Zinc and prostate cancer

Content provided by the Faculty of the Harvard Medical School

Zinc and prostate cancer

The dietary supplement business is a multibillion-dollar industry. You can buy supplements in virtually every supermarket and drugstore in America, to say nothing of the growing number of health food stores and "body shops" that specialize in them. The array is bewildering, and dramatic health claims and testimonials add to the confusion — and the temptation. But despite their variety, supplements have two things in common: Few have been studied scientifically, and none is regulated by the FDA.

A large number of supplements are sold to protect the prostate and promote its healthy function. Many contain zinc. About 15% of all Americans take supplements that contain zinc — and many men who take them are getting much more zinc than recommended.

Is zinc good for your prostate? Doctors don't know, but two studies provide information on the question.

Zinc and health

Zinc is much more familiar because of its industrial uses than because of its role in the human body. The metal is a micronutrient, one of 11 "trace" minerals that the body needs in very small amounts to maintain health.

In the body, zinc binds to proteins, forming metalloenzymes that regulate many metabolic functions. That's how zinc plays a role in many processes, including normal growth, tissue repair and wound healing, various functions of the immune system, and in the senses of taste and smell.

Zinc is present in many foods. Oysters and other shellfish head the list, but good amounts are also present in the bran of many grains, nuts, legumes, and red meats. On the other hand, poultry, eggs, and dairy products have little zinc. But since the recommended dietary allowance for men is only 11 mg a day, nearly all Americans get enough from food, even without supplements. Zinc deficiency does occur in some remote parts of the world, where it causes delayed growth in children, testicular failure and impotence, an impaired sense of taste, hair loss, immune disturbances, and a variety of other problems.

Doctors have used zinc tablets or lozenges to treat the common cold and to improve impaired taste and smell, but the results have been mixed at best. Ointments that contain zinc make excellent sunblocks, turning away nearly all the harmful ultraviolet rays. Zinc ointments are also used to help heal wounds and to help patients recover from bedsores. The antioxidant vitamins that can slow the progression of age-related macular degeneration contain 80 mg of zinc. Side effects from zinc are rare, but extremely high doses can result in low HDL ("good") cholesterol levels, immune disturbances, gastritis, and deficiency of another trace mineral, copper.

Except for lifeguards, who use zinc oxide ointment to keep their noses pale, and new mothers, who use it to treat diaper rash, most people don't need to give zinc a moment's thought. But men who think about their prostate often turn to zinc, thoughtfully or not.

Zinc and the prostate

Zinc is present in all the body's organs and fluids, but the prostate has a higher concentration than any other tissue except bone. Scientists don't know why it has so much zinc. Among other functions, though, the metal has a role in regulating the balance between testosterone and dihydrotestosterone, the two male hormones that stimulate the growth of prostate cells, both benign and malignant.

Although normal prostate tissue has lots of zinc, cancerous tissue has much less. That may be the reason zinc is turning up in so many supplements for men. The idea has obvious appeal, but does it have scientific merit?

Older studies

Studies of zinc and prostate cancer are incomplete, even contradictory. Some experiments show that zinc slows the growth of cancer cells in the laboratory. On the other hand, zinc has been linked to high levels of two substances that promote the growth of prostate cancer, insulin-like growth factor-1 and testosterone.

When animal and lab experiments are contradictory, researchers turn to human observation. Many studies have asked if a man's consumption of zinc is connected to his risk of prostate cancer. Unfortunately, the results are all over the map. One of the problems is that many studies rely on a dietary history to estimate zinc consumption, but the zinc content of grains and other foods can vary considerably, depending on the metal content of the soil.

Recent studies

Two studies, one based at Johns Hopkins, the other at Harvard, attempt to overcome the problem by measuring the amount of zinc in the body and by evaluating the effects of very large amounts of supplementary zinc.

The Johns Hopkins study explored the possible link between zinc and prostate cancer in men living in Washington County, Maryland. Instead of estimating dietary zinc, the scientists actually measured zinc levels — not in the prostate or blood, but in the toenail clippings of more than 9,000 men. The volunteers were aged 42 to 82 when the study began in 1989, and none had been diagnosed with prostate career. Each man also reported his medical history and health habits.

The researchers tracked the men for seven years, identifying 145 new cases of prostate cancer. They then compared the zinc levels of the men who developed cancer with those who did not. When the results were analyzed, there was no link between zinc and prostate cancer, either helpful or harmful.

The scientists at Harvard reasoned that if zinc influences the risk of prostate cancer, men taking supplements might display the link most clearly, so they evaluated 46,974 men in the Health Professionals Follow-up Study. Between 1986 and 2000, 2,901 men were diagnosed with prostate cancer, including 434 advanced cases. Because all the men provided detailed information about their diets and supplements, the scientists were able to check for a relationship between zinc consumption and prostate cancer. There was no link between dietary zinc and prostate cancer; nor did modest supplement use affect risk. But the prolonged use of high-dose supplements was another matter. Men who consumed supplemen­tary zinc for 10 years or longer were 2.4 times more likely to develop advanced prostate cancer than those who took none, and men who used more than 100 mg a day were 2.3 times more likely to develop advanced forms of the disease.

A work in progress

As of early 2006, it seems likely that a healthy diet will provide all the zinc a man needs for his prostate — and his other organs, too. Moderate amounts of supplementary zinc don't appear to influence the risk of prostate cancer one way or the other, but the prolonged use of large doses may do more harm than good. And there is little evidence that supplements containing zinc have a role in reproductive function. Clearly, more research is needed.

The zinc story is incomplete, but it raises the larger question of supplements in general. If there is a moral, it's that claims and hopes should never replace sound scientific information. That information is lacking for most supplements. Men might be wise to wait for the evidence before springing for unproven supplements.

Human disease is complex. As one wit put it, every complex problem has a simple solution — and it's usually wrong.

Last updated:	September 05, 2008