Changes in health state utilities with changes in body mass in the Diabetes Prevention Program

Abstract

Health utilities are measures of health-related quality of life (HRQL) used in cost-effectiveness research. We evaluated whether changes in body weight were associated with changes in health utilities in the Diabetes Prevention Program (DPP) and whether associations differed by treatment assignment (lifestyle intervention, metformin, placebo) or baseline obesity severity. We constructed physical (PCS-36) and mental component summary (MCS-36) subscales and short-form-6D (SF-6D) health utility index for all DPP participants completing a baseline 36-item short form (SF-36) HRQL assessment (N = 3,064). We used linear regression to test associations between changes in body weight and changes in HRQL indicators, while adjusting for other demographic and behavioral variables. Overall differences in HRQL between treatment groups were highly statistically significant but clinically small after 1 year. In multivariable models, weight change was independently associated with change in SF-6D score (increase of 0.007 for every 5 kg weight loss; P < 0.001), but treatment effects independent of weight loss were not. We found no significant interaction between baseline obesity severity and changes in SF-6D with changes in body weight. However, increases in physical function (PCS-36) with weight loss were greater in persons with higher baseline obesity severity. In summary, improvements in HRQL are associated with weight loss but not with other effects of obesity treatments that are unrelated to weight loss. Although improvements in the SF-6D did not exceed commonly reported thresholds for a minimally important difference (0.04), these changes, if causal, could still have a significant impact on clinical cost-effectiveness estimates if sustained over multiple years.

Figure 1

Mean changes in SF-6D scores at year 1 by DPP treatment group and baseline BMI category. Mean changes >0 indicate an improvement in health-related quality of life with weight loss; P values shown are for weight change-by-treatment group interaction term, from separate multivariable linear regression models, stratified by BMI subgroup and adjusting for sex, race/ethnicity, marital status, employment, educational attainment, baseline Beck Anxiety Inventory, baseline Beck Depression Inventory, baseline percent calories from fat, baseline MET-hours per week of modifiable physical activity, and baseline body weight (see text for details). ^aP < 0.05 and ^bP < 0.01 for tests of change in SF-6D within individual BMI × treatment group substrata. DPP, Diabetes Prevention Program; SF-6D, short-form-6D.