Risk of Diabetes Correlates with Duration of Obesity

The degree and duration of obesity, expressed as excess body mass index (BMI)-years, increases the risk of developing type 2 diabetes more than BMI alone, but race and young age also are risk factors, according to the results of a longitudinal analysis [Arch Pediatr Adolesc Med. 2012;166(1):42-48]. Using the National Longitudinal Survey of Youth 1979, a nationally representative, longitudinal, cohort study, researchers from the University of Michigan conducted a study of 8157 adolescents and young adults 14 to 21 years of age at study onset to assess the correlation between excess BMI-years and incident diabetes. Individuals provided self-reported data on height, weight, and onset of diabetes from 1981 through 2006. The retention rate was 80.9% by 2006. Of the 8157 patients, 55% were white, 15% black, and 6% were Hispanic. At baseline, 80% had normal weight, 15.8% were overweight, and 4.2% were obese. More blacks and Hispanics than whites were overweight and obese (16.4% vs 19.9% vs 14.9% for overweight, and 5.4% vs 5.7% vs 3.7% for obese, respectively). After 25 years of follow-up, the mean age for the participants was 44.9 years; 337 individuals had developed diabetes. The researchers defined excess BMI as the participant’s actual BMI minus the reference BMI. The reference BMI was defined as BMI threshold for overweight of 25.0 for adults and 85th percentile for adolescents. The investigators explained, “For example, 23.4 corresponds to the 85th percentile for a boy aged 15 years; the degree of obesity for a BMI of 25.0 would be 25.0 – 23.4=1.6.” They then totaled excess BMI for all previous years to calculate excess BMI-years. The findings showed that by 2006, 4.2% of the participants had developed diabetes (mean age at onset, 37.2 years), including a greater percentage of Hispanics (7%) and blacks (4.8%), compared with whites (2.4%). Furthermore, more women than men developed diabetes (4.4% vs 3.8%). Overall, the researchers found that a higher level of excess BMI-years was associated with a greater risk of developing diabetes. On average, white men in their 40s with 200 cumulative excess BMI-years versus a similar cohort with 100 excess BMI-years had higher odds of self-reported diabetes (odds ratio [OR], 2.94; 95% confidence interval [CI], 2.36-3.67). The combination of race and excess BMI-years also correlated with incident diabetes. Hispanics at 30 years of age with 200 excess BMI-years had a greater risk compared with whites with the same level of excess BMI-years (OR, 1.96; 95% CI, 1.09-3.54). The trend continued for blacks, compared with whites at 30 years of age with the same level of excess BMI-years (OR, 1.13; 95% CI, 0.71-1.81). However, the risk of diabetes was similar for all races at higher levels (300-500) of excess BMI-years. The authors also noted that their findings may be reflective of the pathophysiologic development of type 2 diabetes during an individual’s life. The combination of insulin resistance and beta-cell function is required for developing diabetes; however, the relative influence of these factors may differ with age. Beta-cell failure has been associated with diabetes diagnosed at older ages, whereas insulin resistance is largely determined by obesity and may represent a “more important determinant for individuals diagnosed with diabetes at younger ages.” Study limitations included the use of self-reported data for weight and height and no specification of diabetes type. The researchers said, however, that it was unlikely that those within the age bracket at baseline would be unaware of having type 1 diabetes. “Because younger compared with older individuals have a higher risk of self-reported diabetes for a given level of excess BMI-years, and cumulative exposure to excess BMI is increasing among younger US birth cohorts, future public health interventions focused on diabetes prevention may need to target younger nonwhite individuals to prevent a further acceleration in diabetes rates,” concluded the researchers.