Zinc is an essential trace mineral, which is second only to iron as the most plentiful trace element in the body. It aids in the absorption of B vitamins, and is a constituent of at least 25 enzyme systems, as well as being a component of insulin and of superoxide dismutase. Zinc is essential for growth and development of sexual organs, is important in nucleic acid synthesis and plays a role in carbohydrate and phosphorus metabolism. There is concern regarding the absorption of zinc, both from dietary intake and nutritional supplements, because of zinc's essential role in human nutrition and metabolism, and because of evidence that some population groups have a marginal-to-deficiency condition: not all zinc preparations have equal bioavailability.