Does a parsimonious measure of complex body mass index trajectories exist?

Abstract

Background: A single measure that distills complex body mass index (BMI) trajectories into one value could facilitate otherwise complicated analyses. This study creates and assesses the validity of such a measure: average excess BMI.

Methods: We use data from Waves I-IV of the National Longitudinal Study of Adolescent to Adult Health (n = 17,669). We calculate average excess BMI by integrating to find the area above a healthy BMI trajectory and below each subject-specific trajectory and divide this value by total study time. To assess validity and utility, we (1) evaluate relationships between average excess BMI from adolescence to adulthood and adult chronic conditions, (2) compare associations and fit to models using subject-specific BMI trajectory parameter estimates as predictors, and (3) compare associations to models using BMI trajectory parameter estimates as outcomes.

Results: Average excess BMI from adolescence to adulthood is associated with increased odds of hypertension (OR = 1.56; 95% CI: 1.47, 1.67), hyperlipidemia (OR = 1.36; 95% CI: 1.26, 1.47), and diabetes (OR = 1.57; 95% CI: 1.47, 1.67). The odds associated with average excess BMI are higher than the odds associated with the BMI intercept, linear, or quadratic slope. Correlations between observed and predicted health outcomes are slightly lower for some models using average excess BMI as the focal predictor compared to those using BMI intercept, linear, and quadratic slope. When using trajectory parameters as outcomes, some co-variates associate with the intercept, linear, and quadratic slope in contradicting directions.

Conclusions: This study supports the utility of average excess BMI as an outcome. The higher an individual's average excess BMI from adolescence to adulthood, the greater their odds of chronic conditions. Future studies investigating longitudinal BMI as an outcome should consider using average excess BMI, whereas studies that conceptualize longitudinal BMI as the predictor should continue using traditional latent growth methods.

Figure 1.. Excess body mass index for four respondents.

Dashed lines indicate sex and age specific healthy body mass index ranges for males as established by the Centers for Disease Control and Prevention, where the top lines indicate the overweight threshold (sex/age-specific 85th percentile for individuals below 20 years and a BMI of 25 kg/m2 for 20+ years) and the bottom lines indicate the underweight threshold (sex/age-specific 5th percentile for individuals below 20 years and a BMI of 18 kg/m2 for 20+ years). Solid lines indicate respondents’ model-implied body mass index trajectories. Diagonal lines represent each respondent’s excess BMI including no (a), low (b), moderate (c), and high (d), excess BMI.

Figure 2.

Histogram of average excess BMI.