Measuring BMI May Miss 25% of Obese Kids

Fran Lowry

July 06, 2014

Using body mass index (BMI) to detect excess body weight fails to identify more than a quarter of children with excess body fat percentage who may, in fact, be obese, according to new research published online June 24 in Pediatric Obesity.

"BMI has been used in the clinical setting despite carrying the inherent flaw of failing to distinguish between lean and fat mass, both of which contribute to BMI," Asma Javed, MD, from the Department of Pediatric and Adolescent Medicine, Division of Pediatric Endocrinology, Mayo Clinic, Rochester, Minnesota, and colleagues write.

"Despite ethnic and racial differences in adiposity, recommendations from several organizations, including the American Academy of Pediatrics, advise the use of BMI for age with national reference data to diagnose pediatric obesity in the clinical setting," they write.

Researchers have assessed the performance of BMI in predicting adverse cardiovascular risk in a systematic review, but there has been no meta-analysis on the estimates of pooled sensitivity and accuracy of BMI compared with reference standard methods of measuring excess body fat or adiposity, such as bioelectrical impedance analysis, dual-energy X-ray absorptiometry, hydrostatic weighing, air-displacement plethysmography, and skin-fold thickness measurement.

The aim of the current study was to assess an overall estimate of BMI validity for the detection of obesity, as defined by excess adiposity in children and adolescents up to the age of 18 years, while recognizing that BMI is not used to diagnose obesity in children younger than 2 years.

The researchers performed a meta-analysis that included 37 studies comprising a total of 53,521 children aged 4 through 18 years.

They found that BMI has a high specificity, but moderate sensitivity, in identifying pediatric obesity.

"Commonly used BMI cutoffs for obesity showed a pooled sensitivity to detect high adiposity of 0.73 (95% confidence interval [CI], 0.67 - 0.79) and a specificity of 0.93 (95% CI, 0.88 - 0.96)," the authors write.

The BMI measure had lower sensitivity in boys (pooled sensitivity, 0.67; 95% CI, 0.56 - 0.76) than in girls (pooled sensitivity, 0.71; 95% CI, 0.62 - 0.79).

The pooled results from the 37 studies showing a sensitivity of 73% suggests more than a quarter of children not labeled as obese by BMI might indeed have excess adiposity, the authors write in their discussion.

They also cite some of their study's limitations, including the risk for publication bias inherent in meta-analyses and the moderate heterogeneity of the study sample.

Because BMI fails to identify obesity in a significant percentage of children, there is a need for pediatric studies to develop reference values for skin-fold thickness, air-displacement plethysmography, and dual-energy X-ray absorptiometry and to assess their additive or independent ability to predict development of obesity-associated comorbidities, the authors conclude.

"If we are using BMI to find out which children are obese, it works if the BMI is high, but what about the children who have a normal BMI but do have excess fat? Those parents may get a false sense of reassurance that they do not need to focus on a better weight for their children," Francisco Lopez-Jiminez, MD, senior study author and director of preventive cardiology at the Mayo Clinic, said in a clinic news release.

Dr. Javed noted that childhood obesity can lead to an increased risk for type 2 diabetes and cardiovascular disease. In the news release, Dr. Javed said: "Our research raises the concern that we very well may be missing a large group of children who potentially could be at risk for these diseases as they get older. We hope our results shine a light on this issue for physicians, parents, public health officials and policymakers."

The authors have disclosed no relevant financial relationships.

Pediatric Obesity. Published online June 24, 2014. Abstract

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