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Sunday, June 23, 2013

What Is a Normal BMI for Children?

The body mass index, or BMI, is a number derived from a comparison of weight and height. When considering the BMI of children and teens (up to 20 years of age), much finer measurements are considered than on the adult scale, and gender is also included as a variable. The BMI compares measurements taken of an individual child, compares them against averages and then places the child within an underweight, healthy weight, overweight or obese category. Although variation exists across all individuals and you should work with a medical professional to determine the healthy weight for your child, the BMI can be used generally as a barometer of health and can help families make nutrition and exercise decisions as well as predict risks.

History

    According to an article by G. Eknoyan, the BMI was developed by Adolphe Quetelet in 1832 (it was called the Quetelet Effect until 1972). The measurement is obtained by the following formula (in imperial units): (weight in pounds x 703) / height in inches squared. Numbers obtained for adults are generally discussed in terms of below 18.5 as underweight, between 18.5 and 25 as healthy weight, between 25 and 30 overweight, over 30 obese and 40 and over being morbidly obese.
    As is to be expected, numbers for children skew lower, but because BMI for children takes finer measurements and considers age and sex, percentiles are usually discussed: according to the CDC, children in the 5th percentile and under are underweight, children between the 5th and 85th percentile have a healthy weight, children between 85th and 95th are overweight and children are obese over the 95th percentile.

Function

    BMI helps to classify all individuals against one standard of health in relationship to weight and height. In particular, BMI measurement for children (commonly called BMI for age) is calibrated to reflect the many changes the bodies of children of both sexes undergo as they age; BMI will change with a child's age even if no change in habits occurs. There is a correlation between health risks such as cardiovascular disease and people whose BMIs fall outside the BMI healthy standard. However, BMI cannot be used as a predictive tool and generally does not discern health information about individuals in its healthy parameters.

Benefits

    The parents and caretakers of children with BMIs outside healthy percentiles can use this information to take action to get under- and overweight children back in healthy parameters. The BMI is not a catch-all measurement and does not predict specific health risks, but it is a quick measurement that can highlight the possibility of problems. Because BMI varies with sex and age, no standard number can be used for healthy BMI in children; instead, the CDC offers a BMI for age calculator, for which you will need to enter the child's birth date, the date the measurement is taken, the child's sex, height rounded to the nearest fraction of an inch and weight to the nearest fraction of a pound. Then, this can be checked against the charts for children's BMI percentiles found in Resources.

Prevention/Solution

    To prevent weight gain, it is important to eat a diet commensurate with level of physical activity, to encourage healthy amounts of exercise and to limit inactivity (for instance, viewing television). If your child is under- or overweight, work with a nutritionist and her pediatrician to return to a healthy weight level for your child's sex and age.

Considerations

    The BMI is limited as a measurement because it is not a measure of the level of body fat vs. muscle mass. A test of adiposity (the amount of body fat) will yield better information about the health of a child, and the BMI may provide a misleading perspective on the health of very athletic children.

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