Prostate cancer is the most common cancer in men in North America. In 2008, approximately 186,000 men in the United States and 25,000 men in Canada were diagnosed with prostate cancer, which accounts for 25% of all cancers in men.^{1, 2}

Compared to most cancers, prostate cancer progresses very slowly, and many men live with it for years and even decades. Once diagnosed, some men decide to undergo treatment to halt the progression of the disease, and others refrain from treatment, preferring instead to closely monitor the cancer’s progression. Those who choose “watchful waiting” do this because the medical and surgical treatments for prostate cancer can cause debilitating side effects, and because most men with prostate cancer will die from something else. This strategy is especially likely for older men in the earliest stage of the disease.

At one time, it was unheard of to suggest that diet might have a role to play in battling prostate cancer. But there is now evidence that certain foods and dietary supplements have an impact on prostate health—both positive and negative. Some foods or supplements appear to promote prostate health and prevent cancer cells from developing, but others should not necessarily be taken by men who already have prostate cancer.

The role of diet drew researchers’ attention when they noticed that prostate cancer rates vary greatly from one country to another, with the highest rates appearing in countries where people tend to eat a lot of fat. Studies also show that men who are obese or have a high fat diet are more likely to have prostate cancer.³  Diets high in saturated fats, such as the animal fats found in red meat, may pose the greatest risk. The lowest rates of prostate cancer are found in Asian countries where men eat a lot of soy foods, a rich source of naturally occurring phytoestrogens. It was hoped that by increasing men’s intake of phytoestrogens, they might reduce their risk of prostate cancer or slow its progression, but at least two studies have failed to find any protective benefit from phytoestrogens.^{4, 5}

As more and more people take dietary supplements containing antioxidants, studies have been conducted to determine their effect on reducing the risk and growth of cancers, including prostate cancer. The antioxidants that have received the greatest attention with regard to prostate health are vitamin E and selenium.

Studies comparing men who live in areas of the country with high levels of selenium to men in areas with low levels suggest that this mineral protects against prostate cancer. Selenium is believed to reduce the risk of developing prostate cancer because it keeps cells from proliferating or dying off in a rapid or unusual way. An analysis in 2002 of the Nutritional Prevention of Cancer Trial revealed that the men who took selenium supplements daily were half as likely to be diagnosed with prostate cancer.⁶ However, in 2008, the Selenium and Vitamin E Cancer Prevention Trial (SELECT) indicated that neither selenium nor vitamin E, alone or in combination, was effective for the primary prevention of prostate cancer.^{7, 8}

So do antioxidants prevent prostate cancer or not? The case of selenium is an interesting one that helps shed light on this question. Based on the newest research by Philip Kantoff, June Chan, and their colleagues at the Dana-Farber Cancer Institute in Boston, it seems that higher selenium levels in the blood may worsen prostate cancer in many men who already have the disease.⁹

In his earlier research, Dr. Kantoff had found that the risk of developing prostate cancer was modified by a strong interaction between a mitochondrial enzyme (SOD2) and selenium.¹⁰ In his most recent study published in 2009, Dr. Kantoff and his research team measured selenium in the blood of men with prostate cancer and determined which of the two forms of SOD2 the men had: AA or V.⁹ Among the men with the AA genotype, those with a higher level of selenium in their blood had a lower risk of aggressive prostate cancer. In contrast, the men with the much more common V genotype who had higher levels of selenium in their blood were at an increased risk for aggressive prostate cancer. Unless a man knows which of the two genotypes he has, he may want to avoid taking supplements with selenium, particularly if he has already been diagnosed with prostate cancer.

Dr. Kantoff and his colleagues believe that antioxidants such as selenium can be used to prevent cancer, but that when a person already has cancer, antioxidants may be harmful. Why is this? In addition to helping healthy cells live longer, antioxidants appear to make cancer cells more resilient, too. Recent research on vitamin C, for instance, has shown that, in large doses, it can interfere with chemotherapy, and when cancer cells are implanted in mice and treated with vitamin C, the tumors grow more rapidly.¹¹

More studies are needed in order to determine exactly how diet and dietary supplements can be used to prevent prostate cancer and slow its spread. Meanwhile, men should reduce saturated fats as much as possible. While the jury is still out on phytoestrogens, men may benefit from eating more soy products—especially if they are eating them in place of red meat!