings, chocolate, and a variety of baked items. Fatty acids are components of many household products including lubricants, cooking oils, soaps, and detergents.

Dietary and Biosynthetic Sources of Fat

Fatty acids found in mammals are derived from both dietary sources and intracellular biosynthesis. Humans can synthesize all of the necessary fatty acids with the exception of the essential fatty acids. Fatty acids are synthesized in most cells from excess dietary carbohydrate, amino acids, and from other fatty acids. Palmitic acid (16:0) is synthesized by mammals and is a precursor for the synthesis of all other nonessential fatty acids. The carbon chain of palmitic acid is extended by the sequential addition of two carbons to the carboxy terminal end of the molecule. This is an enzyme catalyzed reaction that uses acetyl coenzyme A (CoA) as a source of the two carbon atoms. Mono-and polyunsaturated fatty acids are synthesized by the desaturation of saturated fatty acids. The first double bond is formed between the C9 and C10 of palmitate or stearate to form palmitoleic or oleic acid. This is the first step in the synthesis of polyunsatirated fatty acids. This reaction is inhibited by dietary polyunsaturated fatty acids but activated by insulin and thyroid hormone.

Triglycerols are synthesized by most tissues from glycerol 3-phosphate, an intermediate in carbohydrate metabolism, and chemically activated fatty acids known as fatty acyl CoAs. This reaction occurs most frequently in the liver and white adipose tissue. In the liver, triacylglycerol synthesis is necessary for the assembly of lipoproteins, whereas triacylglycerol synthesis in adipose tissue functions to create long-term energy stores for mammals. Although a storage form of energy, fat is a dynamic tissue. Triacylglycerols constantly undergo hydrolysis and resynthesis in adipocytes. Newly synthesized triacylglycerol molecules remain intact for only a few days.

Digestion and Transport

About 90 percent of dietary lipid is in the form of triacylglycerols, and typical adults consume about 60 to 150 g/day. During digestion, dietary lipids aggregate and form water-insoluble particles in the gut that must be disrupted before absorption. Specific enzymes in the stomach, called gastric lipases, and in the intestine, called pancreatic lipases, bind to the lipid droplets and catalyze the hydrolysis or removal of fatty acids from triacylglycerols resulting in the liberation of free fatty acids, diacylglycerols, and monoacylglycerols. Fatty acids are also liberated from phospholipids by pancreatic phospholipases. The products of triglycerol hydrolysis are made soluble by bile acids, which are negatively charged detergents that are synthesized from cholesterol in the liver and secreted into the duodenum. Bile acids form micelles, which are disc-shaped particles with a negatively charged exterior that is water soluble and a hydrophobic center that sequesters fatty acids. During digestion, liberated fatty acids are continuously transferred from lipid droplets to micelles. Virtually all free fatty acids are transported from the micelles into intestinal epithelial cells by passive diffusion. Lipids that cannot be made soluble are not absorbed and are excreted.

Once absorbed into the intestinal cells, short-and medium-chain fatty acids are released directly into blood and taken up by the liver. Long-chain fatty acids are resynthesized into triacylglycerols and complex with apolipoproteins to form lipid globules known as chylomicrons. Chylomicrons travel through the lymphatic system and then through the venous plasma. Most triacylglycerol in chylomicrons is metabolized by lipoprotein lipase that is bound to the surface of adipose and muscle cells.

Metabolism of Fat

Most fat cells are derived in infancy and adolescence except in instances of severe childhood obesity. As fat stores accumulate, adipocytes increase in size but generally not in number. Normal fat stores provide sufficient energy to sustain humans for several weeks during total starvation. During fasting, fatty acids are catabolized or broken down to acetyl-CoA, which is an intermediate in the citric acid cycle. This reaction requires carnitine, a derivative of the amino acid lysine. The oxidative breakdown of fatty acids occurs in mitochondria through a series of reactions known as beta-oxidation. Fatty acids are rich sources of energy; 44 moles of ATP are generated by the complete oxidation of 1 mole of a six-carbon fatty acid, whereas only 38 moles of ATP are generated from 1 mole of glucose, a six-carbon sugar. During starvation, acetyl-CoA can be converted to ketone bodies, which include acetone, acetoacetate and alpha-hydroxybutyrate. These compounds are produced exclusively in the liver but readily enter the circulatory system by passive diffusion. The odor associated with the generation of these ketones becomes apparent in the breath and urine of individuals. Ketone bodies are an alternative energy source for glucose during starvation, and are utilized by the brain and other tissues. Normally, ketones are rapidly metabolized by the peripheral tissues and do not accumulate in blood. However, if the citric acid cycle is depressed by low glucose due to starvation, diabetes mellitus, or a high-fat, low-carbohydrate diet, ketones accumulate in serum and a state of ketosis can result. High concentrations of ketones in blood can lower its pH and result in metabolic acidosis, which can be fatal during diabetic ketosis.

Fatty Acid Regulation of Gene Expression

Polyunsaturated fatty acids and eicosanoids are informational or signaling molecules that can influence the expression of certain genes involved in lipid synthesis, breakdown, and transport. Omega-3 and omega-6 polyunsaturated fatty acids lower the accumulation of triacylglycerol in muscle by inhibiting triacylglycerol synthesis in the liver and accelerating the breakdown of fatty acids in the liver and skeletal muscle. Linoleic and linolenic acid, as well as certain pharmaceuticals, bind to and activate the transcriptional activity of a family of related nuclear receptors known as the peroxisome proliferatoractivator receptors (PPARs). These receptors are transcription factors that can directly bind DNA and elevate the transcription of genes. The target genes are involved in the metabolism, storage, and transport of lipids, triacylglycerol, and fatty acids. These receptors also regulate the differentiation of immature adipocytes into mature fat cells.

Individual members of the PPAR family have different functions. In the fasting liver, PPAR-alpha activates genes that encode enzymes that metabolize lipids to ketone bodies and decreases expression of genes involved in fatty acid synthesis. As fatty acids are hydrolyzed from triacyglyceride, PPAR-alpha is further activated. PPAR-alpha activates the expression of genes in fat cells that are necessary for fatty acid uptake, triacylglycerol synthesis, and fat storage.

Determinants of Total Body Fat

Fat is a storage form of energy, and as such only accumulates when energy intake exceeds energy output. Total body fat accumulation is determined by complex interactions among genes, environment, and behavior. The human body can adjust to a wide range of fat intake, but both deficiency and excess are associated with disease. In a normal, healthy individual, fat stores constitute 12 to 18 percent of total body weight in males and 18 to 24 percent in females. Excessive consumption of high-calorie foods and/or a lack of exercise elevate fat stores. In some cases, the genetic background alone can determine total body fat in the absence of strict dietary control. Children with obese parents are at higher risk of becoming obese, and studies of identical twins also indicate that risk for obesity has a strong hereditary component. Furthermore, more than 75 percent of the Pima Indians are obese, again indicating a strong influence of genetics on fat accumulation. Many genes have been identified that control weight gain. The products of these genes regulate energy balance and expenditure and are signaling hormones that regulate appetite and fat metabolism. Some studies indicate that genetic factors, and the metabolic signals they generate, balance energy expenditure and appetite to form an individual's "set point" that specifies body weight. These signals include the satiety hormones such as serotonin and leptin. The neurotransmitter serotonin is responsible for "cravings" that can increase consumption of particular food types. Leptin is a peptide hormone that is secreted by fat cells and signals the hypothalamus. Leptin secretion is proportional to fat cell size, and increased leptin concentrations in blood signal the brain to increase energy expenditure and decrease food intake. Mice lacking the leptin gene or the leptin receptor become obese. Human mutations in the leptin gene are rare but result in obesity.

Dietary Fat and Disease Risk

Lipids constitute about 33 percent of total energy intake in the typical North American diet, whereas Japanese diets have a lower fat intake (11 percent of energy from fat). Western-style diets are deficient in omega-3 fatty acids and contain excess omega-6 fatty acids. Some evidence indicates that prehistoric diets that were consumed through much of human evolution contained an omega-6/omega-3 fatty acid ratio that was near 1.0, whereas this ratio is about 20 in the typical Western diet. Vegetarian diets also tend to contain excess omega-6 fatty acids. Diets deficient in omega-3 fatty acids or diets that contain an elevated omega-6/omega-3 ratio may increase risk for cardiovascular disease and cancer.

Research over the past few decades has indicated that excess consumption of saturated fat increases risk for disease including heart disease (arteriosclerosis), obesity, diabetes, and certain cancers (see "Fat and Heart Disease"). Obesity is a clinical condition defined as having a body weight that is greater than 20 percent above a desirable body weight standard or a body mass index that exceeds 30 kg/m2. Obesity occurs in epidemic proportions in the United States and other Western societies, especially in individuals from lower socioeconomic level. Its prevalence is rapidly increasing in developing societies that are adapting Western lifestyles. The combination of increased fat intake and sedentary lifestyle (otherwise referred to as excess energy intake) increases risk for overweight and obesity. Increased body fat, in turn, is an independent risk for heart disease, diabetes, and high blood pressure. Elevated fat intake can also increase risk for cancers of the colon, prostate, and breast. The incidence of cancers of the breast is high in populations with high intakes of either natural saturated fat or trans -fatty acids, but not diets rich in olive oil, which contains high levels of monounsaturated fatty acids. High polyunsaturated fat intake in the form of linoleic acid (omega-6) increases risk for breast cancer incidence in mice, compared to diets high in omega-3 fatty acids.

Cultures in which traditional foods have high concentrations of monounsaturated fats, products that include olive oil and fish, have lower incidence of heart disease compared to the United States. The prevalence of heart disease in Mediterranean countries is only 50 percent of that found in the United States, even when fat represents almost 40 percent of total energy intake. However, the decreased rates of heart diseases in these countries also reflects other dietary patterns including a high consumption of fresh fruits and vegetables and other lifestyle differences.

See also Assessment of Nutritional Status; Body Composition; Cholesterol; Gene Expression, Nutrient Regulation of; Mediterranean Diet; Nutrition.

BIBLIOGRAPHY

Berdanier, Carolyn D., and James L. Hargrove. "Nutrient Receptors and Gene Expression." In Nutrition and Gene Expression, edited by Carolyn D. Berdanier and James L. Hargrove, pp. 207226. Boca Raton, Fla.: CRC Press, 1993.

Devlin, Thomas M. Biochemistry, 5th ed. New York: Wiley-Liss, 2002.

Kersten, Sander, Beatrice Desvergne, and Walter Wahli. "Roles of PPARs in Health and Disease." Nature 405 (2000): 421424.

Simopoulos, Artemis P. "The Mediterranean Diets: What Is So Special About the Diet of Greece?" Journal of Nutrition 131 (2001): 3065S3073S.

Smolin, Lori A., and Mary B. Grosvenor. Nutrition, Science and Application. Philadelphia: Saunders College Publishing, 2000.

Stipanuk, Martha H. Biochemical and Physiological Aspects of Human Nutrition. Philadelphia: W. B. Saunders, 2000.

Patrick J. Stover


Fat and Heart Disease

Risk for heart disease results from excess fat consumption and the type of fat that is present in the diet. Diets high in saturated fatty acids, especially those found in animal fat, increase the concentration of lowdensity lipoprotein (LDL) cholesterol or "bad" cholesterol. Elevations in serum LDL concentrations increase risk for arteriosclerosis. Consumption of trans- fatty acids, although only representing between 2 and 4 percent of calories in Western diets, also increases risk for heart disease, but the pathogenic mechanisms are not certain. Trans -fatty acids may be as efficient as natural saturated fat in increasing serum LDL concentrations, and their consumption replaces foods that contain beneficial unsaturated fatty acids. Consumption of omega-3 and omega-6 fatty acids, especially when they replace consumption of saturated fat, decreases risk for heart disease, in part by lowering LDL cholesterol levels. Omega-3 fatty acids are more protective than omega-6 fatty acids. Omega-6 fatty acids may lower serum HDL cholesterol, which is harmful because HDL protects the heart from disease. Omega-3 fatty acids may prevent heart disease by improving immune function, lowering blood pressure, and inhibiting the growth of plaques on blood vessel walls. Omega-3 fatty acids obtained from whole food sources such as fatty fish seems to be more beneficial than dietary supplements.



Pharmaceuticals That Target F>at Metabolism

Many of the most prevalent diseases in Western cultures are related to excessive caloric intake and sedentary lifestyles, diseases that include obesity, hyperlipidemia, diabetes, and arteriosclerosis. These states often occur in combination, and are diagnosed as syndrome x. Pharmaceutical have been developed to manage these disorders. These agents either inhibit intestinal fat absorption or affect fat metabolism by manipulating the activity of PPARs.

Fibrates (gemfibrozil, bezafibrate, fenofibrate) are pharmaceuticals that target and inhibit the function of PPAR-alpha. Thiazolidinediones target PPAR-alpha. Fibrates are effective in the treatment of cardiovascular disease. They function to elevate HDL levels by increasing the expression of proteins necessary for its structure, and decreasing plasma triglyceride by accelerating fatty acid oxidation in the liver. TZDs are effective in the treatment of Type 2 diabetes because they have a hypolipidemic and hypoglycemic effect.

Nondigestible commercial lipids have also been developed to limit total fat intake. One product, Olestra, contains fatty acids linked to the sugar sucrose. These products replace natural fat in foods, and were designed to taste like natural fat. However, they cannot be hydrolyzed in the gut and therefore are not absorbed. Other pharmaceuticals target and inhibit pancreatic lipase, such that natural dietary lipids are not broken down to fatty acids and therefore are not absorbed.


Cite this article
Pick a style below, and copy the text for your bibliography.

  • MLA
  • Chicago
  • APA

"Fats." Encyclopedia of Food and Culture. . Encyclopedia.com. 27 May. 2017 <http://www.encyclopedia.com>.

"Fats." Encyclopedia of Food and Culture. . Encyclopedia.com. (May 27, 2017). http://www.encyclopedia.com/food/encyclopedias-almanacs-transcripts-and-maps/fats

"Fats." Encyclopedia of Food and Culture. . Retrieved May 27, 2017 from Encyclopedia.com: http://www.encyclopedia.com/food/encyclopedias-almanacs-transcripts-and-maps/fats

Fats

Fats

Lipids are organic substances consisting mostly of carbons and hydrogen atoms . They are hydrophobic, which means that they have little or no affinity to water. All lipids are soluble (or dissolvable) in nonpolar solvents, such as ether, alcohol, and gasoline. There are three families of lipids: (1) fats, (2) phospholipids, and (3) steroids.

Fatty acids and glycerol make up the larger molecule of fats. A fatty acid consists of a long carbon skeleton of 16 or 18 carbon atoms, though some are even longer. The carbonyl group, which is a carbon atom double-bonded to an oxygen atom and single-bonded to an oxygen attached to a hydrogen (OH-C=O), is the acidic group of the fatty acids. The acidic property is determined by the ability of the hydrogen to dissociate, or break away, from the oxygen atom. The carbonyl group is followed by a long chain of carbon atoms bonded to hydrogen, which is referred to as the hydrocarbon "tail." The long hydrocarbon tail gives fatty acids their hydrophobic, or "water-fearing" property. Fats cannot be dissolved in water because fats are nonpolar (an equal distribution of electrons) and water is polar (an unequal distribution of electrons). The polarity of water is unable to form bonds and break down the nonpolar fatty acid molecule.

There are different types of fatty acids, which vary in length and the number of bonds. Saturated fatty acids have single bonds between the carbon atoms that make up the tail. The carbon atoms are "full" or saturated, and therefore cannot take up any more hydrogen. Most animal fat, such as butter, milk, cheese, and coconut oil, are saturated. Unsaturated fatty acids have one or more double bonds between carbon atoms. A double bond is the sharing of four electrons between atoms, while a single bond is the sharing of two electrons. The double bond has the ability to lend its extra two electrons to another atom, thereby forming another bond. Monounsaturated fatty acids contain only one double bond, such that each of the carbon atoms of the double bond can bond with a hydrogen atom. An example of monounsaturated fatty acids is oleic acid, which is found in olive oil. Polyunsaturated fatty acids contain two or more double bonds, such that four or more carbon atoms can bond with hydrogen atoms. Most vegetable fats are polyunsaturated fatty acids. The double bonds change the structure of the fatty acid, in that there is a slight bend where the double bond is located.

Foods high in saturated fatty acids include whole milk, cream, cheese, egg yolk, fatty meats (e.g., beef, lamb, pork, ham), coconut oil, regular margarine, and chocolate. Foods high in polyunsaturated fatty acids include vegetable oils (e.g., safflower, corn, cottonseed, soybean, sesame, sunflower), salad dressing made from vegetable oils, and fish such as salmon, tuna, and herring.

Triglycerides are the basic unit of fat and are composed of three ("tri-") fatty acids individually bonded to each of the three carbons of glycerol. Fatty acids rarely exist in a free form in nature because they are highly reactive, and therefore make bonds spontaneously.

Fat Function, Metabolism, and Storage

Fats and lipids play critical roles in the overall functioning of the body, such as in digestion and energy metabolism. Usually, 95 percent of the fat in food is digested and absorbed into adipose, or fatty, tissue. Fats are the body's energy provider and energy reserve, which helps the body maintain a constant temperature. Fats and lipids are also involved in the production and regulation of steroid hormones , which are hydrophobic (or "water-fearing") molecules made from cholesterol in the smooth endoplasmic reticulum, a compartment within a cell in which lipids, hormones, and proteins are made. Steroid hormones are essential in regulating sexuality, reproduction, and development of the human sex organs, as well as in regulating the water balance in the body. Steroid hormones can also freely flow in and out of cells, and they modify the transcription process, which is the first step in protein synthesis, where segments of the cell's DNA , or the genetic code, is copied.

Fats and lipids also have important structural roles in maintaining nerve impulse transmission, memory storage, and tissue structure. Lipids are the major component of cell membranes. The three most common lipids in the membranes of eukaryots, or nucleus-containing cells, are phospholipids, glycolipids, and cholesterol. A phospholipid has two parts: (1) the hydrophilic ("water-loving") head, which consists of choline, phosphate, and glycerol, and (2) the hydrophobic ("water-fearing") fatty acid tail, which consists of carbon and hydrogen. The hydrophilic head is the part of the phospholipids that is in contact with water, since it shares similar chemical properties with water molecules. The hydrophobic tail of the phospholipids faces inward, and therefore is able to avoid any contact with water. In this particular arrangement, the phospholipids arrange themselves in a bilayer (double layer) alignment in aqueous solution.

Fats are metabolized primarily in the small intestines because the enzymes of the stomach cannot break down fat molecules due to their hydrophobicity. In the small intestines, fat molecules stimulate the release of cholecystokinin (CCK), a small-intestine hormone, into the bloodstream. The CCK in the blood triggers the pancreas to release digestive enzymes that can break down lipids. The gallbladder is also stimulated to secrete bile into the small intestines. Bile acids coat the fat molecules, which results in the formation of small fat globules, which are called micelles. The coating prevents the small fat globules from fusing together to form larger fat molecules, and therefore the small fat globules are more easily absorbed. The pancreatic enzymes can also break down triglycerides into monoglycerides and fatty acids. Once this occurs, the broken-down fat molecules are able to diffuse into the intestinal cells, in which they are converted back to triglycerides, and finally into chylomicrons.

Chylomicrons, which are composed of fat and protein, are macromolecules that travel through the bloodstream into the lymphatic capillaries called lacteals. The lymphatic system is a special system of vessels that carries a clear fluid called lymph, in which lost fluid and proteins are returned to the blood. The lacteals absorb the fat molecules and transport them from the digestive tract to the circulatory system, dumping chylomicrons in the bloodstream. The adipose and liver tissues, which release enzymes called lipoprotein lipase, break down chylomicrons into monoglycerides and fatty acids. These molecules diffuse into the adipose and liver cells, where they are converted back to triglycerides and stored as the body's supply of energy.

Fat Nutrition

The energy value of fats is 9 kcal/gram (kilocalories per gram), which supplies the body with important sources of calories. Calories are units of energy. The breaking of bonds within fat molecules releases energy that the body uses. A kilocalorie is the unit used to measure the energy in foods. It is the equivalent of "calories" listed on Nutrition Facts labels on food packaging.

Some of the foods known to contain large amounts of fat include the obvious examples, such as butter on toast, fried foods, and hamburgers. But many of the foods that people consume on a daily basis have hidden sources of fat that may not be obvious to the person eating them. These foods include cookies and cakes, cheese, ice cream, potato chips, and hot dogs. One way to avoid foods that contain high amounts of fat is to look at the Nutrition Facts label located on the packages of most foods, where the total fat content of the food is listed.

Actual intake of fat can vary from 10 percent to 40 percent of the calories consumed daily, depending on personal or cultural regimens. Limiting one's daily fat intake to less than 30 percent of total calorie intake and increasing consumption of polyunsaturated fatty acids have been shown to be beneficial in maintaining a healthful diet .

Effects of Excess Dietary-Fat Intake

The recommended intake of fats in the American diet is to limit fats to below 30 percent of the total daily caloric intake. One-third of fats should come from saturated fats, with the other two-thirds split evenly between monounsaturated and polyunsaturated fat. It is estimated that in the average American diet (as of 2002), fats make up 42 percent of calories, with saturated fat making up between a third and a half of that amount.

The effects of this excess intake of dietary fat has some well-established implications for the health of overweight Americans. For instance, the consumption of excess amounts of saturated fats has been recognized as the most important dietary factor to increase levels of cholesterol. A high cholesterol level is detrimental to health and leads to a condition known as atherosclerosis. Atherosclerosis is the build-up of cholesterol on the walls of arteries , which may eventually result in the blocking of blood flow. When this occurs in the arteries of the heart, it is called coronary artery disease. When this process occurs in the heart, a myocardial infarction, or heart attack , may occur.

Besides the cholesterol implications due to high fat intake, obesity is a factor in the causation of disease. Being overweight or obese is highly associated with increasing the risk of type II diabetes , gallbladder disease, cardiovascular disease, hypertension , and osteoarthritis .

Fat-Replacement Strategies

The purpose of fat-replacement strategies is to reduce the percentage of fat in various foods, without taking away the appealing taste of the food. There are three broad categories of fat-replacement strategies: (1) adding water, starch derivatives, and gums to foods, (2) using protein-derived fat replacements, and (3) using engineered fats.

The addition of water to foods lowers the quantity of fat per serving in the selected food item. When starch derivatives are added to food, they bind to the water in the food, thus providing a thicker product that simulates the taste and texture of fat in the mouth. Examples of specific starch derivatives include cellulose , Z-trim, maltrin, stellar, and oatrim. The problem with starch derivatives, however, is their limitations as a fat replacement in foods that require frying.

Protein-derived fat replacements are made from egg and milk proteins, which are made into a microscopic globule of protein. They give the sensation of fat in the mouth, although they contain no fatty acids. One such product is Simplesse, which is used mostly in frozen desserts. Because its chemical structure is easily destroyed by cooking or frying, its use is limited in most other foods.

The third fat-replacement strategy includes the use of engineered fats, which are made by putting together various food substances. One popular engineered fat is olestra, which is made by adding fatty acids to regular table sugar molecules (sucrose). This process results in a product that can neither be broken down in the digestive tract nor absorbed. It therefore cannot provide energy, in terms of carbohydrates or fatty acids, to the body. Olestra is the first engineered fat to be used in fried foods. It does have its drawbacks, however. Olestra can cause abdominal cramping, loose stools, and it can bind beneficial substances that are normally absorbed, such as the fat-soluble vitamins (vitamins A, D, E, and K) and carotenoids .

In addition to fat-replacement strategies, there are low-fat or fat-free versions of many foods on the market. Some products made to be low-fat or fat-free include milk, yogurt, some cheeses, and deli meats. As a general rule, products that claim to have reduced amounts of fat should conform to the following stipulations: (1) a product labeled "reduced-fat" must have at least 25 percent less fat than the normal product, (2) a "low-fat" product can have no more than three grams of fat per serving, and (3) a "fat-free" product most have less than 0.5 grams of fat per serving. But one does not always need to look for foods made to contain less fat than normal, as there are plenty of natural foods that contain very little fat, or no fat at all, including most fruits and vegetables. Other foods that fit into the category of low-fat or nonfat foods include egg whites, tuna in water, skinless chicken, and pasta.

Foods that are low in fat are important for a healthful diet. While fats are essential components for bodily function, excess consumption of fats can lead to health problems such as obesity and heart disease . A healthful diet therefore consists of balanced proportions of proteins, fats, and carbohydrates.

see also Fat Substitutes; Lipid Profile; Omega-3 and Omega-6 Fatty Acids.

Jeffrey Radecki

Susan Kim

Bibliography

Campbell, Neil A., et al. (2000). Biology, 4th edition. San Francisco: Benjamin/Cummings.

Must, A., et al. (1999). "The Disease Burden Associated with Overweight and Obesity." Journal of the American Medical Association 282: 1523.

Robinson, Corinne H.; Weigley, Emma S.; and Mueller, Donna H. (1993). Basic Nutrition and Diet Therapy, 7th edition. New York: Macmillan.

Wardlaw, Gordon M., and Kessel, Margaret (2002). Perspectives in Nutrition, 5th edition. Boston: McGraw-Hill.

Cite this article
Pick a style below, and copy the text for your bibliography.

  • MLA
  • Chicago
  • APA

"Fats." Nutrition and Well-Being A to Z. . Encyclopedia.com. 27 May. 2017 <http://www.encyclopedia.com>.

"Fats." Nutrition and Well-Being A to Z. . Encyclopedia.com. (May 27, 2017). http://www.encyclopedia.com/food/news-wires-white-papers-and-books/fats

"Fats." Nutrition and Well-Being A to Z. . Retrieved May 27, 2017 from Encyclopedia.com: http://www.encyclopedia.com/food/news-wires-white-papers-and-books/fats

fats

fats or, more technically, lipids, together with carbohydrates and proteins, are one of the three staples of the diet. As the seventeenth-century nursery rhyme relates, ‘Jack Sprat he ate no fat/His wife she ate no lean’. This does not mean, however, that Jack Sprat could not get fat, because excess carbohydrates and proteins can be broken down in the body and the fragments synthesized into fat. But had he eaten no fat whatsoever he would have been deprived of certain essential fatty acids, and of the fat-soluble vitamins. Fats serve many purposes: they not only provide thermal insulation and a source of energy, with stores in reserve, but also are involved in important functions, providing for example the materials for components of cell membranes, of the myelin sheaths that electrically insulate nerve fibres.

Chemically, fats are esters — that is, formed by combination of an alcohol with an acid. In this case the alcohol is glycerol, which has three alcohol groups, allowing it to combine with three acidic groups. The acids are fatty acids: long chains of carbon atoms linked to a carboxyl (acidic) group. Three fatty acids therefore combine with one glycerol molecule to form a triglyceride or neutral fat molecule.

Triglycerides are the main fats in the diet. After breakdown and absorption in the alimentary system, they are resynthesized and stored in special cells in which fat globules coalesce to form a large droplet, almost filling the cell. Aggregations of fat cells form adipose tissue (as in the white fat of uncooked meat). White fat can be deposited in a variety of tissues, especially beneath the skin and in muscles — but a great deal of human endeavour is today devoted to getting rid of adipose tissue collected around the waist, even to the extent of having liposuction. Fat acts as a rich fuel reservoir that can be utilized during starvation. Oxidation of fat yields about twice the energy that can be derived from equal amounts of carbohydrate or protein. However, white fat is not easily got rid of physiologically, since it is poorly perfused with blood. Hence hormones that mobilize fat do not reach the target in high concentration, nor is there a high flow rate to carry the energy-giving molecules in the blood to the tissues, such as muscles, where they can be burned.

There is a second type of fat deposit, namely brown fat, found at the base of the neck and between the shoulder blades. This tissue is specialized for thermogenesis, i.e. the rapid mobilization of fat to generate heat. The cells contain many small lipid droplets, a pigment, and many mitochondria, the latter essential for breaking down the lipids into simple two carbon fragments from which energy can be rapidly generated. Furthermore the tissue is well perfused with blood and has a rich innervation by nerves liberating noradrenaline, one of the hormones that can rapidly mobilize the breakdown of fats. Brown fat is important for heat production in infants, and is retained variably into later life. Those who retain the most brown fat into adulthood find it easier to avoid putting on weight. Hibernating animals lay down large amounts of brown fat to see them through the dormant period.

Many different fatty acids are found among the lipids. They vary in the number of carbon atoms in the chain, which in some cases are branched, and also in the number of double bonds they contain, if any. Those with double bonds are known as unsaturated, and those without as saturated. For example palmitic acid and palmitoleic acid both have 16 carbon atoms, but the latter has one double bond (monounsaturated). Oleic, linoleic, and arachidonic acid have respectively 16, 18, and 20 carbon atoms in the chain and 1, 2, and 4 double bonds. Oleic and linoleic acids are essential fatty acids, that is they cannot be synthesized in the body and are therefore essential dietary constituents. Readers will be familiar with the term ‘rich in polyunsaturates’ applied to many supermarket products like margarines, sunflower oils, etc. There is evidence that diets rich in polyunsaturated fats are less likely to cause atherosclerosis than ones rich in animal fats, with their predominance of saturated fats.

Fatty acids are essential constituents of phospholipids and glycolipids. In phospholipids the glycerol moeity is combined with only two fatty acids, the remaining alcohol group being combined with a phosphate group linked to an alcohol (e.g. to serine, choline, or inositol, to give the phospholipids phosphatidylserine, phosphatidylcholine, and phophatidylinositol). These compounds are ‘amphipathic’, that is, they have a polar head group, which is compatible with an aqueous environment, and a non-polar tail, which is not. Such molecules can form films (as does oil spread on the surface of water) in which the hydrophobic tails interact with each other, projecting out of the aqueous surface, while the polar head groups remain in the water. Now imagine such films are brought together so that the hydrophobic tails of one film are opposed to those of the second. This is a very close approximation to the structure of all cell membranes, where the polar head of one lipid layer contacts the extracellular environment, while the head groups of the other layer contact the aqueous environment within the cell. These structures, so-called lipid bilayers, form flexible membranes, which are very impermeable to the movement of substances across them, whilst particular permeability properties are provided by the inclusion of protein molecules in the membrane. Some membrane fatty acids can be mobilized as autacoids (released to affect other cells) and as intracellular messengers. For example, arachidonic acid gives rise to prostaglandins in cell membranes, and phosphatidylinositol is the source of the important ‘second messenger’ inositol triphosphate, implementing an internal response to a chemical message from outside the cell.

Alan W. Cuthbert


See also body weight; cell membrane; metabolism; obesity.

Cite this article
Pick a style below, and copy the text for your bibliography.

  • MLA
  • Chicago
  • APA

"fats." The Oxford Companion to the Body. . Encyclopedia.com. 27 May. 2017 <http://www.encyclopedia.com>.

"fats." The Oxford Companion to the Body. . Encyclopedia.com. (May 27, 2017). http://www.encyclopedia.com/medicine/encyclopedias-almanacs-transcripts-and-maps/fats

"fats." The Oxford Companion to the Body. . Retrieved May 27, 2017 from Encyclopedia.com: http://www.encyclopedia.com/medicine/encyclopedias-almanacs-transcripts-and-maps/fats

fats and oils

fats and oils, group of organic substances that form an important part of the diet and also are useful in many industries. The fats are usually solid, the oils generally liquid at ordinary room temperatures. Some tropical products, liquids in their sites of origin, become solids in cooler climates; in commerce these often retain the name originally given, e.g., palm oil and coconut oil. Chemically fats and oils are either simple or mixed glyceryl esters of organic acids belonging to the fatty-acid series (see triglycerides; fatty acids). Fats and oils are derived from both plant and animal sources.

Commercial Processing of Fats

Among the vegetable oils of greatest commercial importance are cottonseed, linseed, olive, palm, corn, peanut, soybean, and castor oils. The method of obtaining the oils is similar for all: the fruits or seeds after being cleaned are crushed and pressed cold to obtain the highest grade of oil and then pressed warm, yielding a grade suitable for industrial use. Sometimes solvents are used to remove the remaining oil from the crushed mass. Edible oils are those used in foods, and for these the highest grade is utilized; these must be pale in color, free from disagreeable odor and taste, and wholesome. The lower grades are suitable for making soap and for other industrial purposes. The chemical property that makes fats solid and oils liquid is the amount of saturation in the ester (see saturated fats). Animal fats are esters of saturated fatty acids; vegetable oils are esters of unsaturated fatty acids.

Conversion of liquid vegetable oils into solid fats is an important chemical industry. This process, sometimes called hardening, involves hydrogenation of the unsaturated fatty-acid portion of the oil molecule by heating the oil with hydrogen in the presence of a metal catalyst; by controlling the extent of hydrogenation, various products can be obtained. For example, controlled hydrogenation of cottonseed oil produces a solid vegetable cooking fat. Most fats become rancid upon standing; since a major factor leading to rancidity is air oxidation of double bonds (to form foul-smelling aldehydes), saturated fats are much more resistant to rancidity than unsaturated fats.

Fats as Food

Animal fats used in foods include butter, lard, chicken fat, and suet. Cod-liver oil and some other fish oils are used therapeutically as sources of vitamins A and D. Nutritionally fats and oils are valued as a source of energy. Because they contain less oxygen than other nutrients, they oxidize more readily and release more energy. Fats are digested in the human body chiefly by the enzyme lipase (in the pancreatic juice) aided by the bile. There are several theories to explain the method of absorption of fats; favored by many is the view that they are absorbed by the epithelial cells of the lining of the small intestine in the form of the fatty acids and glycerol into which they are split by digestion and that a recombination to re-form the fat occurs within the cells. Most of the fat then enters the lymphatic system through the villi in the lining of the small intestine, although some is probably absorbed directly by the blood vessels of the villi. Medical research indicates the possibility that saturated fats in the diet contribute to the incidence of arteriosclerosis; such fats may raise the blood's level of cholesterol, which is deposited in the arteries.

See oils; petroleum.

Cite this article
Pick a style below, and copy the text for your bibliography.

  • MLA
  • Chicago
  • APA

"fats and oils." The Columbia Encyclopedia, 6th ed.. . Encyclopedia.com. 27 May. 2017 <http://www.encyclopedia.com>.

"fats and oils." The Columbia Encyclopedia, 6th ed.. . Encyclopedia.com. (May 27, 2017). http://www.encyclopedia.com/reference/encyclopedias-almanacs-transcripts-and-maps/fats-and-oils

"fats and oils." The Columbia Encyclopedia, 6th ed.. . Retrieved May 27, 2017 from Encyclopedia.com: http://www.encyclopedia.com/reference/encyclopedias-almanacs-transcripts-and-maps/fats-and-oils

fat

fat / fat/ • n. a natural oily or greasy substance occurring in animal bodies, esp. when deposited as a layer under the skin or around certain organs. ∎  a substance of this type, or a similar one made from plant products, used in cooking. ∎  the presence of an excessive amount of such a substance in a person or animal, causing them to appear corpulent. ∎  Chem. any of a group of natural esters of glycerol and various fatty acids, which are solid at room temperature and are the main constituents of animal and vegetable fat. ∎  something excessive or unnecessary: fat in the state budget. • adj. (fat·ter , fat·test ) (of a person or animal) having a large amount of excess flesh: the driver was a fat, wheezing man. ∎  (of an animal bred for food) made plump for slaughter. ∎  containing much fat: fat bacon. ∎  large in bulk or circumference: a fat cigarette. ∎ inf. (of an asset or opportunity) financially substantial or desirable: a fat profit. ∎  inf. used ironically to express the belief that there is none or very little of something: fat chance she had of influencing him a fat lot of good that'll do him. ∎  ∎  (of wood) containing a high proportion of resin: fat pine. • v. (fat·ted , fat·ting) archaic make or become fat: [as adj.] (fatted) a fatted duck. PHRASES: live off (or on) the fat of the land have the best of everything.DERIVATIVES: fat·ness n. fat·tish adj.

Cite this article
Pick a style below, and copy the text for your bibliography.

  • MLA
  • Chicago
  • APA

"fat." The Oxford Pocket Dictionary of Current English. . Encyclopedia.com. 27 May. 2017 <http://www.encyclopedia.com>.

"fat." The Oxford Pocket Dictionary of Current English. . Encyclopedia.com. (May 27, 2017). http://www.encyclopedia.com/humanities/dictionaries-thesauruses-pictures-and-press-releases/fat-2

"fat." The Oxford Pocket Dictionary of Current English. . Retrieved May 27, 2017 from Encyclopedia.com: http://www.encyclopedia.com/humanities/dictionaries-thesauruses-pictures-and-press-releases/fat-2

Fats

FATS

Fats, or lipids, are a group of chemical substances in food that are generally insoluble in water. There are several classes of fats. The triglycerides (triacylglycerols) are the predominant constituent of vegetable and animal fats and oils. They are composed of a 3-carbon glycerol backbone and three fatty acids of various types. The character of the fat is determined by these fatty acids. Saturated fatty acids tend to make the fat "hard" or solid at room temperature and are associated with an increased risk of heart disease. Monounsaturated fatty acids are prominent in olive and canola oils and do not increase the risk of heart disease. The third group of fatty acids, the polyunsaturated fatty acids, are important components of omega-3 fish and plant oils. They play a role in blood clotting and in inflammatory responses in the body.

Phospholipids are closely related to triacylglycerols, except that one of the carbons on the glycerol contains one of several phosphate groups; the other two carbons have fatty acids. A third group, related to the first two, is the sphingomyelins and other complex brain lipids.

Cholesterol and its precursors are another group of fats that are essential for membranes; these are chemically composed of several 5-and 6-membered carbon rings. Steroids make up a fifth group of fats. Steroids are derived from cholesterol and include the androgens and estrogens, among others.

George A. Bray

(see also: Blood Lipids; HDL Cholesterol; LDL Cholesterol; Lipoproteins; Triglycerides; VLDL Cholesterol )

Cite this article
Pick a style below, and copy the text for your bibliography.

  • MLA
  • Chicago
  • APA

"Fats." Encyclopedia of Public Health. . Encyclopedia.com. 27 May. 2017 <http://www.encyclopedia.com>.

"Fats." Encyclopedia of Public Health. . Encyclopedia.com. (May 27, 2017). http://www.encyclopedia.com/education/encyclopedias-almanacs-transcripts-and-maps/fats

"Fats." Encyclopedia of Public Health. . Retrieved May 27, 2017 from Encyclopedia.com: http://www.encyclopedia.com/education/encyclopedias-almanacs-transcripts-and-maps/fats

fat

fat Semi-solid organic substance made and used by plants and animals to store energy. Fats dissolve in organic solvents such as ether, carbon tetrachloride, chloroform and benzene. Most common fats are triglycerides: esters in which one molecule of glycerol is bound to three molecules of fatty acids, each having 12 to 18 carbon atoms. Animal fats are esters of saturated fatty acids. Vegetable oils are esters of unsaturated fatty acids, that is, they have a higher proportion of molecules with double carbon–carbon bonds in the chain. In animals, fat resides in the subcutaneous layer beneath the skin and deep within the body as a specialized adipose tissue. It serves as an insulator and protects internal organs. Research indicates that the consumption of high levels of saturated fat can increase the risk of heart disease. Foods high in fat include butter, margarine, and most oils. Almost all fats found in plant sources are unsaturated. See also lipid

Cite this article
Pick a style below, and copy the text for your bibliography.

  • MLA
  • Chicago
  • APA

"fat." World Encyclopedia. . Encyclopedia.com. 27 May. 2017 <http://www.encyclopedia.com>.

"fat." World Encyclopedia. . Encyclopedia.com. (May 27, 2017). http://www.encyclopedia.com/environment/encyclopedias-almanacs-transcripts-and-maps/fat

"fat." World Encyclopedia. . Retrieved May 27, 2017 from Encyclopedia.com: http://www.encyclopedia.com/environment/encyclopedias-almanacs-transcripts-and-maps/fat

fat

fat fat cat a wealthy and powerful person, especially a businessman or politician; the term is recorded from the late 1920s in the US, but has become frequent in the UK since the early 1990s, in relation particularly to what are perceived as inflated salaries paid to senior executives of formerly nationalized industries.
the fat is in the fire something has been said or done that is about to cause trouble or anger. In this current sense with reference to the sizzling and spitting resulting from a spillage of cooking fat into an open flame. The expression is recorded from the mid 16th century in the sense that something has gone irretrievably wrong.
the fat of the land the best of everything as one's resource for living; originally with reference to Genesis 45:18, ‘Ye shall eat the fat of the land.’

see also a green Yule makes a fat churchyard, the opera isn't over till the fat lady sings.

Cite this article
Pick a style below, and copy the text for your bibliography.

  • MLA
  • Chicago
  • APA

"fat." The Oxford Dictionary of Phrase and Fable. . Encyclopedia.com. 27 May. 2017 <http://www.encyclopedia.com>.

"fat." The Oxford Dictionary of Phrase and Fable. . Encyclopedia.com. (May 27, 2017). http://www.encyclopedia.com/humanities/dictionaries-thesauruses-pictures-and-press-releases/fat

"fat." The Oxford Dictionary of Phrase and Fable. . Retrieved May 27, 2017 from Encyclopedia.com: http://www.encyclopedia.com/humanities/dictionaries-thesauruses-pictures-and-press-releases/fat

fat

fat A mixture of lipids, chiefly triglycerides, that is solid at normal body temperatures. Fats occur widely in plants and animals as a means of storing food energy, having twice the calorific value of carbohydrates. In mammals, fat is deposited in a layer beneath the skin (subcutaneous fat) and deep within the body as a specialized adipose tissue (see also brown fat). The insulating properties of fat are also important, especially in animals lacking fur and those inhabiting cold climates (e.g. seals and whales).

Fats derived from plants and fish generally have a greater proportion of unsaturated fatty acids than those from mammals. Their melting points thus tend to be lower, causing a softer consistency at room temperatures. Highly unsaturated fats are liquid at room temperatures and are therefore more properly called oils.

Cite this article
Pick a style below, and copy the text for your bibliography.

  • MLA
  • Chicago
  • APA

"fat." A Dictionary of Biology. . Encyclopedia.com. 27 May. 2017 <http://www.encyclopedia.com>.

"fat." A Dictionary of Biology. . Encyclopedia.com. (May 27, 2017). http://www.encyclopedia.com/science/dictionaries-thesauruses-pictures-and-press-releases/fat-1

"fat." A Dictionary of Biology. . Retrieved May 27, 2017 from Encyclopedia.com: http://www.encyclopedia.com/science/dictionaries-thesauruses-pictures-and-press-releases/fat-1

fat

fat
1. Chemically fats (or lipids) are substances that are insoluble in water but soluble in organic solvents such as ether, chloroform, and benzene, and are actual or potential esters of fatty acids. The term includes triacylglycerols (triglycerides), phospholipids, waxes, and sterols.

2. In more general use the term ‘fats’ refers to the neutral fats which are triacylglycerols, mixed esters of fatty acids with glycerol.

Cite this article
Pick a style below, and copy the text for your bibliography.

  • MLA
  • Chicago
  • APA

"fat." A Dictionary of Food and Nutrition. . Encyclopedia.com. 27 May. 2017 <http://www.encyclopedia.com>.

"fat." A Dictionary of Food and Nutrition. . Encyclopedia.com. (May 27, 2017). http://www.encyclopedia.com/education/dictionaries-thesauruses-pictures-and-press-releases/fat

"fat." A Dictionary of Food and Nutrition. . Retrieved May 27, 2017 from Encyclopedia.com: http://www.encyclopedia.com/education/dictionaries-thesauruses-pictures-and-press-releases/fat

fat

fat adj. and sb. OE. fæt(t) = MDu., MLG. vett (Du. vet), OHG. feizzit (G. feist) :- WGmc. *faitiða, pp. formation on Gmc. *faitjan fatten (OHG. veizzen, ON. feita), f. *faitaz adj. fat, repr. by OS. feit, OHG. feiz, ON. feitr.
So vb. surviving in arch. fatted calf; OE. fǣttian. fatling XVI. fatten. OE. (ġe)fættnian (see -EN5). fatty XIV.

Cite this article
Pick a style below, and copy the text for your bibliography.

  • MLA
  • Chicago
  • APA

"fat." The Concise Oxford Dictionary of English Etymology. . Encyclopedia.com. 27 May. 2017 <http://www.encyclopedia.com>.

"fat." The Concise Oxford Dictionary of English Etymology. . Encyclopedia.com. (May 27, 2017). http://www.encyclopedia.com/humanities/dictionaries-thesauruses-pictures-and-press-releases/fat-3

"fat." The Concise Oxford Dictionary of English Etymology. . Retrieved May 27, 2017 from Encyclopedia.com: http://www.encyclopedia.com/humanities/dictionaries-thesauruses-pictures-and-press-releases/fat-3

fat

fat (neutral fat) (fat) n. a substance that consists chiefly of triglycerides and is the principal form in which energy is stored by the body (see adipose tissue). It also serves as an insulating material beneath the skin and around certain organs. See also brown fat, fatty acid, lipid.

Cite this article
Pick a style below, and copy the text for your bibliography.

  • MLA
  • Chicago
  • APA

"fat." A Dictionary of Nursing. . Encyclopedia.com. 27 May. 2017 <http://www.encyclopedia.com>.

"fat." A Dictionary of Nursing. . Encyclopedia.com. (May 27, 2017). http://www.encyclopedia.com/caregiving/dictionaries-thesauruses-pictures-and-press-releases/fat

"fat." A Dictionary of Nursing. . Retrieved May 27, 2017 from Encyclopedia.com: http://www.encyclopedia.com/caregiving/dictionaries-thesauruses-pictures-and-press-releases/fat

fat

Cite this article
Pick a style below, and copy the text for your bibliography.

  • MLA
  • Chicago
  • APA

"fat." The Columbia Encyclopedia, 6th ed.. . Encyclopedia.com. 27 May. 2017 <http://www.encyclopedia.com>.

"fat." The Columbia Encyclopedia, 6th ed.. . Encyclopedia.com. (May 27, 2017). http://www.encyclopedia.com/reference/encyclopedias-almanacs-transcripts-and-maps/fat

"fat." The Columbia Encyclopedia, 6th ed.. . Retrieved May 27, 2017 from Encyclopedia.com: http://www.encyclopedia.com/reference/encyclopedias-almanacs-transcripts-and-maps/fat

fat

fat See LIPID.

Cite this article
Pick a style below, and copy the text for your bibliography.

  • MLA
  • Chicago
  • APA

"fat." A Dictionary of Plant Sciences. . Encyclopedia.com. 27 May. 2017 <http://www.encyclopedia.com>.

"fat." A Dictionary of Plant Sciences. . Encyclopedia.com. (May 27, 2017). http://www.encyclopedia.com/science/dictionaries-thesauruses-pictures-and-press-releases/fat

"fat." A Dictionary of Plant Sciences. . Retrieved May 27, 2017 from Encyclopedia.com: http://www.encyclopedia.com/science/dictionaries-thesauruses-pictures-and-press-releases/fat

fat

fat See LIPID.

Cite this article
Pick a style below, and copy the text for your bibliography.

  • MLA
  • Chicago
  • APA

"fat." A Dictionary of Zoology. . Encyclopedia.com. 27 May. 2017 <http://www.encyclopedia.com>.

"fat." A Dictionary of Zoology. . Encyclopedia.com. (May 27, 2017). http://www.encyclopedia.com/science/dictionaries-thesauruses-pictures-and-press-releases/fat-0

"fat." A Dictionary of Zoology. . Retrieved May 27, 2017 from Encyclopedia.com: http://www.encyclopedia.com/science/dictionaries-thesauruses-pictures-and-press-releases/fat-0

fat

fatat, bat, brat, cat, chat, cravat, drat, expat, fat, flat, frat, gat, gnat, hat, hereat, high-hat, howzat, lat, mat, matt, matte, Montserrat, Nat, outsat, pat, pit-a-pat, plait, plat, prat, Rabat, rat, rat-tat, Sadat, sat, scat, Sebat, shabbat, shat, skat, slat, spat, splat, sprat, stat, Surat, tat, that, thereat, tit-for-tat, vat, whereat •fiat • floreat • exeat • caveat •Croat, Serbo-Croat •Nanga Parbat • brickbat • dingbat •combat, wombat •fruitbat • numbat • acrobat • backchat •whinchat • chitchat • samizdat •concordat • Arafat • Jehoshaphat •butterfat • Kattegat • hard hat •sun hat • fat cat • hellcat • requiescat •scaredy-cat • Magnificat • copycat •pussycat • wildcat • bobcat • tomcat •Sno-Cat • polecat • muscat • meerkat •mudflat • cervelat •doormat, format •diplomat • laundromat • Zermatt •Donat • cowpat

Cite this article
Pick a style below, and copy the text for your bibliography.

  • MLA
  • Chicago
  • APA

"fat." Oxford Dictionary of Rhymes. . Encyclopedia.com. 27 May. 2017 <http://www.encyclopedia.com>.

"fat." Oxford Dictionary of Rhymes. . Encyclopedia.com. (May 27, 2017). http://www.encyclopedia.com/humanities/dictionaries-thesauruses-pictures-and-press-releases/fat-1

"fat." Oxford Dictionary of Rhymes. . Retrieved May 27, 2017 from Encyclopedia.com: http://www.encyclopedia.com/humanities/dictionaries-thesauruses-pictures-and-press-releases/fat-1

FAT

FAT Computing file allocation table

Cite this article
Pick a style below, and copy the text for your bibliography.

  • MLA
  • Chicago
  • APA

"FAT." The Oxford Dictionary of Abbreviations. . Encyclopedia.com. 27 May. 2017 <http://www.encyclopedia.com>.

"FAT." The Oxford Dictionary of Abbreviations. . Encyclopedia.com. (May 27, 2017). http://www.encyclopedia.com/humanities/dictionaries-thesauruses-pictures-and-press-releases/fat-0

"FAT." The Oxford Dictionary of Abbreviations. . Retrieved May 27, 2017 from Encyclopedia.com: http://www.encyclopedia.com/humanities/dictionaries-thesauruses-pictures-and-press-releases/fat-0