Body mass index has a curvilinear relationship with the percentage of body fat among children.

Background: Body Mass Index (BMI), which is defined as the ratio between weight (in kg) and height (in m2), is often used in clinical practice as well as in large scale epidemiological studies to classify subjects as underweight, normal weight, overweight or obese. Although BMI does not directly measure the percentage of Body Fat (BF%), it is widely applied because it is strongly related with BF%, it is easy to measure and it is an important predictor of mortality. Among children, age and sex-specific reference values of BMI, known as percentiles, are used.