NUTRIENT BIOAVAILABILITY. The chemical analysis of a food gives us values for the total amounts of particular nutrients that are present. Commonly, when a food is to be analyzed, it is first treated with a strong acid, or an enzyme, that breaks up complex materials into simpler, soluble products. Thus, the starch content of a food may be measured as the amount of additional sugar found after the treatment of the food with an enzyme that acts specifically on starch, and nothing else, to break the large molecules down to glucose. Usually, if different analytical methods are compared, the one giving the highest values is chosen, on the grounds that it has extracted the nutrient in question most completely.
However, from a nutritional point of view our interest is in how much our digestive system will be able to extract in a form that can be absorbed into the bloodstream. The term "digestibility" was formerly used to designate this, but nowadays it is becoming more common to use the term "bioavailability." It must be remembered that, after a period of preliminary storage and preparatory digestion in the stomach, ingested food passes into the small intestine and it is there that final digestion occurs and the smaller, digestible molecules are absorbed. The residual gut contents are then passed into the large intestine, whose major function is the removal of water, though the presence of a large bacterial population ferments some of the otherwise indigestible components of food. Since the fermented material is not recovered in the feces it may, wrongly, be considered to have been digested, even though it is not "bioavailable."
The final measure of bioavailability must therefore be some overall response of the subject to a test dose of the food in question. If the subject is a human "guinea pig," this might be the increased level in the blood of the nutrient in question; if young, growing animals are used, it could be their rate of growth when receiving a supplement of the test food, compared with that of other animals receiving the pure nutrient. We will cite some examples below to illustrate why the subject has some importance, and how it can be studied with different nutrients.
Proteins are enormous molecules composed of chains of hundreds of amino acids. Our bodies can only build up their own proteins from single amino acids, and the absorptive system of the gut will only allow single amino acids to pass into the bloodstream. This breakdown, which is very efficient for most of the protein in ordinary foods, requires a complex series of enzymes, beginning with pepsin, which is secreted into the stomach. It is only very resistant proteins, like those in hair, that fail to be digested by the combined attacks of digestive enzymes.
In general, cooking makes proteins more digestible by loosening and breaking up some of their natural coatings. However, there is one exception–—the so-called Maillard (or Browning) reaction, which occurs when a reducing sugar is heated in contact with protein under conditions of relatively low moisture. These reactions are limited to a subset of amino groups. This reaction is not very common but can occur, for example, if dried skim milk is used to fortify cookies intended for undernourished toddlers. During the baking the reactive group of the lactose in the dried milk combines with reactive groups on the surface of the protein molecules present. One of these reactive groups is attached to the essential amino acid lysine and the resulting compound is resistant to digestion, so that the protein has lost much of its nutritional value.
The disease associated with a deficiency of the vitamin niacin (also called nicotinic acid) is pellagra, which is characterized by terrible dermatitis on parts of the skin exposed to the sun and by mental deterioration. The disease has nearly always been associated with poor people eating a diet based on corn (that is, maize) and little in the way of meat or milk. It is now known that, as the grain matures, the niacin combines with structural materials to form a complex that is not readily broken down by digestive enzymes. This gives the vitamin a very low bioavailabilty, and ordinary cooking in boiling water does not liberate it. Corn is also lower in the amino acid tryptophan than are other grains (and also lower in tryptophan than meat and milk). By means of a series of chemical reactions in the body, tryptophan can provide an indirect source of niacin.
These two characteristics of corn serve to explain its association with pellagra, which began to be seen in southern Europe when corn first became the staple food of many low-income peasant communities. However, this seemed inconsistent with the fact that pellagra was not a problem among low-income communities in Central America where corn had been the staple food for very long periods. It is now realized that a difference in the way that corn is prepared for cooking in the two geographic areas was at least partly responsible for the differing rates of pellagra. In Central America, corn is traditionally soaked in lime water, which is strongly alkaline, before being cooked and made into tortillas. This treatment, which softens the hard grains, also has the effect of releasing the niacin from its B vitamin complex and making it bioavailable. This is an interesting example of a traditional folk custom having a beneficial effect in addition to the obvious one of softening the grain. It has also been recorded that Native American groups in North America who relied on corn as their staple food, but did not have access to lime, used infusions of wood ash, which is also alkaline, for the same purpose.
Maize came from the New World. One could say, therefore, that the thousands of Europeans who suffered and died from pellagra in the 1800s did so because the first explorers who brought maize to Europe failed to bring with them the cooking instructions that had been adopted from long experience where it had been a staple food for many centuries. Furthermore, people of European origin and habits, but living in the American South, were again subject to the disease in the early 1900s.
Pernicious anemia results from a long-term deficiency of vitamin B 12. A normal mixed diet provides enough of this vitamin. However, its absorption requires the secretion of a specific "intrinsic factor" by cells in the stomach wall. These cells may atrophy, particularly in some older people, and then the vitamin in the food remains unabsorbed so that, when the tissue reserves are used up, the result is a macrocytic (large cell) anemia and progressive neurological damage. After the problem has been detected, the vitamin can be supplied by intramuscular injection.
Vitamin A is found naturally only in animal products. Many people, for lack of purchasing power or other reasons, eat little or no animal products and meet their need for vitamin A by ingesting carotene and related compounds (carotenoids) that provide the coloring of carrots and are present in all green leafy material. Carotene can be hydrolyzed by enzymes within the gut wall to form active vitamin A. However, it has very limited solubility and can be absorbed only in conjunction with fats. Unfortunately, low-income children in Third World countries who rely on carotenoids as their source of vitamin A also commonly have diets that are very low in fat, so that even a reasonable intake of carotenoids does not save them from blindness and other effects of vitamin deficiency.
Deficiency of iron and the resulting anemia is a major nutritional problem in many parts of the world. This metallic element is, of course, an essential component of hemoglobin, the oxygen-carrying chemical in our red blood corpuscles, as well as having other functions. However, iron can also be toxic if the body absorbs much more than it needs, because we have no mechanism for excreting the excess. Our bodies therefore have to regulate the absorption of iron so that very little is absorbed when we already have enough, but that we do absorb more after blood loss, or during pregnancy when the growing fetus has to be provided with its hemoglobin. It appears that there is a feedback system that informs the cells on the inner surface of the small intestine how much iron is required to be absorbed.
However, there are limits to this control, and children may die from acute iron poisoning as a result of finding iron pills and eating them as candy. A small proportion of people also absorb more iron than they need even from a normal diet and suffer from a serious disease (hemachromatosis) as a consequence. Iron, therefore, is a nutrient for which a relative low bioavailability is normal, and in fact essential.
It is estimated that adult men need to absorb about 1 milligram of iron per day to replace their losses from rubbed-off skin, and so forth, and that premenopausal women need to absorb some 50 percent more in order to compensate for losses of menstrual blood. It is commonly recommended that people consume 10 times these levels each day, on the assumption that there will be about 10 percent bioavailability. In general, "heme iron" from meat is best absorbed, with a typical value of about 20 percent. The absorption of the iron compounds present in vegetable foods is lower and variable, but is increased if vitamin C is present in the same meal. It is particularly low from green vegetables like spinach that are relatively rich in iron but also in another compound called phytic acid or phytate. This compound can form insoluble salts with iron that inhibit its absorption. Legumes (that is, beans, lentils, and peas) are generally richer sources than the grains, but the iron compounds they contain are also less well absorbed than those in meat.
Wheat fully milled to "white flour," and used to make white bread, is particularly low in iron, and yet it can form a large portion of the diet of many people. Because of this, many technically developed countries legally required that millers add a source of iron, as part of a micronutrient supplement, to all the white flour that they produce. This is a problem for millers because the addition of ferrous sulfate, the common iron salt with relatively high availability, can catalyze the oxidation of fat in foods, producing rancid off-flavors. That can be avoided by adding the actual metal in very fine form. On chemical analysis, this form of iron is fully measured and meets the legislative standard for the iron content of white flour. However, its bioavailability is more doubtful and depends on whether the gastric acid, which it meets in the stomach, is sufficient to react with it to produce soluble salts. Even iron with the finest particle size seems to have only 20 percent of the bioavailability of the iron in ferrous sulfate.
Other Trace Minerals
Zinc provides the best example of deficiency of a trace mineral occurring in practice as the result of low bioavailability, rather than as a result of an absolute deficiency of the mineral in the diet. In the Middle East, cases were observed of young men who had failed both to grow to normal height and to reach puberty. Tests showed that these subjects did respond to dietary supplements of zinc; even though their diet was low in zinc they were not really deficient in the element. The traditional diet in the area was based on wheat, but it was not customary there when cooking wheat flour to first "raise," or leaven the dough by fermenting the flour with yeast, as in Western baking. We now realize that yeast, in addition to producing bubbles of carbon dioxide that "raise" the dough, secretes enzymes that hydrolyze the phytate present in the flour. Unfermented products, like "falafel," still contain a significant amount of phytate and, when they are eaten, the phytate binds with zinc present in the same meal to make it largely unavailable. When a diet includes more meat or fish, which are richer sources of zinc, enough remains uncombined to provide what is required for normal functions; but the young men who had failed to mature and grow were typically from poor families who ate little in the way of animal products.
Another problem with trace minerals is that they share absorption pathways and are, in a sense, in competition with each other. As a consequence, very high intakes of one element can reduce the bioavailability of another. Thus, if one were to take a high supplement of an iron salt, in order to avoid any risk of an iron deficiency, one could precipitate a deficiency of zinc since that element would no longer be so well absorbed. Equally, a particularly high intake of zinc could reduce one's absorption of copper, and so on.
There is no one "ideal diet." Our bodies can tolerate and adapt to considerable differences in the intake of nutrients, but there are limits, and it is clearly dangerous to supplement the diet with a very large amount of a single nutrient. The simplest way to maintain a reasonable balance in one's diet is to follow the traditional advice of eating a wide variety of foods.
See also Assessment of Nutritional Status; Cooking; Disease: Metabolic Diseases; Dietary Guidelines; Ecology and Food; Functional Foods; Iron; Maize: Maize as a Food; Nutraceuticals; Nutrition; Nutritional Biochemistry; Trace Elements; Vitamins.
Kenneth John Carpenter
Fairweather-Tait, S. J. "Trace Element Bioavailability." Bibliotheca Nutritio et Dieta 54 (1998): 29–39.
Hallberg, Leif. "Perspectives on Nutritional Iron Deficiency." Annual Review of Nutrition 21 (2001): 1–21.
Southgate, D. A. T., I. T. Johnson, and G. R. Fenwick, eds. Nutrient Availability: Chemical and Biological Aspects. Royal Society of Chemistry Special Publications 72. Cambridge, U.K.: Royal Society of Chemistry, 1988.