Title of article:

Validity of the body mass index as an indicator of the risk and presence of overweight in adolescents

Authors: Malina RM, Katzmarzyk PT.
Journal: Am J Clin Nutr, Jul 1999;70(1):131S-6S

Abstract

The validity of the body mass index (BMI) as an indicator of the risk of becoming overweight and of the presence of overweight was evaluated in 6 groups of adolescents comprising several ethnic groups (n=1570, aged 9-19 yrs). With use of triceps skinfold thickness and estimated percentage body fat as the criteria for adiposity, BMI had high specificities (86.1-98.8% for risk of overweight and 96.3%-100% for presence of overweight) and lower but variable sensitivities (4.3-75% for risk of overweight and 14.3-60% for presence of overweight). Thus, almost all adolescents who were not at risk for overweight or who were not overweight were classified correctly. In contrast, many adolescents who were at risk of overweight or who were overweight were not correctly identified as measured by BMI. Partial correlations, controlling for age, between BMI and the triceps skinfold thickness and estimated percentage body fat were generally moderate to moderately high, whereas BMI and triceps skinfold thickness appeared to be equally related to estimated total body fatness and percentage body fat in Mexican American and Austrian white males. BMI was better correlated with trunk skinfold thicknesses, but when relative subcutaneous fat distribution was statistically controlled, the trunk-extremity contrast in the correlations was no longer apparent.

Comments and Key points

This article indicates that the current CDC criteria for defining overweight and obesity, are generous to adolescents, by letting kids be fatter than necessary before labelling them as overweight or obese.

The body fat percentage criteria they used were:

- "at risk of overweight" is body fat >= 20% for boys and >=25% for girls.
- "overweight" is body fat >= 25% for boys and >=30% for girls.

Notice that they are using the kind & gentle terminology, but in reality,
"at risk of overweight" (in kids) means the same as "overweight" in adults.
"overweight" (in kids) means the same as "obese" in adults.

This article shows some data comparing adolescents BMI between different countries, and different races. It lists the sensitivity and specificity of the cut-off criteria for all these different sets of data from different countries and races. In most cases, the specificities listed are above 90% ( which is very good), and the sensitivities are a little lower.

For "overweight" (obesity), the sensitivities are generally in the 70s% range, which is also quite good. For "overweight", the sensitivities are a bit too low, around 50%.

So, I personally conclude that the 85th percentile line is a good body mass index cut-off threshold for adolescents, but the 95th percentile line threshold is probably a bit too high for adolescents.

 

Review & comments by Steven B. Halls, MD, Modified it on: 23-June, 2008, Copyright.
Back to Bibliography of BMI articles.