Percent Body Fat and Chronic Disease Risk Factors in U.S. Children and Youth

Background

The dramatic increase in pediatric obesity has renewed interest in accurate methods and screening indexes for identifying at-risk children and youth. Whether age-specific standards are needed is a factor that remains uncertain.

Purpose

This study was designed to describe the age-specific fatness–risk factor relationship in boys and girls across a wide age range.

Methods

Data were from 12,279 white, black, and Mexican-American children and adolescents from the National Health and Nutritional Examination Surveys (NHANES) III (1998–1994) and IV (1999–2004). Children were grouped based on percent fat, estimated from subscapular and triceps skinfolds, and the age-specific relationships between percent fat and chronic disease risk factors (e.g., blood pressure, lipids and lipoprotein levels, glucose, insulin, and circulating C-reactive protein levels) were described in boys and girls, aged 6–18 years.

Results

Percent fat was significantly related to risk factor levels. At higher levels of percent fat, the prevalence of adverse cardiovascular disease risk factors was higher, particularly above 20% fat in boys and above 30% fat in girls. In boys and girls, the interaction term age by percent fat was a significant predictor of risk factors, whereas the percent fat by race interaction term was nonsignificant.

Conclusions

The results demonstrate a strong relationship between chronic disease risk factors and percent fat in children and youth that varies by age in boys and girls.

Introduction

Childhood and adolescent overweight and obesity are important public health concerns. The most recent U.S. national surveys indicate that 31.7% of youth are overweight,¹ and more than half of this group, 16.9%, are obese. The high prevalence of overweight and obesity in children and youth, with its attendant health risks,2, 3, 4, 5, 6, 7 has renewed interest in the development of accurate methods for body composition assessment and screening indexes for identifying children and youth at risk for obesity-related comorbidities.

Common definitions of pediatric overweight and obesity are based on the BMI. Early recommendations set the 85th percentile of age- and gender-specific BMI as the level at which children and youth were considered at risk of overweight (now called overweight instead of at risk of overweight), and the corresponding 95th percentile of BMI was defined as overweight (now called obesity).⁸ These definitions continue to be used today, although present-day surveys compare children and youth to BMI distributions that existed in the 1960s and 1970s.⁹

Although BMI is a practical, easy-to-obtain index, its application assumes that differences in BMI among individuals reflect differences in adiposity and that individuals with identical BMI have identical body composition. Clearly these assumptions are not valid,10, 11 although it is the degree of variation that matters when setting a screening standard. A more important limitation may be the arbitrariness of a definition based on the population distribution of the BMI rather than an a priori criterion, such as disease risk.

Although studies have shown that increasing levels of BMI are associated with higher levels of disease risk especially in youth, some studies have shown that worrisome levels of risk occur at percentiles higher than the 95th percentiles of BMI,⁹ whereas only modest risk is associated with the current recommendations.6, 12 Age-, gender-, and race/ethnicity-related variation in the BMI–body fat relationship potentially limit the widespread use of BMI for assessing disease risk.13, 14, 15 An alternative approach to setting obesity standards is to study the association of a more direct measure of adiposity with risk factors to determine the level of fatness associated with high levels of risk factors, although the challenges of measuring fatness directly outside of the laboratory are acknowledged. Using this approach, body fat levels ranging from 20%–25% in boys and 30%–35% in girls have been shown to be associated with health risk.16, 17, 18, 19 These ranges are remarkably similar, given the differences in samples, sample sizes, and methodology in these studies.

In one of the largest studies of percent fat and risk factors, Williams et al.¹⁹ reported that body fat levels ≥25% in boys and ≥30% in girls were associated with over-representation (excess risk) in the highest fifth of age-, gender-, and race-specific distributions of cardiovascular disease (CVD) risk factors. These fatness levels have been used to differentiate between unhealthy and healthier levels of body fat in children and youth ranging from age 5–18 years (www.cooperinstitute.org/). The appropriateness of applying the same standard across such a wide age range is a factor that has remained uncertain. Developing standards against a surrogate for adiposity such as BMI in absolute units of kg/m² is confounded by differential changes in fat and fat-free mass at different stages of maturation that likely influence the relationships between BMI and risk factors, which is why age- and gender-specific BMI percentiles are used.9, 20

When risk is defined against more direct measures of body fat, the level of body fatness that confers risk may be very similar at different ages. However, due to the changes in hormonal profile and fat-free mass that accompany maturation, the relationships between body fat and risk factors may also vary with age, thereby suggesting that health-related adiposity standards for boys and girls should be defined differently at different ages. Given the uncertainty concerning the influence of age on the fatness–risk factor relationship, the present analysis was undertaken to describe the age-specific percent fat–risk factor relationship in boys and girls across a wide age range in a large sample representative of U.S. boys and girls.