Another Defect Detected in BMI: Classifies Tall Kids as Overweight

by Tanya Thomas on Sep 30 2008 10:19 AM

Another Defect Detected in BMI: Classifies Tall Kids as Overweight
A new study claims that Body Mass Index (BMI) calculation is unfair to children, especially tall pre-pubescent children who are often classified as obese.
An Australian study into children's health and lifestyles has found that tall pre-pubescent children are being incorrectly classified as overweight or obese.

Lead author Richard Telford, an Australian National University (ANU) adjunct professor, says that using BMI to calculate the body size of pre-pubescent children is skewing results.

The results of the study highlighted that six to 12 percent of eight-year-olds are misclassified when BMI is used to determine body size or adiposity, he adds.

Telford claims that BMI could incorrectly classify significantly tall and short eight-year-old children, as the adiposity measurements of BMI and percent body fat are both related to height.

"BMI is biased as a measure and unfairly would predict a taller child to be fatter than he or she is," ABC Online quoted him as saying.

Giving an example of two eight-year-old girls with a 19 cm difference in their heights, he said that, if calculated by current measurement system, the taller girl's BMI would be 1.6 points higher and her percent 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," he said.

According to Telford, the biggest drawback 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.

However, the researchers admitted that the height bias could easily be overcome by changing the formula by which adiposity is calculated.

The team has now proposed a new system for measuring eight-year-olds called body mass function, which is body weight divided by height cubed. On the other hand, BMI is calculated by dividing body weight by height squared.

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 years.

The study appears in a paper published in the journal Obesity.


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