The body mass index, or BMI, is a comparison of height to weight. BMI does not measure body fat, but gives a general assessment of healthy body weight in comparison to other children in the same age range. Although not exact, it has been found by the Center of Disease Control and Prevention to be closely associated to the percentage of body fat in children. The National Institutes of Health sets forth recommendations for BMI, and notes that health risks are related to BMI outside of the healthy range.
BMI Calcuation
BMI is calculated with a height-to-weight formula which can be figured using kilograms and meters or pounds and inches:
BMI= mass (kg) divided by height in meters squared
BMI= mass (lbs) multiplied by 703 divided by height in inches squared
This calculation is then plotted on an age/gender-based chart to compare with other children of the same age and sex.
BMI Percentile
Use BMI to compare a child to other children of the same age and sex. By figuring a child's percentile ranking on the CDC's growth rate chart, a doctor can determine the relative healthy weight of the child. Anything less than five percent is considered underweight, while the 85th to 95th percentile is considered overweight. A reading in the 95th percentile or above is obese.
Recommendations
The Center for Disease Control and Prevention recommends that BMI readings for children begin at two years of age and continue, using the percentile charting, until 19 years of age. This allows for the change of body fat that occurs as children mature, as well as the difference in body fat between boys and girls. A straight calculation of BMI using the adult levels would be inaccurate as these factors are not taken into consideration. The BMI percentile ranking allows a doctor to asses the need for further body fat testing, such as skin fold or underwater weighing.
Risk Factors
Risk factors are associated with a BMI that falls outside of the acceptable range. Overweight children are at risk for elevated lipid levels, blood pressure and are at higher risk of becoming overweight adults. According to the March 2, 2009, issue of "Pediatrics," the official journal of the American Association of Pediatrics, waist-to-height ratio has a greater impact on risk factors than BMI alone. Children with a high waist/height ratio were two to three times more likely to experience risk factors than those with low waist/height ratio.
Making a Difference
Staying within normal BMI and healthy weight range is vital to long-term health. This can be accomplished through making healthy food choices, focusing on the USDA recommendations of plenty of fruits and vegetables, whole grains, lean meats, nuts and low-fat dairy, and meeting the activity guidelines set forth for children by the American Heart Association of 30 minutes of daily exercise. Parental involvement and positive role modeling play a key role in a child's food and activity choices.
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