BMI-for-age, also known as body mass index for age, is a concept that uses the standard body mass index calculation but, in evaluating that BMI, takes the person's age into account. The body changes as we grow older, so the amount of fat content that is normal changes as we age. Taking this into account is important when using BMI to determine whether a person is overweight or underweight. BMI for age has important implications for adults as they age but is a term that is most common when discussing issues of childhood obesity.
Calculating the BMI of any person, whether an adult or child, is relatively simple. The subject begins by measuring his or her height and weight. To calculate the BMI, the subject simply multiplies the weight in pounds by 703, then divides this value by the height in inches squared. When using metric values to calculate the BMI, the subject simply uses the weight in kilograms and divides that number by the height in meters.
Adjusting the BMI-for-age is particularly important when using BMI to evaluate the health of a child because children's bodies undergo such substantial changes as they grow and develop. A healthy BMI for a fully developed adult might be very different from the healthy BMI for a child. As a result, rather than adhering to the standard chart of BMI values, evaluating a baby's BMI is done using a percentile system to determine whether a child might be overweight, underweight or obese.
The percentile system that BMI-for-age uses compares the BMI of a child to other children of that same age group. Generally, when a child's BMI is in or above the 95th percentile for his or her age group, this means that the child is overweight. When the BMI is at or below the fifth percentile, then the child is underweight. When the BMI is between the 85th and 95th percentile, this can indicate that a child is at risk for becoming overweight.
BMI-for-age is not important only when determining healthy BMI levels for children, though. The body development of adults is not as extreme or as fast as that of children, but adults' bodies do, nevertheless, undergo changes as they age. As men and women enter middle age, they tend to see a rise in BMI. When they reach old age, BMI can drop, sometimes to unhealthy levels. To account for these bodily changes, it is important to use BMI-for-age values to determine the healthiest BMI levels for people of all ages.