Measured height and weight parameters can be used to calculate a child’s body mass index (BMI), which is then plotted on a BMI growth chart to assess whether a child is underweight, normal weight, overweight or obese. In adults, BMI ranges are set: any adult with a BMI greater than 25 is considered overweight, while those with a BMI over 30 are considered “obese.” Children are continually growing and experience spurts at certain ages; for example, a typical child will have a decrease in BMI around age four and then progressively increase throughout childhood. For this reason, BMI is plotted on an age- and gender-appropriate growth chart.
A BMI below 10 percent is considered underweight, 10 to 84 percent normal weight, 85 to 94 percent overweight, and >95 percent “obese.” Even if a child is considered to have a normal BMI, a rapid change in trajectory across percentile lines is cause for alarm and may trigger further investigation into the cause of the change.
Low BMI in children and adolescents can be caused by genetic factors and a predisposition to be small; it may also be caused by a restriction of calories (especially in preadolescent and adolescent girls, but certainly not limited to them). This can happen on the part of the child in response to perceived increased weight. It can also occur accidentally in some cases, such as in very active children who do not eat enough to support physical demands.
A disproportionate number of children have a BMI greater than the 85th percentile. The percentile norms are based on the distribution of children in 1976, when only 15 percent of children exceeded the 85th percentile. Now more than one-third of children are classified as overweight or obese. Largely due to poor nutrition habits and low levels of physical activity, childhood overweight and obesity can negatively affect nearly every organ of the body, causing a wide range of complications, including asthma, sleep apnea, infertility in girls, gallstones, liver dysfunction and bone fractures.
While BMI is a commonly used method of approximating body composition, a more accurate assessment of a child’s actual fat mass is the direct measurement of body composition with skinfold measurements or bioelectrical impedance analysis (the same is true for measuring adult fat mass). This is the method commonly employed by school districts to measure body composition as part of their fitness assessment. If these methods are used, norms for children are available at fitnessgram.net.