Measuring and tracking obesity inequality in the United States: evidence from NHANES, 1971-2014

Abstract

BackgroundBecause people care about their weight relative to peers and society, obesity inequality plays a role in explaining obesity incidence and the impacts of being obese on subjective well-being. While the increase in obesity prevalence and mean body mass index (BMI) is well documented, the measurement of distributional changes and corresponding obesity inequality is yet to be fully explored.MethodsThe present study analyzed BMI data for adults aged 20 to 74 from the National Health and Nutritional Examination Survey (NHANES) I (1971-1974), II (1976-1980), III (1988-1994), and continuous NHANES (1999-2014). We applied tools developed to measure income inequality to analyze the inter-temporal variation in the BMI distribution among US adults. Using stochastic dominance tests, we construct partial orderings on cumulative BMI distributions during the study period. Shapley decompositions and inequality indices are employed to quantify the source and extent of temporal variation and decompose the inequality into within and between-group components considering age, gender, and race.ResultsThe BMI distribution of each NHANES study first-order stochastically dominated the BMI distribution of the previous wave from 1971-1974 to 2003-2006, whereas more recent comparisons failed to reject the null hypothesis of non-dominance. The Shapley decomposition analysis revealed that horizontal shifts of BMI distributions accounted for a majority of the increase in obesity prevalence since 1988-1991. Especially in recent years when the rate of obesity growth has slowed down, the contribution of the redistribution component dropped significantly and even became negative between 2007-2010 and 2011-2014. The inequality indexes consistently show a worsening of obesity inequality from the mid-1970s to the mid-2000s regardless of population subgroups, and this disproportionate shift of the BMI distribution is unlikely to be a result of a changing ethnic composition of the US population.ConclusionOur findings demonstrate that seemingly similar increases in obesity prevalence can be accompanied by very different patterns of distribution change. We find that the early phase of the obesity epidemic in the US was largely driven by increasing skewness, whereas more recent growth is a population-wide experience, regardless of demographic characteristics. Increasing morbid obesity certainly played an important role in the initial phase of the epidemic, but more recently the BMI distribution has largely horizontally shifted to the right.

Obesity, underweight and BMI distribution characteristics of children by gross national income and income inequality: results from an international survey

“…In 6-7 year females and males the medians were similar to the reference population but the dispersion (S values) was greater than the reference population ( Figure 1A and 1B). [13][14] year old females, the median values and dispersion (S values) were both similar to the reference population ( Figure 1C). [13][14] year old males, the M values and dispersion (S values) were greater than the reference population ( Figure 1D).…”

“…[13][14] year old females, the median values and dispersion (S values) were both similar to the reference population ( Figure 1C). [13][14] year old males, the M values and dispersion (S values) were greater than the reference population ( Figure 1D). BMI distribution curve characteristics are shown in Table 2 and actual BMI distribution histograms are shown for each country and centre in Supplementary Figure 3.…”

“…ISAAC is a multi-centre, multi-country, multi-phase cross sectional study originally designed to measure time trends in the prevalence and severity of asthma, rhinoconjunctivitis and eczema and their risk factors (20). ISAAC Phase Three was undertaken between 2000 and 2003 and involved 6-7 year old children and [13][14] year old adolescents from a random sample of schools, representative of the centres they were selected from, as described in the published protocol (22). Height and weight were reported by the parents of the 6-7 year old children and were self-reported by adolescents.…”

“…In contrast, childhood underweight is likely more prevalent in lower income countries predominantly due to nutritional scarcity. High economic development and income inequality of a nation has been associated with increases in mean BMI and proportions of obesity in adults (11)(12)(13)(14)(15) and children (16). National income and income inequality may also be associated with the dispersion of childhood BMI distributions.…”

“…This study investigated whether among female and male children and adolescents from different countries (a) the prevalence of underweight and obesity (b) the skewness, median, and dispersion of the BMI distribution curves, differed by gross national index (GNI) or the Gini index (to estimate national wealth and income inequality), using data from 6 to 7 year old and [13][14] year old boys and girls collected between 2000 and 2003 in the International Study of Asthma and Allergies in Childhood (ISAAC) (20). This study also investigated how these BMI distribution curve characteristics from the ISAAC study participants differed from those derived from nationally representative datasets from Brazil, United Kingdom (UK), Hong Kong, Netherlands, Singapore and the United States of America (USA) between 1963 and 1993 (21).…”