Context Physical inactivity and body mass index (BMI) are established independent
risk factors in the development of type 2 diabetes; however, their comparative
importance and joint relationship with diabetes are unclear.
Objective To examine the relative contributions and joint association of physical
activity and BMI with diabetes.
Design, Setting, and Participants Prospective cohort study of 37 878 women free of cardiovascular
disease, cancer, and diabetes with 6.9 years of mean follow-up. Weight, height,
and recreational activities were reported at study entry. Normal weight was
defined as a BMI of less than 25; overweight, 25 to less than 30; and obese,
30 or higher. Active was defined as expending more than 1000 kcal on recreational
activities per week.
Main Outcome Measure Incident type 2 diabetes, defined as a new self-reported diagnosis of
Results During the follow-up, 1361 cases of incident diabetes occurred. Individually,
BMI and physical activity were significant predictors of incident diabetes.
Compared with normal-weight individuals, the multivariate-adjusted hazard
ratio (HR) was 3.22 (95% confidence interval [CI], 2.69-3.87) for overweight
individuals and 9.09 (95% CI, 7.62-10.8) for obese individuals. For overall
activity (kilocalories expended per week), compared with the least active
first quartile, the multivariate-adjusted HRs were 0.91 (95% CI, 0.79-1.06)
for the second quartile, 0.86 (95% CI, 0.74-1.01) for the third, and 0.82
(95% CI, 0.70-0.97) for the fourth (P for trend =
.01). In the combined analyses, overweight and obese participants, whether
active or inactive, had significantly elevated risks, compared with normal-weight
active individuals. The multivariate-adjusted HRs were 1.15 (95% CI, 0.83-1.59)
for normal-weight inactive, 3.68 (95% CI, 2.63-5.15) for overweight active,
4.16 (95% CI, 3.05-5.66) for overweight inactive, 11.5 (95% CI, 8.34-15.9)
for obese active, and 11.8 (95% CI, 8.75-16.0) for obese inactive participants.
Conclusions Although BMI and physical inactivity are independent predictors of incident
diabetes, the magnitude of the association with BMI was greater than with
physical activity in combined analyses. These findings underscore the critical
importance of adiposity as a determinant of diabetes.