BMI 'misdiagonising' children as overweight
Posted
Tall pre-pubescent children are being incorrectly classified as overweight or obese, an Australian study into children's health and lifestyles has found.
The lead author, Adjunct Professor Richard Telford of the Australian National University (ANU), says the use of body mass index (BMI) to determine the body size of pre-pubescent children is skewing results.
In the article published in the journal Obesity, Professor Telford and co-author statistician Ross Cunningham, also of the ANU, say between 6 to 12 per cent of eight-year olds are misclassified when BMI is used to determine body size or adiposity.
He says because the adiposity measurements of BMI and per cent body fat are both related to height, significantly tall and short eight-year-old children tend to be incorrectly classified.
"BMI is biased as a measure and unfairly would predict a taller child to be fatter than he or she is," Professor Telford said.
He points to an example of two eight-year-old girls, one of whose is 10 cm taller than the other.
Using the current measurement systems, the taller girl's BMI would be 1.6 points higher and her per cent body fat value would be 3.8 points higher.
"That would easily tip a girl from being normal into the mid-range of being overweight," Professor Telford said.
He says the danger of the bias is that medical staff using BMI to measure young patients "may be giving misinformation".
"If a child is told they are overweight [when they are not] that can have serious consequences," he said.
In the paper the two researchers say the height bias can easily be overcome by changing the formula by which adiposity is calculated.
They propose a new system for measuring eight-year-olds called body mass function, which is body weight divided by height cubed.
BMI is calculated by dividing body weight by height squared.
Broader study
Professor Telford says the problem with using BMI as a measure for eight-year-olds is highlighted in a broader research project currently underway.
The four-year Lifestyle Of Our Kids project (LOOK), funded by the Commonwealth Institute, is following the health, academic achievement and physical activity of 830 Canberra schoolboys and schoolgirls from the age of eight.
Researchers are examining health-related issues such as cardiovascular structure and function, bone density and strength, family influences, medical history, and level and type of physical activity including motor control, balance, nutrition and academic performance.
Professor Telford says one aspect of the study is to assess whether specialist physical education teachers are needed in primary schools.
In a paper yet to be published, he reports they have found those children with specialised physical education classes "have enhanced balance skills".
He says this has broad ramifications, as previous studies have shown a strong correlation between good motor coordination and strong literacy and numeracy skills.
