A recent large study of children with high body mass indexes (BMI) found that many children of normal body mass had high fat percentages while 25% of children with high BMI were not obese by fat percentage criteria.
BMI (weight in kilograms divided by height in meters squared) does not distinguish between the weight of muscle, fat or bone and has a statistical artifact that tends to classify tall children as overweight.
“One out of six children in our study who had a BMI in the normal range had an unhealthy level of body fat,” said Dr. Kenneth Ellis, a Baylor College of Medicine professor of pediatrics who studies growth and body composition at the USDA/ARS Children’s Nutrition Research Center in Houston. “And one out of four with a BMI in the at-risk-to-obese range had a body-fat level that was normal.”
There is an assumption when using BMI as a criteria for obesity that individuals who have a BMI within the normal range have an average amount of body fat. And another assumption is that every ounce of body weight over the standard weight for height is fat.
While the BMI/fitness disparity can be true of adults- many pro athletes have BMIs that are very high- in children there is also often a weight gain just before growth spurts. High levels of physical activity, early maturity, genetics and ethnicity can all contribute to an elevated childhood BMI with a healthy amount of body fat.
BMI is not the only measure of obesity that has challenges, although as a quick and dirty method for looking at large populations it is easily problematic when applied to individuals. Electrical impedence studies, as found on scales, have low levels of accuracy, even when from the same manufacturer or in hospital quality machines. Dual X-ray absorptiometry with its $150,000 price tag has various methods and two machines may give very different results. It also has problems with small adults and children.
An Australian Lifestyle of Our Kids review published in late 2007, in Medicine and Science of Exercise and Sport, concluded that more evidence existed to implicate physical inactivity than body fat as a cause of chronic diseases such as type 2 diabetes. Another of their studies showed that changes in body fat in children aged eight to 10 had no influence on changes in insulin resistance. This is because children are in a state of dynamic growth and may gain fat to fuel growth spurts.
None of this is to disparage the effect of poor diets, full of low quality fats, excess carbohydrates or high fructose corn syrup on poor health. It speaks more to the difficulty in standardizing measures of children. At some point in adulthood, excess fat is better correlated with health problems. But we should spend more energy getting all children active than worrying about BMI .
It appears that fitness is more significant than weight in assessing health. At a time when underfunded schools lack recess or physical education, when children are kept indoors after school as safety measures, and when sedentary electronic games have replaced climbing trees, fitness takes on a new importance. And fitness measures can be objectively assessed.