Body Mass Index across the Life-Course: Emergence of Race by Sex Disparities in Early Childhood.

In the U.S. 35% of adults aged 20 years or older are obese and the obesity epidemic represents a critical public health issue.  There are marked disparities in body mass index (BMI) and obesity prevalence by race/ethnicity and sex.  Among men the age adjusted prevalence of obesity is modestly higher among Non-Hispanic Blacks and Hispanics than among non-Hispanic Whites, while among women the prevalence of obesity is substantially higher among Non-Hispanic Blacks and Hispanics than among non-Hispanic Whites.  Non-Hispanic Black women have the highest prevalence of obesity of any racial, ethnic, sex group, a disparity that has been in place for several decades, with an age adjusted obesity prevalence of 57% in 2011-2012.

Work by Andrew Rundle and colleagues recently published in the Annals of Epidemiology used data from the Child Health and Development Studies (CHDS) to assess when in the life course the race by gender disparity in BMI for Blacks and Whites begins. CHDS participants were born in the early 1960s and height and weight data were collected at ages 5, 9-11 and 15-17.  Six hundred and five CHDS participants were recently follow-up again at age ~50 and height and weight were measured. Analyses of these data showed that the race by gender disparity in BMI was present by age 9-11 years and continued at ages 15-17 and 50 years, with Black women having the highest BMI scores.  A large proportion of the race by sex disparity in BMI at age 50 could be accounted for by the participant’s BMI at age 9-11.

1. Socio-demographic covariates were: paternal and maternal education, maternal pre-pregnancy BMI, participant obtained a college degree, participant’s smoker/non-smoker status at age 50 and participant age of assessment

One possible explanation for race by sex disparities in BMI in adulthood is that the effects of discrimination that are patterned by race, sex and body size place Black women at a unique economic disadvantage and in turn at risk for weight gain. Among women BMI is often found to be inversely associated with income and this correlation is thought to reflect both systematic occupational discrimination against overweight and obese women in regards to hiring, salary and promotion, and lower income being causally associated with poorer quality diets and lower levels of physical activity.  Furthermore, patterns of discrimination in hiring and pay by race and sex are expected to particularly affect Black women.  However, race by sex disparities in body size were already in place by age 9-11 and largely explained disparities in BMI at age 50.  This suggests that, while workplace discrimination might perpetuate these disparities, disparities in BMI by race and sex begin prior to entry into the workforce.

Another possible explanation for the higher BMI observed among Black women relates to differences in attitudes regarding a desirable body size. Studies have found that Blacks are more likely to underestimate their weight category compared to whites, particularly Black women.  When asked to describe their ‘ideal’ weight Black women also tend to report a higher weight than whites, while whites tend to experience greater dissatisfaction with their body size.  These differences in perceptions regarding an ideal or desirable weight may create greater normative support, or reduce stigma, for obesity among Black women. However, the race by sex disparity in adult BMI was largely explained by differences in BMI already in place at age 9-11.  If indeed, race and sex differences in attitudes or norms about body size explain race by sex disparities in BMI in adulthood, these attitudes or norms are established early in life and have long term persistent effects.

In the data analyses, adjustment for BMI at age 9-11 diminished the race by sex disparity in BMI at age 50.  Further adjustment for covariates diminished the difference in BMI by race among men, with adjustment for paternal education accounting for the bulk of the change in the association between race and age 50 BMI among men.  While the difference in BMI by race among women remained significant after further adjustment for other covariates, adjustment for the covariates also somewhat diminished the difference in age 50 BMI by race among women.  Adjustment for paternal education accounted for almost all of the observed change in the association between race and age 50 BMI among women. Considering paternal education in the 1960s to be a marker of familial socioeconomic status, these analyses suggest that socioeconomic conditions experienced during childhood have persistent effects on BMI that account for some of the differences in BMI associated with race, particularly for men.

The results are based on a cohort that entered its adolescence and teen years as the obesity epidemic in the U.S. was beginning and may have experienced an obesogenic environment that differs from the one that exists today.  However, these findings emphasize the importance of early prevention of obesity and unhealthy weight gain trajectories in childhood and suggests that such prevention programs could have lasting beneficial consequences.

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