Zinc is an important trace mineral that is required as a co-factor by more enzymes than any other metal. Zinc plays an important role in the immune and reproductive system, and evidence suggests that Western diets are too low in zinc, although deficiency diseases are rare. Absorption of zinc is highest when dietary intakes are low, and as intakes rise absorption levels are reduced. This mechanisms, along with control of excretion, maintains zinc homeostasis to provide consistent levels of zinc to tissues. For example, researchers1 fed post menopausal women a control diet containing around 5mg of zinc, with supplementation increasing zinc intakes to 14 (low), 32 (medium), or 47mg/d (high). The data showed that increasing intakes resulted in the absorption of relatively uniform amounts of zinc irrespective of the dose of tablet.
At baseline, absorption of zinc was 4.6, 8.7 and 10.3mg/d in the low, medium and high zinc groups, respectively. However, by week 8, the differences in absorption were not significant (5.4, 5.8 and 6.4mg/d in the low, medium and high zinc groups, respectively), and had almost disappeared by week 16 (5.0, 5.0 and 5.1mg/d in the low, medium and high zinc groups, respectively). This suggests that the subjects had normalised their zinc intake to a uniform absorption of around 5mg/d. This may be due to a saturation response with regard uptake and is consistent with models of zinc absorption. Interestingly, the zinc plasma levels did not differ between subjects, suggesting that homeostatic controls were able to maintain uniform plasma levels despite different intakes of zinc. Very high zinc diets may therefore not be beneficial to zinc status in postmenopausal women.
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