Zinc is an essential trace mineral and intakes have generally been shown to be too low in Western populations. In particular, studies have identified chronic low zinc intakes that can be clinically defined as insufficient. Insufficiencies suggest an intake low enough to reduce optimal function, yet high enough to prevent outright deficiencies. Zinc is required as a cofactor for a number of enzymes in humans, more than any other trace mineral. In this regard zinc is required for correct reproductive, growth, development and immune functions. Zinc is also an essential element in other mammals including rats, and much of the understanding of how zinc functions at a molecular level has been derived from studies using rodents. Zinc deficiency in rodents causes skin lesions, anorexia, growth retardation, impaired keratogenesis, hypogonadism, hypogeusia, lethargy, increased susceptibility to infection and impaired immune function generally. Therefore zinc may play a similar role in rodents and humans.
Starvation of mammals reduces circulating concentrations of triiodothyronine, (T3), thyroxine (T4) and thyroid stimulating hormone and may lower thyroid releasing hormone (TRH) suggesting that a high energy intake is required for correct thyroid hormone production. Some evidence suggests that certain macronutrients, particularly carbohydrate, may affect thyroid function more than other macronutrients. The reduction in thyroid hormones seen following energy restriction is measurable as a reduction in core body temperature, and explains most if not all of the reduction in resting metabolic rate (RMR) seen during such diets. Low levels of circulating thyroid hormones, particularly the active T3, are one of the main reasons that energy restrictive diets do not cause successful long term fat loss, when compared to ad libitum access to healthy food. While initial weight loss does occur, the fall in thyroid hormone levels assures that this weight loss does not continue as long as energy restriction proceeds.
The role of zinc in rodent hypothalamic-pituitary-thyroid function has been investigated1. One group of rats were fed zinc deficient diet, while another control group received a zinc replete diet. The rats fed the zinc deficient diet failed to gain weight compared to the control rats, but rats fed a calorie constricted diet showed similar stunted growth curves. The zinc deficient rats and the calorie restricted rats had depressed thyroid function as measured by circulating levels of triiodothyronine and thyroxine when compared to the rats fed a zinc replete ad libitum diet. Hypothalamic thyrotropin-releasing hormone was also suppressed in the zinc deficient rats compared to the ad libitum zinc replete rats. The hypothalamic-pituitary-thyroid axis therefore appears to be adversely affected by zinc deficiency, possibly to a greater extent that energy restriction. Zinc may therefore be required for correct production of triiodothyronine, which may explain some of its effects on the growth of mammals.
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