Body mass index

The body mass index (BMI) is a measure of body fat. It is defined as the weight of a subject in kilograms (kg) divided by the square of their height in metres (m) ¹. The resultant quantity is measured in kg/m². 

A body mass index greater than 25 kg/m² is the medical definition for being overweight, when greater than 30 kg/m², then obesity is present.

Limitations of body mass index

The use of the body mass index has clear limitations as a measure of body fat. It does not take into account, the age, gender, skeletal structure, lean body (muscle) mass or distribution of body fat ³.

 a measure In view of this, the body mass index may give an erroneous impression of an individual's level of body fat, however, BMI has serious flaws. It does not, for example, take age, sex, bone structure, fat distribution or muscle mass into consideration. For these reasons and others, BMI can misrepresent the quantity it is used to measure. There are three main issues to consider when using BMI, namely (1) errors stemming from the fact that BMI is an indirect measure of obesity, (2) errors in self-reported data and (3) the poor sensitivity and specificity of BMI. These problems result in misclassification of individuals with respect to body fat, and that misclassification, can in turn, introduce bias in studies that deal with body fat, such as those estimating the effects of obesity on health outcomes.