BMI measurement in children
Calculation of Body Mass Index (BMI), being a person's weight in kilograms divided by the square of their height in meters.
Although BMI does not measure body fat directly, research has shown that adult BMI is closely correlated with direct measures of body fat, such as skinfold thickness measurements, bioelectrical impedance, densitometry (underwater weighing) and dual energy x-ray absorptiometry (DXA) (1,2,3).
BMI is regularly referred to by doctors, dietitians, midwives and personal trainers and although not perfect, it is widely considered to be a convenient and reliable measurement of the degree to which an adult maybe over or under weight and consequently most people are happy with its use.
But what about in children?
In 2006 Public Health England started a monitoring programme in schools. The National Child Measurement Programme (NCMP) is considered an important element of the Government’s work programme on child obesity and is operated by Pubilc Health England and the Department of Health (DH). Every year, as part of the NCMP, children in Reception (aged 4-5 years) and Year 6 (aged 10-11 years) have their height and weight measured during the school year and this is causing an increase in the use of BMI by teachers and school nurses, with letters of advice being sent out to parents (4).
On the surface this isn’t a problem
Obesity is increasing in primary schools and is seen as a particular problem in year 5 and 6 (*5), there is some evidence that parents are not able to identify when their children are at risk (6) and parental knowledge and support is key for bringing about the necessary lifestyle changes (7). However, what is a worry, is the possibility that many of the staff calculating BMIs and sending out the advice are not qualified to do so and may be misinterpreting the tools that they are using.
BMI in children is a very different measure than that in adults. In adults we can use one formula and set of bands for everybody but this is not possible in children because of the huge differences that both sex and maturity can make to the numbers, even in same aged children.
A UK review of the issues around the use of BMI in children and the thresholds for underweight, overweight and obesity was published April 2012 (8). It recommended that when looking at BMI in children not only should the results be calculated taking into account of age and sex but that they should then be given as a z score or percentile of the population rather than as a numerical band. A BMI above the 91st centile may suggest a child is overweight but it can also be reflective of a high muscle mass or an upcoming growth spurt. Below the 2nd centile may reflect undernutrition, but may simply reflect a small build.
I have seen an increase in the number of active parents asking me to look at their child’s diet after they have been told their child is overweight by the school. I would advise that its outside the training of a nutritional advisors to do this properly but that if you are interested in looking at the numbers an excellent resource can be found on the Royal College of Paediatrics and Child Health website (9) where they provide detailed charts and articles, including training materials. I am not suggesting that there is not an obesity problem in primary age children, but as far as BMI is concerned, I feel the numbers should viewed with caution. Children all grow at different times and different rates and research has consistently shown that BMI measurement is not particularly effective in children under 18yrs (10). The NHS advice is to always consider other facts alongside the BMI score in children such as fitness, diet and physical activity.
Learn more about nutrition for children and calculating BMI in our Childhood Nutrition & Obesity Prevention course
Written by Tilly Spurr
1. Garrow, J.S. & Webster, J., 1985. Quetelet’s index (W/H2) as a measure of fatness. Int. J. Obes., 9(2), pp.147–153.
2. Freedman, D.S., Horlick, M. & Berenson, G.S., 2013. A comparison of the Slaughter skinfold-thickness equations and BMI in predicting body fatness and cardiovascular disease risk factor levels in children. Am. J. Clin. Nutr., 98(6), pp.1417–24.
3.Wohlfahrt-Veje, C. et al., 2014. Body fat throughout childhood in 2647 healthy Danish children: agreement of BMI, waist circumference, skinfolds with dual X-ray absorptiometry. Eur. J. Clin. Nutr., 68(6), pp.664–70.
4. National Child Measurement Programme. NHS Digital http://content.digital.nhs.uk/ncmp.
5. *. figures, for 2015/16, show that 19.8% of children in Year 6 were obese and 14.3% were overweight.
6.Black JA, Park M, Gregson J, et al. Child obesity cut-offs as derived from parental perceptions: cross-sectional questionnaire. British Journal of General Practice. Published online March 30 2015
7. Waters et al (2011) Interventions for preventing obesity in children. Cochran review. http://onlinelibrary.wiley.com/doi/10.1002/14651858.CD001871.pub3/abstract
8. SACN & RCPCH Position statement : Consideration of issues around the use of BMI centile thresholds for defining underweight, overweight and obesity in Children. https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/339411/SACN_RCPCH_defining_child_underweight__overweight_and_obesity_in_the_UK_2012.pdf
9. Royal College of Paediatric and Child Health. http://www.rcpch.ac.uk/child-health/research-projects/uk-who-growth-charts/uk-growth-chart-resources-2-18-years/school-age#2-18
10.Javed A, Jumean M, Murad MH, et al. Diagnostic performance of body mass index to identify obesity as defined by body adiposity in children and adolescents: a systematic review and meta-analysis. Pediatric Obesity. Published online June 24 2014