News article

Don't rely solely on BMI to measure childhood obesity, new research concludes

Posted:  Tuesday, December 10, 2013

Relying solely only on Body Mass Index (BMI) may provide inaccurate estimates of the childhood obesity epidemic and associated health risks, according to new research by Leeds Metropolitan University, published in Public Health journal. The results show that the estimated prevalence of obesity defined by BMI remained stable in both boys and girls over the course of the five years (18%, 12% and 10% for boys in year 7, 9 and 11 respectively and 14%, 15% and 11% for girls). However, the prevalence defined by WC increased sharply, especially in girls, (from 20% to 60% over the 5 year period).

The study, led by Dr Claire Griffiths, with childhood obesity expert Professor Paul Gately, Dr Paul Marchant and Professor Carlton Cooke in collaboration with Leeds City Council, compared BMI, waist circumference (WC) and waist-to-height ratio (WHtR) data in Leeds school children over the course of five years. This longitudinal study builds on the research team's 2012 study, published in Obesity.

The results show that the estimated prevalence of obesity defined by BMI remained stable in both boys and girls over the course of the five years (18%, 12% and 10% for boys in year 7, 9 and 11 respectively and 14%, 15% and 11% for girls). However, the prevalence defined by WC increased sharply, especially in girls, (from 20% to 60% over the 5 year period).

Speaking about the study, Dr Griffiths commented: "BMI is a useful measure however, it may be more appropriate to use a combination of both BMI and waist circumference, as is recommended in adults. Children appear to be getting fatter and the extra fat is being carried around the waist, which is not being detected by BMI alone."

Dr Griffiths added: "It is important to acknowledge that these measures are looking at different aspects of obesity, which may in part explain the differences in the patterns observed. BMI and WC interact in a complex fashion, especially during childhood and it is not yet possible to estimate the impact of the increase in standardised WC on current and future morbidity, but such increases should be a cause for concern. Most shocking is the fact that by the time the children reach 15-16 years, 25% of the girls exceed adult cut points (23) for overweight (80cm) and 10% actually exceed the adult cut points for obesity (88cm). This can be considered a robust finding given that these adult cut points have the most evidence in relation to future health risk.

"It should be noted that there is considerable concern with the use of standardised WC measurements in children, and so these findings should be interpreted in light of the potential limitations. The WC charts have not been validated against longitudinal data, so it is unknown if they accurately adjust for growth changes. One interpretation from these data is that they do not."

Both studies demonstrate the strong collaboration between Leeds Metropolitan University and Leeds City Council and the proactive approach to public health which they have established in Leeds.

The studies are part of an annual programme Leeds Metropolitan University carries out in partnership with Leeds City Council called the Rugby League and Athletics Development Scheme (RADS), which has been running for several years. Through working with secondary schools in Leeds collecting data on a range of sports performance skills as well as height, weight, BMI and waist circumference, the council and its partners are able to effectively plan activities to support prevention and target intervention strategies. This research also provides additional information for participating schools.

C. Griffiths, P. Gately, P.R. Marchant, C.B. Cooke, A five year longitudinal study investigating the prevalence of childhood obesity: comparison of BMI and waist circumference, Public Health, Available online 20 November 2013

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