BMI 'may not reflect child obesity'

First published in Health News by

Assessing children’s weight using body mass index may mean that the obesity epidemic is being underestimated, said The Daily Telegraph. Body mass index, or BMI, looks at how appropriate a person’s weight is for their height. However, according to the Telegraph, it doesn’t take into account where children carry extra weight, and may therefore fail to detect cases where children are carrying too much body fat.

The news is based on a UK study that looked at trends in child obesity using three different measures:  BMI, waist circumference (WC), or waist-to-height ratio (WtHR). The researchers compared how the three measures estimated obesity rates among nearly 15,000 children aged 11–12 years over three years to see how well they matched each other.

Using BMI, they found little change in the prevalence of obesity over the three years, which affected around 19–20% of boys and 16–18% of girls. When they used WC to define obesity, they found that the prevalence was strikingly higher, particularly in girls: 20–26% in boys and 28–36% in girls across the three years. WtHR produced a similar pattern.

The study demonstrates varying child obesity levels when different measures are used to define obesity. However, it is not possible to conclude from this study which is the best measure of obesity to use in children, as the study did not relate these different measurements to the chance of experiencing ill health. However, as the researchers say, it highlights the need for research that assesses this question, to allow a consistent public health message on how weight, height and waist circumference relate to health in children.

 

Where did the story come from?

This study was carried out by researchers from Leeds Metropolitan University, and did not state any sources of funding. The study was published in the peer-reviewed medical journal Obesity.

The news stories generally described the study correctly. However, the stories didn’t highlight that this study does not prove that waist circumference is a “better” way to measure obesity than BMI. This judgement is likely to need studies that follow children up over time to look at which measure best predicts the health outcomes they experience.

 

What kind of research was this?

This cross-sectional study looked at the prevalence of obesity during three years – 2005, 2006 and 2007 – using three different measures to define obesity. The three measures used were:

  • body mass index (BMI) - measured by weight (kg) divided by height in metres (m) squared
  • waist circumference (WC) - distance around the waist in cm
  • waist-to-height ratio (WtHR) - measured by dividing waist circumference in cm by height in cm

The authors say previous research has suggested that child obesity levels have stabilised in recent years. However, this observation has been based on studies mainly using BMI as a measure of obesity. Although BMI is a well-established measure of obesity in children, the authors note there is emerging evidence that children’s “central adiposity” (fat around the middle of the body) may be more relevant to health outcomes than overall body fat. Therefore, they propose that WC may be a better measure. However, there is a concern that WC doesn’t take the child’s height into account, and so WtHR is also considered as perhaps a more appropriate indicator.

 

What did the research involve?

The source of data for this study was the Rugby League and Athletics Development Scheme, a collaboration between Leeds City Council, Leeds Metropolitan University and the Education authority (Education Leeds). The scheme collected BMI and WC measures from children over three years, with a participation pattern reportedly similar to that seen in national surveys. This study had data on 14,697 children: 5,143 in 2005, 5,094 in 2006, and 4,460 in 2007. The children assessed in the three years were around 11–12 years of age.

Data collection took place in schools during physical education lessons. The children’s height, weight and waist circumference were measured by the lead researcher of this study. Height was measured to the nearest 0.1cm, weight to the nearest 0.01kg, and WC measured to the nearest 0.1cm. WC was measured at a point in between the bottom of the ribcage and the top of the hip bone (a thin t-shirt or vest was allowed, and 0.5cm subtracted for it). The researchers took repeat measures from a sample of children to confirm whether the measures were accurate.

Standard growth charts were used to see whether the child was overweight or obese according to BMI or WC. Children in the top 15% of these charts for BMI or WC were considered to be overweight, and those in the top 5% were considered to be obese.

For WtHR measures, the authors say that a cut-off value of 0.5 in adults has been proposed as a way of indicating whether the amount of upper body fat accumulation is excessive and poses a risk to health. Having a WtHR value greater than 0.5 would mean a person’s waist circumference is greater than half their height. For example, a child with a height of 100cm and a waist measuring 65cm would have a WtHR of 0.65 and be considered overweight. Though this measure has not been closely examined in children, the authors say other studies suggest that the same cut-off can be used in children to identify those “at risk”.

 

What were the basic results?

The researchers found that when using BMI to define obesity, the prevalence of obesity has changed little over the three years, and was slightly higher in boys than it was in girls.

Obesity prevalence defined by BMI for boys:

  • 20.6% in 2005
  • 19.3% in 2006
  • 19.8% in 2007

Obesity prevalence defined by BMI for girls:

  • 18.0% in 2005
  • 17.3% in 2006
  • 16.4% in 2007

Using BMI, the chances of being obese were lower for girls than boys. Conversely, obesity prevalence defined by WC was considerably higher, particularly in girls.

Obesity prevalence defined by WC for boys:

  • 26.3% in 2005
  • 20.3% in 2006
  • 22.1% in 2007

Obesity prevalence defined by WC for girls:

  • 35.6% in 2005
  • 28.2% in 2006
  • 30.1% in 2007

Using WC, the chances of being obese were higher for girls than boys. Prevalence of obesity according to WC changed over the three years, peaking in 2005, falling in 2006 and slightly increasing in 2007.

The prevalence of those deemed “at risk” according to WtHR was somewhere in between the prevalence of obesity according to BMI and WC.

Boys at risk according to WtHR:

  • 23.3% in 2005
  • 16.7% in 2006
  • 17.6% in 2007

Girls at risk according to WtHR:

  • 21.1% in 2005
  • 15.6% in 2006
  • 17.2% in 2007

With WHtR, the chances of being obese were slightly lower in girls than boys. As with WC, prevalence of being “at risk” reduced in 2006 and slightly increased in 2007, but not to the peak levels seen in 2005.

 

How did the researchers interpret the results?

The authors say that their results are consistent with past reports that there has been a “levelling off” in obesity prevalence in children in recent years, when measured according to BMI. They found that the prevalence of obesity based on waist circumference is considerably higher than estimates based on BMI, especially in girls.

The researchers do not, however, judge any of the examined measures to be better or more accurate than the others. Instead, they conclude that there is a need to understand how BMI and waist circumference relate to health risk to establish a consistent public health message.

 

Conclusion

This study assessed nearly 15,000 children in a three-year period, examining the prevalence of obesity according to three different definitions: the clinically preferred method of body mass index, a simple measurement of waist circumference, and the ratio between waist circumference and height. The study demonstrates that these measures indicate varying child obesity levels.

While BMI was found to give an obesity prevalence of around 19–20% in boys and 16–18% in girls aged  11–12 years old, WC suggested a much higher prevalence, particularly in girls: 20–26% in boys and 28–36% in girls. It is important to note that the prevalence of obesity using WC did reduce between 2005 and 2006, with a slight increase in 2007, but not to 2005 levels.

This study benefits from using consistent and valid measurements to assess all children, and from using standard growth charts to define overweight and obesity according to BMI and WC. The researchers also rightly acknowledge that WtHR has not been fully validated as indicating health risks in children.

However, despite these strengths, it is important to note that this is a select sample from a particular region in the country, and all participants were around 11–12 years old. It is not known how prevalence estimates would vary using different samples or different age groups. Also, this study did not examine how these body measures relate to health in children, either currently or in the longer term.

As a next step, the authors call for studies that collect these measures and follow children over time to assess their health outcomes. Such research is important to determine what cut-offs of BMI, WC and WtHR are appropriate for identifying children who could be at risk of health problems, so they can be given help to achieve a more healthy weight.

Overall, it is not possible to determine from this study the best measure of obesity to use in children. However, as the researchers rightly say, in tackling the obesity problem, there is a need to give the public a consistent health message on how weight, height and waist circumference relate to health in children. As lead researcher Claire Griffiths is quoted as saying in an accompanying press release: “Although the choice of BMI as a measure of obesity in children is well-established, and even recommended, widespread use of BMI to assess fatness in children may conceal differences in body composition and central adiposity which potentially pose a greater health risk. Conclusions linking BMI, WC and WHtR as measures of obesity to health risk in children cannot be drawn from the data; however the data could have serious implications for public health, suggesting that there is a need to understand the relationship between BMI and WC, with growth and health risk.”

Griffiths C, Gately P, Marchant PR and Cooke CB. Cross-Sectional Comparisons of BMI and Waist Circumference in British Children: Mixed Public Health Messages. Obesity (2012); 20 6, 1258–1260.

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