Waist circumference-to-height ratio has been identified as an inexpensive measure of obesity in children and adolescents that could replace body mass index (BMI).

The new study – conducted by the University of Exeter in collaboration with the University of Bristol and University of Eastern Finland and published in Pediatric Research – has discovered this new way to accurately detect obesity in children, which is critical to initiating timely interventions.

Obesity affects almost one in four children and adolescents and is associated with cardiovascular, metabolic, neurological, musculoskeletal diseases and premature death in adulthood. For nearly a generation, weight-to-height ratio charts and BMI for age and sex have been used to diagnose children with obesity, but these surrogate assessment tools are inaccurate in childhood and adolescence as they do not distinguish fat mass from muscle mass. For instance, two children with similar BMI might have different proportions of fat and muscle mass which makes obesity diagnosis difficult.

Dr Andrew Agbaje of the Children’s Health and Research Centre at the University of Exeter said: “This study provides novel information that would be useful in updating future childhood obesity guidelines and policy statements. Unlike BMI, the average waist circumference-to-height ratio in childhood, adolescence, and young adulthood does not vary with age and among individuals – which is why it might be preferable to BMI assessment in children and adolescent clinics as an inexpensive tool for detecting excess fat.

“It also means parents can easily and quickly confirm whether an increase in their child’s BMI or weight is due to excess fat, by examining their child’s waist circumference-to-height ratio.”

This study is the largest and the longest follow-up DEXA-measured fat mass and muscle mass study in the world using the University of Bristol’s Children of the 90s data (also known as the Avon Longitudinal Study of Parents and Children). The study included 7,237 children (51-percent female) aged nine years who were followed-up until the age of 24. Their BMI and waist circumference-to-height ratio were measured at ages nine, 11, 15, 17, and 24.

Expensive tools such as the dual-energy Xray absorptiometry (DEXA) scan accurately measures fat and muscle content of the body, but this device is not readily available in primary health care centres. Recently, the American Academy of Pediatrics (AAP) published a clinical guideline on childhood obesity and requested urgent research on inexpensive and accurate alternative measures of obesity.

Emerging studies in adults appear to suggest that waist circumference-to-height ratio predicts premature death better than BMI and could improve the diagnosis of obesity. However, there has been no former evaluation of how much waist circumference-to-height ratio measurements agree with DEXA-measured fat mass and muscle mass during growth from childhood to young adulthood. In addition, the threshold of waist circumference-to-height ratio needed to detect excess fat in children is not clear, hence this study.

Waist circumference-to-height ratio could detect 15-percent more fat mass and 25-percent less muscle mass compared with BMI measures. Since BMI had a high agreement with DEXA-measured muscle mass, it is difficult to specify whether BMI measures excess fat or muscle mass. The optimal waist circumference-to-height ratio cut points that predicted the 95th percentile of total fat mass in males was 0.53 and 0.54 in females. This cut point detected eight out of ten males and seven out of ten females who truly had excess DEXA-measured fat. The cut point also identified 93 out of 100 males and 95 out of 100 females who truly do not have excess fat.

The paper titled ‘Waist-circumference-to-height-ratio had better longitudinal agreement with DEXA-measured fat mass than BMI in 7237 children’ is published in Pediatric Research.

Dr Agbaje’s research group (urFIT-child) is supported by research grants from Jenny and Antti Wihuri Foundation, the Finnish Cultural Foundation Central Fund, the Finnish Cultural Foundation North Savo Regional Fund, the Orion Research Foundation, the Aarne Koskelo Foundation, the Antti and Tyyne Soininen Foundation, the Paulo Foundation, the Yrjö Jahnsson Foundation, the Paavo Nurmi Foundation, the Finnish Foundation for Cardiovascular Research, Ida Montin Foundation, Eino Räsänen Fund, Matti and Vappu Maukonen Fund, Foundation for Pediatric Research, and Alfred Kordelin Foundation.