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BMI myths

Posted by on Jan 6, 2009 in Blog Posts |


From parenting magazines, Wii fit, to your child’s teachers and doctors, it seems all anyone is talking about these days is BMI. BMI or body mass index– for those of you who have been blissfully living under a rock– is a measure relating weight to height.  It is sold as a way to estimate body fat composition and predict health outcomes. It can be a useful tool, but mostly to follow populations– not individuals.

 The latest of a long list of studies to illustrate the problems using BMI appeared in The Journal of the American Dietetic Association in 2008 (author Brann.) It concluded that roughly 20% of boys in the study were incorrectly diagnosed as ‘overweight.’ In addition, about the same percent of boys in the “normal” range actually had higher fat to lean body mass ratio. A focus on BMI alone as an indicator of health is misguided.
BMI labels can be dangerous when a single number is used to label a child and as a proxy for health status. One can see how arbitrary the labels are when you consider that for a six-year-old, a range of 5 pounds can span three categories (normal, overweight, obese), and a height measurement off by as little as 1/8 of an inch can take a child from ‘normal’ to ‘overweight’.  (Robison)
Think its not a big deal to label a six year old girl ‘overweight’ or ‘obese’ regardless of her BMI? (More on that in later installations…)
So, this is mostly a cautionary note. Following weight and height is part of an overall evaluation of the health of the child. Seeing how your child grows over time is important. A child growing consistently at the 90% can be healthy, and the label of ‘obese’ may be totally inappropriate. Same is true for a child growing at the 10% with a label of ‘underweight’ also providing no useful information, and potentially causing parents to worry and try to make the child eat more, which most often makes matters worse!
Is your doctor asking about healthy habits? How much screen time do your kids enjoy? Are they grazing on convenience foods all day long? Are they eating a variety of foods that include fruits, vegetables and yes, treats? Do they have time and opportunities to be physically active?  Focusing on behaviors means kids who are ‘normal’ weight with health risks won’t be ignored, and active children with well-rounded diets and consistent, normal growth won’t be misdiagnosed as ‘overweight’ and possibly harmed with unnecessary intervention.
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