Dr. Hernandez and colleagues studied the relationship in a multiethnic population consisting of blacks, Japanese, Hispanics, Native Hawaiians and whites, and compared differences among age groups using the Multiethnic Cohort, a longitudinal study of men 45-75 years of age established in Hawai'i and California from 1993-1996. Of the 83,879 men who participated in the study, 5,554 developed prostate cancer. Overall, men who were overweight or obese by age 21 had a decreased risk of localized and low-grade prostate cancer, according to Dr. Hernandez.
Their results suggested that being overweight in older adulthood was associated with increased prostate cancer risk among white and Native Hawaiian men, but a decreased risk among Japanese men. While excessive weight gain between younger and older adulthood was observed to increase the risk of advanced and high-grade prostate cancers in white men and increase the risk of localized and low-grade disease in black men, it appeared to decrease the risk of localized prostate cancer in Japanese men.
Dr. Hernandez said, "The relationship of certain characteristics, such as body size, with cancer risk may vary across ethnic groups due to the combined influence of both genes and lifestyle."
Obesity is a known risk factor in other common cancers, including colorectal cancer and breast cancer in post-menopausal women. However, the relationship between body size and prostate cancer risk is not entirely understood.
Excess fat is associated with a number of conditions that contribute to cancer development including low-grade chronic inflammation, insulin resistance, metabolic abnormalities, and hormone imbalances. These conditions may in turn contribute to more aggressive prostate malignancies.
Ethnic differences in cancer risk may be explained by differences in the distribution of stored body fat that could have a differential effect on the development of prostate cancer. For example, in comparison to whites, Asian men and women tend to accumulate more fat in their upper bodies and less fat in their lower bodies (including legs and buttocks). These differences in the distribution of body fat may influence the specific way that excess fat influences cancer risk.
Elizabeth A. Platz, Sc.D., M.P.H., associated professor of epidemiology at the Johns Hopkins Bloomberg School of Public Health, Baltimore, and an editorial board member for Cancer Epidemiology, Biomarkers & Prevention, emphasized as a strength of this study, in spite of its inconsistency across racial/ethnic groups, that it was conducted prospectively and consisted of large numbers of men in most of the ethnic groups studied. An estimated 30 percent of prostate cancer cases occurred among Japanese men, 25 percent among white men, 27 percent among Hispanic men, 13 percent among black men, and seven percent among Native Hawaiian men. According to Dr. Hernandez, their findings do not warrant a change in public health messages about obesity that all men of normal weight should be encouraged to avoid weight gain and those who are overweight or obese should be encouraged to lose weight to achieve good health.