Zinc absorption and metabolism

On average, absorption of zinc is around 20 to 40 per cent of dietary intake, improving when zinc intake is low. Absorption also depends on the food source. More zinc is available from animal and fish sources as these high protein foods contain amino acids which bind to zinc and make it more soluble. Zinc from vegetables, fruits and cereals is less well absorbed as these foods contain compounds such as phytates and oxalates which binds zinc and reduces the amount available for absorption. Food additives and chemicals such as EDTA, which are used in food processing, can also reduce zinc absorption as can large amounts of textured vegetable protein. Zinc absorption decreases with age. People over 65 may absorb half as much zinc as those between 25 and 30 years old.

Zinc is combined in the intestines with picolinic acid which is secreted by the pancreas. This compound requires vitamin B6 for production. The zinc picolinate complex is transported across the absorptive cells of the intestine, then to the liver where some is stored. Vitamin B6 deficiency or a decrease in pancreatic secretion, which is often seen in elderly people, will therefore affect zinc absorption.

Zinc absorptionExcretion of zinc is mainly via the feces but some is lost in the urine. Excessive sweating can cause losses of up to 3 mg per day. Zinc is not well stored in the body and a reduction in dietary intake leads to deficiency fairly quickly. Plasma or serum zinc levels may not reflect body levels. Red or white blood cell measures of zinc may be the most accurate way to assess body stores.

Zinc absorption does not seem to increase during pregnancy but, according to a 1997 study, can increase nearly two-fold during lactation, presumably in response to the demand for zinc to synthesize breast milk.

Hormone replacement therapy has been shown to decrease zinc excretion. In a study done in 1996, Israeli researchers assessed the effect of estrogen treatment on the excretion of several minerals, including zinc in 37 postmenopausal women. They found that zinc excretion decreased 35 per cent utter three months and 26 per cent after one year of treatment.

 
 
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