To date, most studies exploring the relationship between excess weight and cancer risk looked at cross-sectional information on overweight and obesity. Using multiple BMI measurements over time from approximately 74,000 women (with over 6,000 cancers diagnosed during the 12.6-year mean follow-up), Arnold and colleagues examined the association between duration of overweight, obesity and cancer risk, taking into account other factors related to obesity, such as physical activity, diet, smoking, hormone use, and diabetes. They found every ten-year increase in adulthood overweight duration was associated with a 7% (95% confidence interval 6%-9%) increase in the risk of all obesity-related cancers, 5% (3%-7%) increase in risk of postmenopausal breast cancer, and 17% (12%-22%) increase in the risk of developing endometrial cancer. After adjusting for the intensity of overweight, these figures rose to 8% (5%-12%) for postmenopausal breast cancer and 37% (29%-46%) for endometrial cancer.
BMI is not an ideal measure for body fat, and the WHI cohort is dominated by non-Hispanic white women. While these aspects of study design limit generalizability of the findings, the large scale of the study and longitudinal BMI data provide compelling evidence that overweight duration is an important risk factor for cancer. The authors state, “[i]f this is true, health care teams should recognize the potential of obesity management in cancer prevention and that excess body weight in women is important to manage regardless of the age of the patient.”