By Annette M. Boyle
DURHAM, NC – Researchers are grappling with the results of a new study that shows an association between increased exercise and reduced risk of prostate cancer in white men but not black men.
“Bottom line, the underlying reasons why exercise may be beneficial against prostate cancer are still being elucidated,” explained researcher Lionel L. Bañez, MD, of the Veterans Affairs Medical Center in Durham, N.C.
“We have seen reports from other laboratories with regard to the benefit of exercise in terms of preventing cancer development. Those studies indicate that exercise may modify multiple biological pathways, some of which have genetic differences associated with race. We hypothesize that the difference in the genetics in those pathways may explain why exercise is beneficial for one racial group, but not others,” Bañez told U.S. Medicine.
The prospective study, published in Cancer, included 307 veterans who were scheduled to undergo prostate biopsies at the Durham VAMC between January 2007 and June 2011. Of the participants, 164 were white and 143 were black.
Participants completed the Godin Leisure-Time Exercise Questionnaire prior to their biopsy appointments, which measured how often and how long the men participated in mild, moderate and strenuous intensity exercise in a typical week. The results were converted into metabolic equivalent (MET) hours of total current exercise. For example, easy walking was 3 METs, tennis was 5 METs and running was 9 METs. The study established four exercise categories based on METs per week: less than 3 METs (sedentary), 3 to 8.9 (mildly active), 9 to 17.9 (moderately active) and more than 18 (highly active).
Both racial groups reported similar amounts of exercise. In white men, however, a “greater amount of MET hours per week was associated with a decreased risk of a positive biopsy” on a continuum (OR .89, CI .81-.98). More than 9 MET hours per week was significantly associated with a decreased risk of prostate cancer in white men, compared with those who were sedentary (OR .47, CI .22-.99), but white men in the mildly active category did not have the same degree of benefit.
“Our study was not designed to determine whether there was a breakpoint below which exercise is ineffective or whether there was a point at which more exercise conferred no greater protection. When you look at smaller subgroups, there is a risk of being underpowered to determine a relationship,” noted Bañez. “Consequently, it is important for our results to be validated in a larger cohort, with men of various backgrounds.”
Higher rates in black men
For black men, however, the study found no association between any level of exercise and the risk of positive biopsy findings. In addition, while exercise was inversely associated with high-grade disease in white men, that was not true for black men in the study.
“African-American men are more likely to be diagnosed with prostate cancer and more likely to die of prostate cancer than their Caucasian counterparts,” said Bañez. “That was the impetus for us looking at risk of prostate cancer as function of race and exercise. It is generally accepted that the increased risk among African-American men has multifactorial causes.”
The authors identified several potential areas of difference in exercise-induced molecular pathways that might affect the impact of exercise on prostate cancer risk. Overall, men who do not exercise have a 1.7 times greater risk of prostate cancer and other studies indicate that exercise “favorably modulates pathways associated with tumorigensis through changes in the transcription of genes involved in protein metabolism, intracellular protein traffic and protein phosphorylation,” the authors noted.
“Exercise also stimulates antioxidant pathways, repairs oxidation damage and protects against oxidation stress, all linked to cancer development,” added Bañez. “Further, it promotes weight loss, which other data shows may reduce the risk of cancer.”
Racial differences in any of these pathways may explain why exercise does not confer the same reduction in prostate cancer risk on black men as their white counterparts, but the exact reasons are yet to be discovered, Bañez said.
The study noted that the difference in genetic susceptibility to prostate cancer between black and white men suggests “unique race-specific biologic mechanisms at play,” which might lead to the differing impact of exercise noted in the data. In addition, previously identified racial differences in hormonal profiles might result in different effects from the increased levels of sex hormone-binding globulin and other hormones released during exercise in middle-age and older men.
“Our ultimate goal is to find out why there is such a disparity in the risk of prostate cancer between African-American and Caucasian men. Studies such as ours that look specifically at exercise may identify one potential contributor. Exercise is likely not a major factor in the disparity we see, but by understanding the underlying mechanisms that explain the racial disparity in the effects of exercise on prostate cancer risk, perhaps we can come closer to understanding the more significant factors involved,” said Bañez. “If we find out why there’s a difference, then it could be leveraged to reduce the risk of prostate cancer in men of all backgrounds.”
Despite the results of the study, Bañez encouraged all men to continue to exercise regularly. “Aggressive prostate cancer is relatively rare in all groups,” he noted. “Aside from those with aggressive prostate cancer, the course of the disease is more protracted, and the great majority of men are more likely to die from other causes. Exercise generally improves health, and we know it specifically reduces the risk of cardiovascular disease. Most men with diagnosed prostate cancer are more likely to die of cardiovascular disease than the cancer, so continuing to exercise regularly or starting an exercise program are likely to reduce overall mortality.”