Shifts in the Population BMI Distribution

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Researchers can monitor changes in the nature of the obesity epidemic by comparing the BMI distribution curves derived from population-based surveys and noting shifts in any particular distribution over time. A shift toward higher BMIs over the entire distribution would indicate that virtually everyone is becoming heavier, with lean individuals gradually moving into the overweight range, overweight individuals moving into the obese range, and the number of obese individuals becoming more severely obese. However, a graphical analysis comparing NHANES III (1988-1994) with earlier data found that the distributional patterns of BMIs differed among age groups (Flegal and Troiano, 2000).

For adults, there was a general shift upward in the BMI distribution, with the greatest shift occurring at the upper end of the distribution, reflected by the heaviest subgroups becoming heavier. For younger children aged 6 to 11 years, and to a lesser extent in adolescents, the distributions of BMI values were characterized by little or no difference in the lower part of the distribution, though there was also a greater shift at the upper end, as shown schematically in Figures 2.5a and 2.5b (Flegal and Troiano, 2000). The results of this study indicate that the heaviest children and youth were heavier in NHANES III than in earlier surveys; the authors caution, how

Body mass index

FIGURE 2-5 Schematic representations of BMI distribution models. NOTE: Figure 2-5a shows a schematic representation of increased skewness (lack of symmetry) at the upper end of the BMI distribution with little change at the lower end, as has been observed in U.S. children and adolescents. Figure 2-5b shows a schematic representation of both a rightward shift in the distribution and increased skewness at the upper end of the distribution, as has been observed in U.S. adults.

SOURCE: Reprinted, with permission, from Flegal and Troiano, 2000. Copyright 2000 by the International Journal of Obesity and Related Metabolic Disorders.

Body mass index

Body mass index

FIGURE 2-5 Schematic representations of BMI distribution models. NOTE: Figure 2-5a shows a schematic representation of increased skewness (lack of symmetry) at the upper end of the BMI distribution with little change at the lower end, as has been observed in U.S. children and adolescents. Figure 2-5b shows a schematic representation of both a rightward shift in the distribution and increased skewness at the upper end of the distribution, as has been observed in U.S. adults.

SOURCE: Reprinted, with permission, from Flegal and Troiano, 2000. Copyright 2000 by the International Journal of Obesity and Related Metabolic Disorders.

ever, that the unweighted sample sizes for 6- to 17-year-olds, particularly for adolescents, are small (Flegal and Troiano, 2000). Strauss and Pollack (2001) came to a similar conclusion based on their analyses of NLSY data.

Changes in BMI distributions have impacts on the population's health. In adults, the major health-related co-morbidities that occur with obesity do not have a linear relationship with BMI. For example, although relationships between BMI and hypertension, diabetes, dyslipidemia, and even death occur across a wide range of BMIs, these relationships strengthen considerably at the highest levels of BMI (Solomon and Manson, 1997; Must et al.,

Similarly, children at the highest levels of BMI are generally at the greatest risk of adverse health outcomes. Elevated blood pressure and insulin were both observed to be twice as common in children with BMIs above the 97th percentile as in children within the 95th to 97th percentile (Freed-man et al., 1999). But the prevalence of these health outcomes is low between the 25th and 75th BMI percentiles, increasing modestly, if at all, across that span. Thus, with the childhood obesity epidemic characterized by a disproportionate number of children at the extreme ranges of BMI, there are likely to be higher obesity-related morbidity rates in children than if the epidemic mostly resulted from an upward shift in BMI across their entire population.

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