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Criticisms and Reliability of BMI

BMI Basics

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Updated August 23, 2006

Because the BMI formula simply includes a child's height and weight and doesn't directly measure their body fat, a lot of people are critical of using body mass index to measure obesity. However, according to the CDC, 'BMI is a reliable indicator of body fatness for most children and teens.'

The main criticism is that many people who are very athletic and who are overweight because they are very muscular may have a high BMI and be labeled as being overweight or obese. Children who are short and muscular can especially be labeled incorrectly as being overweight by using the BMI formula.

Although most younger children with a high BMI are really overweight and have excess body fat, if you still aren't sure, there are some other measurements or tests for body fat that can be more accurate, including:

  • skinfold thickness measurements
  • waist to hip ratio
  • bioelectric impedance assay (BIA)
  • hydrodensitometry (underwater weighing)
  • dual-energy x-ray absorptiometry (DEXA)
Keep in mind that some of these tests, like hydrodensitometry and dual-energy x-ray absorptiometry, can only be done in research settings.

Another simple test, using a scale to directly measure body fat percentage can be done at home, as many newer home scales include body fat measurements.

Body Fat Percentages

According to the American Council on Exercise, the acceptable body fat percentage for adults is 18-24% for men and 25-31% for women. And although very fit and athletic adults can have lower body fat percentages, adult males with a body fat percentage of 25% or higher and adult females with a body fat percentage of 32% or higher are considered obese.

Unfortunately, as with BMI, a healthy body fat percentage in kids depends on their age, so is a little more difficult to figure out. And while the Pediatric Nutrition Handbook from the American Academy of Pediatrics does show 'the average amount of lean body mass and body fat in non-obese healthy individuals from birth to age 22 years,' they don't publish any ranges for what is healthy or any data on what is considered overweight, which makes the data less useful. Your Pediatrician should be able to help you figure out what a healthy body fat percentage is for your child though.



References:
1CDC. About BMI for Children and Teens.
2Childhood obesity. Speiser PW - J Clin Endocrinol Metab - 01-MAR-2005; 90(3): 1871-87
3Pediatric Nutrition Handbook. 5th Edition. AAP.

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