Linoleic acid is a colorless to straw-colored liquid polyunsaturated fatty acid (C18H32O2) of the omega-6 series. Linoleic and another fatty acid, gamma-linolenic, or gamolenic, produce compounds called prostaglandins. Prostaglandins are substances that are found in every cell, are needed for the body's overall health maintenance, and must be replenished constantly. Linoleic acid is an essential fatty acid, which means that the body cannot produce it, so it must be obtained in the diet.
Linoleic acid is an important fatty acid, especially for the growth and development of infants. Fatty acids help to maintain the health of cell membranes, improve nutrient use, and establish and control cellular metabolism. They also provide the raw materials that help in the control of blood pressure, blood clotting, inflammation, body temperature, and other body functions. Fatty acids are consumed in the greatest quantities in fat. Although many people are encouraged to consume less fat in their diets , fat is still an important component of a healthy body. Fat stores the body's extra calories, helps insulate the body, and protects body tissues. Fats are also an important energy source during exercise , when the body depends on its calories after using up available carbohydrates. Fat helps in the absorption, and transport through the bloodstream, of the fat-soluble vitamins A, D, E, and K.
Conjugated linoleic acid (CLA) is a naturally occurring mixture of various isomers of linoleic acid with conjugated double bonds. The isomers of CLA have different shapes, functions, and benefits. CLA supplements, or fats containing CLA, generally contain a mixture of these isomers. Although CLA is present in many foods and can be synthesized from linoleic acid, it is made naturally in the stomach, especially in ruminant animals. (Ruminants are animals that regurgitate food and chew it, known as "chewing the cud." Cows and sheep are ruminants.) For this reason, CLA is found primarily in dairy and beef products, as well as other foods derived from ruminant animals. Many people have likely decreased their intake of CLA for two reasons. First, beef and dairy fat are usually decreased or deleted from many diets. Second, many cattle are now fed grain diets, which are lower in linoleic acid than the grass on which they used to feed, so there is less CLA in beef and dairy foods. It is possible to increase the CLA in milk by adding a linoleic acid supplement to livestock feed. The supplement also increases lean tissue and decreases fat in the animals, and induces dairy cattle to produce more milk.
Linoleic acid is found in fish oil , meat, milk, and other dairy products. It is also a constituent of many vegetable oils, including evening primrose oil , sunflower oil, and safflower oil. Commercially produced linoleic acid is used in margarine, animal feeds, emulsifying agents, soaps, and drugs.
As mentioned, CLA supplements, or fats containing CLA, generally contain a mixture of CLA isomers. Plant oils, however, contain little CLA, but are a rich source of linoleic acid. While linoleic acid may be taken as a supplement to help with certain conditions, the supplement will not necessarily increase CLA levels in the body.
One particular isomer in CLA, known as cis-9, trans-11, is linked to anticancer benefits. Studies with animals have shown CLA to reduce breast, prostate, stomach, colorectal, lung, and skin cancers. The CLA may slow the growth of cells that give rise to cancer . A human study has shown an association between linoleic acids and a decreased risk for prostate cancer . In addition, a study done in 2001 on human breast cancer cells grown in a laboratory medium showed that linoleic acid works to reduce tumor size through its effects on a gene that controls the rate of apoptosis, or cell self-destruction.
Infants with cystic fibrosis (CF) often have poor weight gain and growth and an inability to absorb fats. Some research suggests that infants with CF can benefit from formula with a high linoleic acid content because it optimizes nutrition , growth, and feeding efficiency.
Multiple sclerosis (MS) is a disease in which demyelination, loss of myelin sheath material, occurs. (The myelin sheath is a fatty substance that surrounds and insulates the axon of some nerve cells.) This leads to disruptions in nerve impulse transmission. Linoleic acid is believed to be helpful because myelin is composed of lecithin , which is made of linoleic and other fatty acids. Many diets recommended for MS patients include supplements. Patients supplementing with linoleic acid show a smaller increase in disability and reduced severity and duration of attacks than those with no linoleic acid supplement. Evening primrose oil is beneficial because of its specialized fatty-acid content, including linoleic acid. Doses of sunflower seed oil or evening primrose oil to provide 17 grams linoleic acid per day may be beneficial.
One study indicated that low doses of linoleic acid and calcium can reduce the incidence of preeclampsia in high-risk women. (Preeclampsia is the development of hypertension with increased protein in the urine or accumulation of watery fluid in cells or tissues or both, due to pregnancy.) Another study showed, however, that linoleic acid consumption can have a negative effect on fetal growth. Pregnant women should talk to their doctors before taking linoleic acid or any other supplement.
Diet and nutrition
CLA helps regulate how the body accumulates and retains fat. It has been shown to reduce body fat, improve muscle tone, improve nutrient usage, and reduce the appetite by improving the way the body extracts energy from less food. These properties are useful not only for those trying to lose weight or tone muscles, but also for people with nutrient absorption disorders and other digestive problems. The CLA isomer linked with reducing body fat and increasing lean muscle mass is trans-10, cis-12.
Linoleic acid helps relieve flaky, itchy, or rough skin and maintain smooth, moist skin. A tablespoon of linoleic acid-rich foods or oils may be added on a daily basis to help improve and moisturize skin. Linoleic acid may also help with skin disorders such as atopic eczema . Evening primrose oil is taken to help with skin, hair, and nail repair.
Animal research suggests that CLA supplementation may limit food allergy reactions and improve glucose tolerance. It is also used as a nutritional supplement for allergic respiratory disease, circulation, arthritis, and inflammatory problems. CLA is also a potent antioxidant and may help reduce plaque formation in arteries and thus help prevent heart disease . Evening primrose oil helps to reduce arthritis pain and depression . It also helps to control diabetes, liver and kidney damage due to alcohol, and several symptoms of premenstrual syndrome (PMS).
Linoleic acid appears to have at least one negative effect on the human body, however. It appears to increase a person's risk of developing age-related macular degeneration (ARMD), a disease of the eye that leads to a progressive loss of vision and eventual blindness.
Evening primrose oil is a fixed oil obtained from the seeds of Oenothera biennis or other spp. (Onagraceae). It contains about 72% linoleic acid and 9% gamolenic acid. Typical doses expressed as gamolenic acid are 320 or 480 mg daily, taken in two or three doses. Safflower oil is the refined fixed oil obtained from the seeds of the safflower, or false (bastard) saffron , Carthamus tinctorius (Compositae). It contains about 75% linoleic acid as well as various saturated fatty acids.
CLA is available in beef and dairy products, but to avoid eating too many fatty animal foods, supplements may be taken. CLA comes in capsules and softgels that range in potency from 600 to 1,000 mg. A specialist should be consulted to determine what is most appropriate.
CLA appears to be safe and nontoxic at supplemental levels. However, using evening primrose oil as a supplement for linoleic acid can cause symptoms of undiagnosed temporal lobe epilepsy and should be used with caution in patients with a history of epilepsy.
CLA may cause gastrointestinal upset in isolated cases, and evening primrose oil can cause minor gastrointestinal upset and headache .
People who take epileptogenic drugs (drugs which cause epilepsy), in particular phenothiazines, may have interactions with evening primrose oil, and should talk to their doctors before using a supplement.
Dox, Ida G., B. John Melloni, and Gilbert M. Eisner. The Harper Collins Illustrated Medical Dictionary. New York: HarperPerennial, 1993.
"Harvard Study Outlines Role of Fats in Blinding Eye Disease." Angiogenesis Weekly (October 12, 2001).
Herbel, Barbara K., Michelle K. McGuire, Mark A. McGuire, and Terry D. Shultz. "Safflower Oil Consumption Does Not Increase Plasma Conjugated Linoleic Acid Concentrations in Humans." American Journal of Clinical Nutrition 67 (1998): 332–7.
Majumder, Barun, Klaus J. Wahle, Susan Moir, and Steven D. Heys. "Conjugated Linoleic Acid Reduces Breast Tumor Growth Both by P53-dependent and P53-independent Pathways." Journal of Nutrition 131 (November 2001): 3140S.
Van Egmond, Andreas W.A., Michael R. Kosorok, Rebecca Koscik, Anita Laxova, and Philip M. Farrell. "Effect of Linoleic Acid Intake on Growth of Infants with Cystic Fibrosis." American Journal of Clinical Nutrition 63 (1996): 746–52.
Melissa C. McDade
Rebecca J. Frey, PhD