Why low-protein diets and vegetarian diets tend to be low in zinc.

The following is extracted from 

http://www.nlm.nih.gov/medlineplus/ency/article/002416.htm

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High-protein foods contain high amounts of zinc. Beef, pork, and lamb contain more zinc than fish. The dark meat of a chicken has more zinc than the light meat.

Other good sources of zinc are nuts, whole grains, legumes, and yeast.

Fruits and vegetables are not good sources, because the zinc in plant proteins is not as available for use by the body as the zinc from animal proteins. Therefore, low-protein diets and vegetarian diets tend to be low in zinc.