Body Mass Index (BMI) is a simple index of weight-for-height that is commonly used to classify underweight, overweight and obesity in adults. It is used to estimate a healthy body weight based on a person’s height. BMI values are age-independent and the same for both sexes. However, BMI may not correspond to the same degree of fatness in different populations due, in part, to different body proportions. The health risks associated with increasing BMI are continuous and the interpretation of BMI grading in relation to risk may differ for different populations.
Differences in body proportions and in the relationship between BMI and body fat content can affect the BMI range considered to be healthy. Adult BMI increases very slowly with age, so age-independent cut off points can be used to grade fatness or underweight. In children, However, BMI changes substantially with age, rising steeply in infancy, falling during the preschool years, and then rising again during adolescence and early childhood. For this reason, child BMI needs to be assessed using age-related reference curves.