The merits of using BMI as an indicator of body fat and obesity are still contested. A key contention to the use of BMI indicators is that it provides a measure of body mass/weight rather than providing a direct measure of body fat. Whilst physicians continue to use BMI as a general indicator of weight-related health risks, there are some cases where its use should be considered more carefully 14 : muscle mass can increase bodyweight; this means athletes or individuals with a high muscle mass percentage can be deemed overweight on the BMI scale, even if they have a low or healthy body fat percentage; muscle and bone density tends to decline as we get older; this means that an older individual may have a higher percentage body fat than a younger individual with the same BMI; women tend to have a higher body fat percentage than men for a given BMI. Physicians must therefore evaluate BMI results carefully on a individual basis. Despite outlier cases where BMI is an inappropriate indicator of body fat, its use provides a reasonable measure of the risk of weight-related health factors across most individuals across the general population.

Obesity and overweight in children are also measured on the basis of body-mass-index (BMI). However, interpretation of BMI scores is treated differently for children and adolescents. Weight categories are defined in relation to WHO Growth Standards – a child is defined as overweight if their weight-for-height is more than two standard deviations from the median of the WHO Child Growth Standards.