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absorbable species being absorbed by the intestine

In theory, zinc must be either in soluble form or bound to soluble, absorbable species to be absorbed by the intestine. The efficiency of mastication and gastric and pancreatic protease digestion of food determines the extraction of zinc from its food matrices. Once liberated and dissolved, the zinc ion needs to remain in solution and proceed in its transit through or around the enterocytes.

Substances that influence intestinal permeability to zinc are also mediators of bioavailability. A series of dietary substances influence the bioavailability of zinc, either by chelating or binding zinc or by competitively inhibiting its absorption. Two common components of whole-grain cereals – phytic acid and dietary fibre – are the most important substances that interfere with zinc absorption. Polyphenolic substances (tannins) in tea and coffee and oxalic acid of green leafy herbs are also inhibitors. Calcium and dietary nonheme iron are two inorganic elements that interfere with zinc uptake by direct competition. (See DIETARY FIBER | Effects of Fiber on Absorption; PHYTIC ACID | Nutritional Impact.)

Alternatively, a series of compounds have been suggested as enhancers or promoters of zinc absorption, including amino acids, sugars, picolinic acid, citric acid, and prostaglandins. Only the first two classes of compound are likely to be of physiological significance. Red wine has been shown to enhance zinc balance under metabolic conditions. Ascorbic acid and lactose have a neutral influence on zinc bioavailability.

It should be remembered that bioavailability refers to a fractional efficiency of uptake, and the total amount of zinc ingested is a codeterminant of how much net nutrient will be absorbed. If the zinc intake is high in a diet of poor zinc bioavailability, the amount absorbed may still be sufficient to support true nutritional requirements.