Charts that take your child's age, weight and height into account are used as a preliminary screening tool to evaluate her risk for medical problems like the diabetes precursor called glucose intolerance, high cholesterol and early maturation. Such charts also are used to predict your child's likelihood of being an overweight adult who suffers health issues. Charts that use formulas based on height and weight to predict health issues also exist for adults, though these do not take age into account.
Growth Charts
One tool that the Centers for Disease Control and Prevention recommends that physicians use is a growth chart that plots a child's height, weight and age. These charts are used for early identification of children who are at risk for becoming obese or overweight when they get older. However, growth charts are not supposed to be a sole diagnostic instrument. They are just one tool for clinically measuring a child.
BMI For Age Charts
Health care providers also use charts that plot body mass index, or BMI, percentile for age. The BMI is calculated based on weight and height and plotted for each age. If your child or teen has a BMI that falls along the 50th percentile, that means he's close to the average for his age group. If your youth comes in between the 85th and 95th percentile he is considered overweight, because 85 to 95 percent of kids in his age group have lower BMIs. If your kid is above the 95th percentile your teen is considered obese, and if he's below the 5th percentile he's considered underweight.
Calculations
The actual calculation for BMI is done the same way for children and adults. The formula used is: weight in pounds, divided by height in inches, squared, and multiplied by 703. However, with children and teens, BMI charts use percentile lines to compare a youth's BMIs to those of a large group of people, because healthy weight ranges change with a youth's height and age. Separate charts exist for boys and girls.
Adult Information
The age and sex factors for the height and weight charts are removed after age 20, so BMI is interpreted via standard weight status categories. These are the same for all ages and for both women and men. A BMI of 18.5 to 24.9 is considered normal. Underweight is a BMI that's below 18.5; overweight is a BMI of 25 to 29.9; and obese is a BMI that's 30 or more. However, the CDC does note that BMI varies by sex and age because men tend to have less body fat than women and older people tend to have more body fat than younger adults.
Interpretation
While the height and weight and BMI charts can raise red flags, they do not always tell the full story, so make sure you have a doctor help you interpret your status. A child with a large frame, for example, may fall into the overweight category though he does not have extra fat. Or, an athlete will have a high BMI because he has more muscle density.
Considerations
Remember that your BMI does not measure your body fat directly. It's used, however, because it's an easy-to-perform and inexpensive measure to screen for weight categories that can lead to health problems. For adults these most often include diabetes, high blood pressure and coronary heart disease. If you have a high BMI, your doctor will perform further screening to assess your risk for health issues.
References
- Centers for Disease Control and Prevention: Overweight Children and Adolescents
- Centers for Disease Control and Prevention: 2000 CDC Growth Charts for the United States; Robert J. Kuczmarski, et al.; 2000
- Child Health and Disability Prevention Program: Body Mass Index for Age Percentile
- Centers for Disease Control and Prevention: Healthy Weight -- About BMI For Adults
- Centers for Disease Control and Prevention: Healthy Weight --About BMI for Children
- Kidshealth.org: Figuring Out Fat Using BMi
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