Zinc is an important trace mineral that is involved as a co-factor to a large number of enzymes in human metabolism. Zinc is needed because it can act as a catalyst, or can maintain the structure of the enzyme by applying charge to the protein. The number of proteins requiring zinc as a co-factor outnumbers all other trace minerals combined. Zinc is known to be absorbed from the small intestine via active transport systems. However this absorption can be impeded by phytic acid present in some plant foods, which results in animals sources of zinc such as meat, shellfish and fish to show superior bioavailability compared to vegetable sources. The current government recommendations for zinc is 7 mg per day in the United Kingdom, but evidence suggests that large percentages of the population still have inadequate zinc intakes.
Absorption of zinc is increased from the gastrointestinal tract during times of lowered intake. In addition, metalloproteins release their zinc stores, and less essential enzymes are catabolised to maintain adequate zinc supply. A fall in dietary zinc from 12.2 to 0.23 mg per day can result in 100% absorption of dietary zinc from the small intestine. If intakes remain low, clinical symptoms of deficiency can occur, including growth retardation and immune deficiency. Although clinical deficiency is rare, long-term marginal deficiency appears to be more common in Western nations. Because of its importance in human nutrition, finding reliable biomarkers for zinc status is highly important. Generally, researchers have used both structural components such as hair and nails, enzymatic markers such as carbonic anhydrase or alkaline phosphatase, or plasma concentrations to assess zinc status.
Researchers1 have reviewed the potential biomarkers of zinc intake to assess their reliability. Analysis of 32 potential biomarkers from 46 publications suggested that plasma zinc concentrations responded in a dose response manner to dietary changes in zinc intake in adults, women, men, pregnant and lactating women and the elderly. This effect was evident in those with moderate baseline plasma levels as well as those with low baseline levels. Urinary zinc status was also responsive to intake in all sub-groups. Hair zinc levels appeared to reflect dietary intakes, but the data was not extensive enough to give sub-group details. The analysis also suggested that platelet, polymorphonuclear cell, mononuclear cell and erythrocyte zinc levels as well as alkaline phosphatise, were not representative of dietary intakes. The authors concluded that plasma, urinary and hair concentrations of zinc are reliable markers of zinc intake.
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