INTERACTION WITH DIETARY COMPONENTS .1 E FFECT ON A BSORPTION

Various nutritional and nonnutritional components of the diet, other nutrients in vitamin–mineral supplements, or assorted medications can interact with minerals in

the gastrointestinal tract and influence their absorption. For example, amino acids may perform as intraluminar binders for some trace minerals. Large, complex, and poorly digestible proteins, on the other hand, may bind minerals tightly and diminish their absorption. Triacylglycerols and long-chain fatty acids derived from triacyl-glycerols may form soaps with calcium and magnesium and decrease the bioavail-ability of these two nutrients.

Lactose has been implicated in the enhanced absorption of calcium from milk.

Pectins, cellulose, hemicellulose, and polymers produced by the Maillard reaction during cooking, processing, or storage may bind minerals in the lumen and thus reduce their biological availability. Interaction between and among minerals, or with anionic species, are important determinants of mineral absorption. Absorption of iron is hindered by fiber and phosphates and promoted by ascorbic acid, copper, and meat protein. Ascorbic acid also enhances absorption of selenium, but reduces the absorption of copper. A high protein intake appears to increase the excretion of calcium, whereas vitamin D ingestion promotes the retention of calcium.

Intestinal parasites, dietary fiber phytates, and excessive sweating interfere with zinc absorption. Phytates, oxalates, and tannates can interfere with the absorp-tion of a number of minerals. Certain medicaabsorp-tions such as tetracycline can also inhibit absorption of minerals, while others such as didoquin or dilatin may actually promote uptake of certain minerals. Apparently, chemically similar minerals share certain “channels” for absorption, and the simultaneous ingestion of two or more such minerals will result in competition for absorption. When unphysiological imbalances among competitive nutrients exist as the result of leaching from water pipes, storage in unlacquered tin cans, or improper formulation of vitamin–mineral supplements, nutritionally important consequences of this mineral–mineral inter-action can result.

Finally, to participate in a nutritionally relevant process for the organism as a whole, a mineral must be transported away from the intestine. The concentration of circulating binding proteins and the degree of saturation of their metallic binding sites may influence the rate and magnitude of transport of recently absorbed minerals (Rosenberg and Solomons, 1984).

Minerals require a suitable mucosal surface across which to enter the body.

Resection or diversion of a large portion of small bowel obviously affects mineral absorption. Extensive mucosal damage due to mesenteric infarction or inflamatory bowel disease or major diversion by jejunoileal bypass procedures reduces the available surface area. Minerals whose absorption occurs primarily in the proximal intestine, e.g., copper or iron, are affected differently than those absorbed more distally, e.g., zinc. In addition, the integrity of the epithelium, the uptake of fluids and electrolytes, the intracellular protein synthesis, the energy-dependent pumps, and the hormone receptors must be intact.

Intrinsic diseases of the small intestinal mucosa may impair mineral absorption.

Such conditions as celiac sprue, dermatitis herpetiformis, infiltrative lymphomas, and occasionally inflammatory bowel disease produce diffuse mucosal damage.

Protein energy malnutrition causes similar damage, and tropical enteropathy affects part of the population of developing countries living under adverse nutritional and hygienic conditions.

As can be seen from above, the absorption of most metals from the gastrointes-tinal tract is variable (Table 4.2) and depends on many external and internal factors.

Thus the quantity of metal ingested rarely reflects that which is bioavailable. In fact, under most circumstances, only a small fraction of ingested metals is absorbed, while the great majority passes out of the gut in the feces.

The Recommended Dietary Allowance (RDA) represents standards of nutrition set by the Food and Nutrition Board of the U.S. National Academy of Sciences (Feltman, 1990). It contains the levels of essential nutrients that are adequate to meet the nutritional needs of the normal, healthy population. Individuals may differ in their precise nutritional requirements. To take these differences into account, the

TABLE 4.2

Mean Daily Intake, RDA or SAI, and Absorption Percentage of Minerals from the Gastrointestinal Tract

Milligram per Adult

Mineral Daily intake RDA^a or SAI^b

Percentage

d0.002 mg of cobalt containing vitamin B₁₂.

RDA provides a “margin of safety,” i.e., it sets the allowances high enough to cover the needs of most healthy people. For additional nutrients that are necessary to keep the body healthy for which the RDA has not yet been established, a “safe and adequate daily intake” (SAI) is estimated.

The functions of minerals in the body involve building tissue and regulating numerous body processes. Their role in the human body is summarized in Table 4.3.

4.2.2 B^UILDING B^ODY T^ISSUEAND R^EGULATING B^ODY P^ROCESSES

Certain minerals, including calcium, phosphorus, magnesium, and fluorine, are com-ponents of bone and teeth. Deficiencies during the growing years cause growth to be stunted and bone tissue to be of poor quality. A continual adequate intake of minerals is essential for the maintenance of skeletal tissue in adulthood.

Potassium, sulfur, phosphorus, iron, and many other minerals are also structural components of soft tissue (Solomons, 1984; Eschleman, 1984).

Minerals are an integral part of many hormones, enzymes, and other compounds that regulate biochemical functions in the organism. For example, iodine is required to produce the hormone thyroxine, chromium is involved in the production of insulin, and hemoglobin is an iron-containing compound. Thus the production of these substances in the organism depends on adequate intake of the involved minerals.

Minerals can also act as catalysts. Calcium is a catalyst in blood clotting. Some minerals are catalysts in the absorption of nutrients from the gastrointestinal tract in the metabolism of proteins, fat, and carbohydrates, and in the utilization of nutrients by the cell.

Minerals dissolved in the body fluids are responsible for nerve impulses and the contraction of muscles, as well as for water- and acid-base balance. They play an important role in maintaining the respiration, heart rate, and blood pressure in normal limits. Deficiency of minerals in the diet may lead to severe, chronic clinical signs of diseases, frequently reversible after their supplementation in the diet, or following the total parenteral nutrition. Their influence on biochemical reactions in living systems also makes it possible to use them intentionally in many food processes.

4.3 ROLE IN FOOD PROCESSES