Although prostate cancer is the second-most common cancer in American men, after skin cancer, it is often slow growing and localized. Instead of treating this type of prostate cancer, doctors simply monitor it through "active surveillance," which involves regular blood tests and examinations.

"Observation and active surveillance are reasonable options for some men with slow-growing cancers because it is not known if treating the cancer with surgery or radiation will actually help them live longer," The American Cancer Society reports on its website.

However, a new study from MD Anderson Cancer Center at the University of Texas has found that men who follow the dietary principles of the Mediterranean diet could reduce the risk of their prostate cancer worsening as they age.

The Mediterranean diet—which is based on vegetables, fruits, grains, legume and fish, with a balance of monounsaturated fats—has been shown to reduce the risk of cardiovascular disease, numerous cancers and early death.

The study: Adherence to the Mediterranean diet and grade group progression in localized prostate cancer: An active surveillance cohort was published online on Jan. 7 in the journal Cancer.

Summary: Men who have been diagnosed with prostate cancer might slow its progression if they follow a Mediterranean-style diet, researchers at the University of Texas MD Anderson Cancer Center found. Researchers calculated a "diet score" based on a 170-item food frequency questionnaire, then classified the men based on their adherence to the diet as high, medium or low.

The findings: The men who followed the Mediterranean diet most closely had the lowest risk of their prostate cancer advancing: For every one-point increase in a participant's diet score, researchers saw the risk of cancer progression decrease at least 10%. Among non-white men, most of whom in the study were Black, the risk decreased more than 30%. Diabetics did not seem to benefit from following the Mediterranean diet, however.

Cancer progressed in 76 of the men, or 18.5%. The median follow-up time was 36 months. During that time, 12 participants died of other causes; none had cancer that had progressed.

Study conclusions: This study shows the potential of the Mediterranean diet to limit the spread of low-risk prostate cancer, particularly when men adhere closely to the diet.

"These and other trial findings targeting largely plant-based, low-fat dietary patterns suggest that promoting overall healthy eating patterns, rather than individual diet components, may be the most relevant in terms of affecting cancer-related outcomes among men with localized, low-risk prostate cancer," the study authors write.

Why the research is interesting: Treatment for prostate cancer can cause incontinence and erectile dysfunction, whether the treatment is surgery, radiation, chemotherapy or hormone therapy. Infertility may also occur. As prostate cancer's side effects might significantly affect a man's quality of life, reducing or slowing the growth of prostate cancer offers tremendous benefits.

“Men with prostate cancer are motivated to find a way to impact the advancement of their disease and improve their quality of life,” Justin Gregg, M.D., assistant professor of Urology and lead author of the study, said in a statement released with the study results.

The increased benefit to Black men is particularly interesting, because they are much more likely to be diagnosed with and die from prostate cancer than non-Hispanic white men, according to 2017 data from the Centers for Disease Control and Prevention:

Points to consider: This was a small study and the men's cancer generally was categorized as low risk. More study is needed, the authors say, to see if the effects are the same in larger, more diverse groups and in men with higher risk prostate cancer.

“Our findings suggest that consistently following a diet rich in plant foods, fish and a healthy balance of monounsaturated fats may be beneficial for men diagnosed with early-stage prostate cancer,” Gregg said. “We are hopeful that these results, paired with additional research and future validation, will encourage patients to adapt a healthy lifestyle.”

How it was done: The study included 410 men whose cancer cells were very similar to or only slightly differentiated from normal prostate tissue—these cells receive grades of 1 or 2 on the Gleason scale—and were on an active surveillance program. Researchers tracked participants' adherence to the Mediterranean diet, changes in their Gleason scores and diabetes status, statin use and more.

Of the 410 men, 15% had diabetes while 44% used statins, which are prescribed to lower cholesterol levels in the blood.

By ethnicity, 82.9% of the men were white, 8.1% Black and 9% other or unknown. The median age was 64.

What was measured: All the men had a biopsy at the start of the study, then physical exams and blood tests every six months. The blood tests measured the levels of prostate-specific antigen (PSA) and testosterone.

Authors: Justin R. Gregg, M.D., Department of Urology, MD Anderson Cancer Center, Houston, Texas; Xiaotao Zhang, Ph.D., Department of Epidemiology, MD Anderson Cancer Center; Brian F. Chapin, M.D., Department of Urology, MD Anderson Cancer Center; John F. Ward, M.D., Department of Urology, MD Anderson Cancer Center; Jeri Kim, M.D., Merck & Co, Inc, Kenilworth, New Jersey, formerly associate professor, Genitourinary Medical Oncology, MD Anderson Cancer Center; John W. Davis, M.D., Department of Urology, MD Anderson Cancer Center; and Carrie R. Daniel, Ph.D., Department of Epidemiology, MD Anderson Cancer Center.

Financial supporters of the research included the Department of Defense Prostate Cancer Research Program, a National Cancer Institute Cancer Center Support grant; and a Research Training Award for Cancer Prevention Post-Graduate Training Program.