Body Mass Index

BMI is a widely used method to define the relationship between weight and height (152). Charts of BMI relative to age are used to determine childhood obesity. BMI percentiles can be downloaded directly from the Centers for Disease Control (153). They are easy to use, nonintrusive, and have been validated against measures of body fat. The BMI is calculated as weight (kg) /height (m2) and provides a practical clinical tool for classification of individuals with normal weight and those with various degrees of obesity. In the adult a grade I obesity is that with a BMI of 25 to 29; grade II is a BMI of 30 to 40; and grade III is one with a BMI of more than 40. The BMI system of classification of obesity is important because it denotes the risk for medical complications of obese patients, which increase at BMI levels above 25 (154). Individuals with a BMI above 27 have a markedly increased risk for hypertension, hypercholesterolemia, and diabetes mellitus. In contrast, when the BMI index is less than 25, there are no apparent physical effects of obesity on the individual, although there may be social and psychological concerns with body appearance. However, the use of BMI has limited applications in the assessment of overweight children since its calculation is based primarily on a stable height, which is not applicable to growing children. Also, the BMI can underestimate the percentage of lean body mass since it does not account for variations in musculature. This BMI paradox was clearly illustrated by Yajnik and Yudkin (155).

Children's BMIs at or above the 85th percentile are considered overweight and those above the 95th percentile are obese. Furthermore, no differences in the relationship between BMI and age exist among boys and girls up to age 20 in the United States, United Kingdom, Japan, and Singapore (156). The BMI charts clearly denote the usual percentiles and will distinguish a stage 1 obese child when the BMI is between the 85th and the 95th percentile for age. However, it will not determine the stage 2 or 3 of obesity as it does for the adult as the excess total BMI will vary with the age of the child. We have utilized an excess of > 50% over the 50th percentile for age to define stage 2 obesity and an excess of > 100% over the 50th percentile to define stage 3 obesity in children.

There are several potential errors associated with BMI as an indicator of obesity in children (157). The increasing height in children from birth until adulthood may cause a difference in the weight-for-height relationship assumed in current BMI-for-age charts. Gender and age also affect body weight and height. Furthermore, puberty may introduce another change in the weight-for-height relationship. Ethnic origin and social class may also affect both body weight and height. Nonetheless, BMI correlates ( > 0.8) with body fat as determined by both skinfold thickness measurements and by densitometry (158). This suggests that BMI is a reasonable criterion for determining obesity in children and adolescents. However in adults, these relations are simpler because adult height is assumed to be fixed. Therefore, body weight is adjusted for height only. However the progression of the BMI as the child grows will be a more appropriate longitudinal measurement, as it denotes the accumulation of excess fat over time and whether this is progressing in the same percentile or if it is crossing percentiles, as in the case of body weight progression mentioned above.

It should be kept in mind that the BMI adjustment for the age of the child may change over time. One study found that the power used to adjust height (Benn index) changed from 2.0 to 3.5 over the first 20 years of life in order to maintain the best relationship with skin-fold thickness. The strongest relationships with skinfold thickness were obtained in children between 9 and 10 years old. Using only a Benn index of 2.0 may introduce subtle errors in BMI when used in younger children. Taller children will tend to have greater BMIs than shorter children. However, trying to correct for age with the Benn index will lead to more errors than if just one standard Benn index of 2.0 is used (159).

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