Purpose of review: The prevalence of overweight, as assessed by a high body mass index (kg/m2), has greatly increased among children and adolescents over the last three decades. Because body mass index is a measure of excess weight rather than excess body fatness, it is important to understand the ability of a high level to identify children who truly have excess adiposity. This review covers the measurement and classification of overweight and obesity, the expression of body composition data, and the relation of body mass index to adiposity.
Recent findings: Although adiposity has typically been expressed as percentage body fat, the use of the fat mass index (fat mass ÷ height2) and the fat-free mass index (fat-free mass ÷ height2) may provide more information. For example, body mass index differences among relatively thin children have been found to largely reflect differences in fat-free mass index, whereas differences among relatively heavy children are primarily due to differences in fat mass index. In addition, the ability of overweight to identify obese children is greatly influenced by the cutpoints selected for body mass index and adiposity. The use of inappropriate cutpoints, rather than the limitations of body mass index, may account for the frequently reported finding that many obese children are not overweight.
Summary: The use of fat mass index and fat-free mass index in expressing body composition data allows one to easily assess the contribution of each to body mass index. If appropriate cutpoints are used, a high body mass index level is a moderately sensitive and a very specific indicator of excess adiposity among children.