Zinc

Daily Value: 15 milligrams

Good Food Sources: Cooked oysters, beef, lamb, eggs, whole grains, nuts, yogurt

When you think zinc, think productivity. From helping to create new skin and sperm cells to boosting the immune system, this mineral works overtime to produce the cells you need to keep healthy.

"Healing, growth, pregnancy, lactation are all situations where there is an increased need for zinc because of the need for more cells," says Adria Sherman, Ph.D., professor and chair of the Department of Nutritional Sciences at Rutgers University in New Brunswick, New Jersey.

A classic example is immune defense. Before your body can battle a foreign invader, zinc and chemicals called zinc-dependent enzymes work together to help build new immune system cells and whip them into fighting trim. That's why zinc is helpful in fending off viral infections.

By the same token, too much zinc--just 25 milligrams a day in one study--has been found to decrease immunity.

Zinc's quick cell replication skills come in handy when you have cuts or wounds. It's vital for the production of collagen, the connective tissue that helps wounds heal, says Richard Wood, Ph.D., associate professor in the School of Nutrition at Tufts University in Medford, Massachusetts, and chief of the mineral bioavailability laboratory at the Jean Mayer USDA Human Nutrition Research Center on Aging at Tufts University in Boston. "And when you don't get enough zinc, normal healing doesn't occur," he says.

Although research findings are slim, some doctors recommend zinc to treat enlarged prostate, a disease that disrupts the flow of urine in men. Zinc hasn't been tested in any large scientific studies, and until it is, many doctors will remain skeptical. It's important to work with a doctor knowledgeable in nutrition if you want to try zinc for prostate problems.

Even several key enzymes that protect and preserve your vision can't be formed without zinc. "Zinc and vitamin A interact in the eyes to maintain the normal process of dark adaptation, where the eyes adjust to low levels of light," says Dr. Wood.

The benefits of zinc aside, it's likely that most Americans simply don't get enough of this mineral. In fact, one study found that 30 percent of healthy elderly people are zinc-deficient. They may not be the only ones: In their zeal to eat low-fat diets, more people than ever are shunning red meat, and red meat is a solid source of this vital nutrient.

Another potential problem: Increased calcium intake, recommended to prevent osteoporosis, removes some zinc from the body, says Dr. Wood. The heavy plant emphasis of some vegetarian diets can interfere with zinc absorption, as can alcoholism, oral penicillin therapy and diuretics (water pills). Low levels of vitamin B₆ have also been found to reduce zinc absorption.

Signs of possible deficiency include impaired immunity, weight loss, bloating, loss of appetite, rashes and other skin changes, bedsores, hair loss, diminished sense of taste or smell, absence of menstrual periods and depression.

Using Zinc Safely

While it's best to get zinc from foods, you can choose from several zinc supplements. But be careful not to take too much. More is not necessarily better. Excessive amounts can cause nausea, headaches, lethargy and irritability. In fact, taking more than 2,000 milligrams of zinc sulfate has been known to cause stomach irritation and vomiting.

Even taking between 30 and 150 milligrams of zinc daily for several weeks interferes with copper absorption and can cause copper deficiency. (For this reason, doctors often recommend that those using zinc supplements take additional copper, in a ratio of 1 milligram of copper to 10 milligrams of zinc.) More than 30 milligrams of zinc a day can increase your risk of developing anemia. Such high doses have also been found to lower levels of HDL, the "good" cholesterol, while raising levels of LDL, the "bad" cholesterol. (A doctor may, however, recommend amounts this high to treat Wilson's disease, a condition involving excess copper in the body.) And increased dietary zinc has been shown to markedly decrease mental functioning in people with Alzheimer's disease. Because of these risks, doctors recommend that zinc supplements in excess of 15 milligrams a day be taken only under medical supervision.

Because zinc can cause stomach upset, it may be taken with food. Dairy products, bran products and foods high in calcium and phosphorus, such as milk, may decrease zinc absorption. Protein-rich foods such as lamb, beef and eggs enhance absorption.