Body fat distribution, weight change during adulthood, and thyroid cancer risk in the NIH‐AARP Diet and Health Study

Thyroid cancer incidence rates increased dramatically in the U.S. over the past three decades. Improvements in the detection and diagnosis of small (≤1 cm), localized papillary thyroid cancers appear to at least partially account for these changes, but the increasing incidence of larger and later stage tumors suggests that environmental factors may also play a role.1 There was a similarly dramatic increase in the prevalence of obesity during the same time period,2 which may have, at least partially, contributed to the rising incidence of thyroid cancer. This hypothesis has been supported by recent evidence from case-control3–6 and prospective7–12 studies finding a positive association between body mass index (BMI) and the risk of thyroid cancer, though some studies found no association.13–15 There are certain limitations of using BMI as a measure of obesity, including the inability of this measurement to distinguish lean from fat mass (e.g. muscle) and central versus peripheral fat distribution.16,17 Without having more detailed measurements of the amount or distribution of body fat besides BMI, it is unclear whether the positive associations observed between BMI and thyroid cancer risk were attributable to excess body fat, overall body size, or some other, related factor. Waist circumference measures have been shown to be more strongly correlated with highly metabolically-active visceral adipose tissue than BMI,17,18 but to date no published studies have been reported on the association between excess abdominal body fat and thyroid cancer risk. The association between weight change during adulthood, which tends to reflect changes in fat rather than lean mass,19 and the risk of this disease has been examined in only a few studies, and results have been conflicting likely due to small numbers of cases and lack of consistent definitions for weight change.5,6,12,13,20 We examined the association of body fat distribution (as measured by waist circumference, hip circumference, and waist-to-hip ratio) and adulthood weight change with thyroid cancer risk in the NIH-AARP Diet and Health Study, a large prospective study of U.S. men and women. A previous publication showed significant positive associations between BMI and thyroid cancer risk in men, but not women, in this cohort.7 After three additional years of follow-up, significant positive associations were observed for both men and women.23