Tuesday, January 6, 2009

BMI myths

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' or 'normal' based on BMI. A focus on BMI alone as an indicator of health is misguided.
BMI can be dangerous  when a single number is used to label a child and assess health status. One can see how arbitrary the labels are when you consider a six-year-old where 5 pounds can span three categories, and a height measurement off by as little as 1/8 of an inch can take a child from 'normal' to 'overweight' or even 'obese.' 
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. BMI 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 very healthy, and the label of 'overweight' may be totally inappropriate. Same is true for a child growing at the 10% with a label of 'underweight' also being misleading. 
Is your doctor asking about healthy habits? How much screen time do your kids have? Are they grazing on convenience foods all day long? Are they eating a variety of foods that include fruits, vegetables and yes, treats? Are they 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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