Calculating overweight and obesity in children and adolescents

For the 2004 Canadian Community Health Survey, Body Mass Index (BMI) was derived from the child’s or adolescent’s weight and height as measured by the interviewer, using the formula BMI = (weight in kg)/(height in m)².

BMI cut-offs of 25 and 30 are used to classify adults as overweight and obese, based on health risks associated with being in these BMI categories.^{6, 23} Recently, the International Obesity TaskForce (IOTF) agreed on a new approach to measure overweight and obesity among children and adolescents.²⁴ Because it is not clear which BMI levels are associated with health risks at younger ages, the group recommended extrapolating the adult cut-offs of 25 and 30 to create sex- and age-specific values. Based on data from the United States, Great Britain, the Netherlands, Brazil, Hong Kong and Singapore, BMI centile curves that passed through the points of 25 and 30 at age 18 were derived. These centile curves were based on data collected between 1963 and 1993.

The overweight and obesity rates presented in this analysis are based on the IOTF criteria. Because sexual maturation influences body fat, the IOTF cut-offs are sensitive to the timing of puberty.⁹

Source: Reference 24

For example, a 7-year-old boy who is 3 feet 11 inches (119 cm) tall would have to weigh at least 56.9 pounds (25.8 kg) (BMI = 17.9) to be considered overweight, and a 13-year-old girl who is 5 feet, 3 inches (160 cm) tall would be considered obese if she weighed 161 pounds (73 kg) (BMI= 28.5).

Many previous studies have used US growth curves and classified BMIs falling over the 85th and 95th centiles for age- and sex-specific categories as overweight or obese. While the two methods generally yield similar results, the IOTF reference values tend to give lower estimates for young children and higher estimates for older children.^{1, 25}