Multiple studies support diet's effect on prostate cancer

San Francisco-Many studies suggest that diet can have a significant impact on the incidence, progression, and mortality of prostate cancer.

"It can be challenging to interpret the literature," said June Chan, ScD, associate professor of epidemiology, biostatistics, and urology at the University of California, San Francisco. "But the existing literature seems clear that diet can reduce the risk of prostate cancer."

Dr. Chan presented an overview of such studies to attendees of the 2008 Genitourinary Cancers Symposium here.

The male offspring of first-generation immigrants had an even higher incidence: 120 in 100,000, Dr. Chan said.

"We saw a 40-fold difference in prostate cancer rates between countries of origin and the offspring of first-generation immigrants," she noted. "It is clear that something in the new environment is playing a role in the development of prostate cancer in this population."

That something appears to be diet, Dr. Chan said. A diet high in calcium, including milk and other dairy products, is associated with an increased risk for prostate cancer. A possible mechanism is the reduction of biologically active forms of vitamin D by an increased calcium intake. Vitamin D has been shown to inhibit the growth of prostate cancer cells.

Additional risk factors appear to include smoking status, increased body fat, and a diet high in processed meats and red meats. Mixed, saturated, and alpha-linolenic fatty acids have all been associated with prostate cancer risk.

Positive effects of diet

On the positive side, some foods and vitamins have been shown to have a positive effect on prostate cancer risk.

Several studies have shown statistically significant reductions of 25% to 80% in prostate cancer risk associated with consumption of lycopene. Vitamin E has been associated with up to a 40% reduction in the risk of prostate cancer and mortality in multiple studies. Other studies have found a 50% to 65% reduction on prostate cancer risk associated with greater intake of selenium or with higher physiologic levels of selenium. A large randomized, controlled trial, the Selenium and Vitamin E Cancer Prevention Trial, is currently under way to examine the effects of these supplements more closely.

Pulses (seeds of various crops), legumes, cruciferous vegetables, fish, and long-chain marine omega-3 fatty acids also appear to convey some protective benefits against the development of prostate cancer. Exercise also may reduce the risk of the disease, but there is continuing debate over both the amount and the intensity of physical activity needed to affect its course.

On the negative side, a large prospective cohort study found an unfavorable association between the intake of multi-vitamins more than once daily and prostate cancer risk. The association was more unfavorable for men who were also taking dietary supplements or who had a family history of prostate cancer.

Disease progression and diet

There are fewer data on the role of diet in the progression of prostate cancer. A large prospective cohort study found that higher intake of tomato products and fish following diagnosis was associated with lower risk for progression or recurrence. The association was independent of tumor features, treatment, and diet before diagnosis, Dr. Chan said.

Multiple small interventional studies have reported declines in PSA levels associated with various low-fat and tomato-rich diets. Lower PSA levels also have been associated with the use of soy, lycopene, and other antioxidant supplements.

Dr. Chan reported that unpublished data from the Cancer of the Prostate Strategic Urologic Research Endeavor Nutrition Study showed specific effects from diet. The first iteration of the study, which began in 1995, shows a protective effect from the increased consumption of vegetables, particularly yellow and cruciferous varieties. Increased consumption of poultry with skin and eggs was associated with higher risk for prostate cancer.

"A generally good, heart-healthy diet may protect against the occurrence of prostate cancer and against the progression or recurrence of existing disease," Dr. Chan said. "The data are not absolute, but it seems clear that dietary habits can affect the course of this disease."