BOSTON, July 23 -- Obesity's link to multiple myeloma has received new corroboration from an analysis of data from two large studies of health care professionals.
Obesity more than doubled the risk of multiple myeloma in men, report Brenda N. Birmann, Sc.D., of Harvard, and co-authors, in the July issue of Cancer Epidemiology, Biomarkers & Prevention.
In addition, women who were overweight or obese had a 20% to 60% greater risk of multiple myeloma compared with women of normal weight.
Although neither finding was statistically significant, the trends added to earlier evidence linking obesity and myeloma.
"We observed increasing risks of multiple myeloma with increasing body mass index in two large prospective cohorts," they wrote. "Those findings were independent of physical activity, which was not significantly related to multiple myeloma risk."
Current BMI and BMI values four to eight years earlier had a similar association with myeloma, suggesting that preclinical disease did not have a major influence on the findings, they added.
Several large population-based studies have demonstrated an increased risk of multiple myeloma with increasing BMI. Collectively, the studies suggested that a BMI of 30 kg/m2 or greater, a widely accepted definition of obesity, conferred a multiple myeloma risk 1.5 to 2.0 times greater than that associated with a normal BMI, generally defined as less than 25 kg/m2.
The role of physical activity in multiple myeloma remained unclear. Additionally, results of prior studies might have been influenced by reliance on a single measurement to determine BMI, the authors noted.
Dr. Birmann and colleagues analyzed data on BMI and physical activity from the Nurses' Health Study and the Health Professionals Follow-Up Study. The analysis included a combined total of 136,623 adults and data on weight, height, and physical activity going back as far as 1976.
The study participants were separated by BMI into four groups, ranging from less than 22 kg/m2 to 30 kg/m2 or greater. Women with the lowest BMI were slightly younger (45 versus 47 to 48 in the higher BMI categories), whereas men had mean age of 54 to 55 across all four BMI groups. Physical activity decreased as BMI increased in men and women, from an average of 3.5 hours weekly in the lowest BMI group to 2.4 to 2.5 hours in participants with the highest BMI.
The investigators identified 129 confirmed cases of multiple myeloma (with complete data) in women during 22 years of follow-up in the Nurses' Health Study. In the Health Professionals Follow-up Study, 86 cases of multiple myeloma were identified in men (with complete data) during about 15 years of follow-up.
For both studies, there was a trend toward an increased risk of multiple myeloma with increasing BMI, but the trend did not reach statistical significance in either study cohort or in a combined analysis. The strongest associations emerged from the comparison of obese men versus those with the lowest BMI (relative risk 2.4) and comparison of overweight women (RR 1.6) and obese women (RR 1.2) versus women in the lowest BMI category. The combined analysis showed that men had a 30% greater risk of multiple myeloma compared with women after adjustment for age, BMI, and physical activity.
The analysis revealed no clear association between physical activity and multiple myeloma. A suggestion of an inverse association was seen in women who reported more than two hours of physical activity per week.
Despite the nonsignificant results, a causal role for obesity in multiple myeloma remains biologically plausible, the authors stated. Adipocytes account for as much as 30% of the body's production of the proinflammatory cytokine interleukin-6 (IL-6), which has proliferative and antiapoptotic effects on multiple myeloma cells. In addition, IL-6 levels predict disease severity in patients with multiple myeloma.