Although more research is needed, the findings suggest that men may be able to modify their risk of aggressive prostate cancer by maintaining a healthy weight.
The results of both studies are reported in the December 22 online edition of the Journal of Clinical Oncology (JCO).
"The primary role of obesity in prostate cancer is still unclear, but it appears to induce the development of more aggressive tumors," said Christopher L. Amling, of the Naval Medical Center's department of Urology in San Diego and lead author of one of the studies. "I would advise patients to maintain a normal body weight to limit the possibility that they would develop clinically significant, more aggressive prostate tumors."
Both studies examined the relationship between obesity and prostate cancer recurrence in large samples of men with localized prostate cancer who had undergone surgery to remove the prostate - a procedure called radical prostatectomy.
While obesity rates in the general adult population are similar between African-American and Caucasian men, both studies found that obese patients in the study groups were more likely to be African American. This finding may help explain why African-American men with prostate cancer generally have more aggressive tumors and worse outcomes compared to Caucasians.
"We suspect that worse outcomes among African-American men with prostate cancer are related to obesity rather than race. If we can target obesity in the African-American community, we may be able to reduce the burden of prostate cancer among black men," explained lead author of the second study, Stephen J. Freedland, based at the multi-institution Shared Equal Access Regional Cancer Hospital (SEARCH) Database and currently at the James Buchanan Brady Urological Institute at the Johns Hopkins University School of Medicine.
While other studies have indicated that obesity influences cancer recurrence for women diagnosed with breast cancer, the current studies are the first to investigate the relationship between obesity and prostate cancer recurrence after surgery.
Dr. Amling's study involved 3,162 prostate cancer patients, including 19 per cent who were obese. Obesity was defined as a body mass index of 30 kg/m2 or higher - the equivalent of a 5 foot, 10 inch man weighing 210 pounds or more. In this study, obesity was associated with an elevated Gleason score - a method used to classify the aggressiveness of prostate tumors - and a higher rate of cancer recurrence (determined by elevated PSA levels). African-American men in the study were more likely to be obese (27 per cent vs. 18 per cent of white men), have higher cancer recurrence rates, and more aggressive disease.
Dr. Freedland's study involved 1,106 patients, including 22 per cent who were obese. In this study, mild obesity was also defined as a body mass index of 30 kg/m2 or greater. However, moderate and severe obesity was defined as a body mass index of 35 kg/m2 or greater- equivalent to a 5 foot, 10 inch man weighing 245 pounds or more - and was associated with a higher Gleason score and higher rates of cancer recurrence (determined by rising PSA levels).
Men with a BMI of 35 kg/m2 or greater had a 60 per cent risk of cancer recurrence within 3 years, the investigators found. In this study, 31 per cent of African-American men were obese, compared to 21 per cent of Caucasian men.
Both Drs. Amling and Freedland suggest that proteins and hormones stored in body fat - such as leptin and insulin-like growth factor-1 - may promote prostate tumor growth in obese men. Also, obese men typically have lower testosterone levels and higher estrogen levels, which may encourage the growth of cancer. In addition, diets high in fat may promote tumor growth.
An accompanying editorial in the JCO commends the studies for presenting a provocative thesis relating obesity to prostate cancer aggressiveness and outcome.
"In light of the rising incidence of obesity worldwide, identifying obesity as a risk factor for aggressive prostate cancer is important, since it may be one of the few modifiable risk factors for prostate cancer," said Alfred Neugut, professor of Medicine and Epidemiology, and head of cancer prevention at Columbia university's Herbert Irving Comprehensive Cancer Center and author of the editorial.
"The number of prostate cancer survivors is steadily increasing," Dr. Neugut added, "and it will be important to investigate if weight loss and other lifestyle changes can improve prognosis in those already diagnosed with prostate cancer."
The full findings of the two studies can be accessed on the JCO, the peer-reviewed journal of the American Society of Clinical Oncology (ASCO).