Body mass index (BMI) is a measurement of weight adjusted for height (kg/m2) and is used in people over twenty years of age. It is not a measure of fat in the body, but does provide some idea of excess or inadequate weight. However, there are many reasons why this measurement is inaccurate.
Age, gender, genetics, ethnicity and muscle mass are all confounding factors when using BMI as a way of assessing a person’s weight. For example, although women have a higher amount of body fat normally, there is no standardisation in BMI specifically for women. Asian people generally have less muscle mass than most Indigenous Africans, but BMI does not take this into account. The elderly lose muscle mass, and as muscle weighs more than fat, here may be another error apparent in interpretation. If a person has a lot of muscle bulk and is lean, their BMI may well suggest they are in the overweight (25-29.9kg/m2), obese (30-34.9kg/m2) or even the morbidly obese (35 kg/m2 plus) range. The normal range for BMI is 18.5 to 25kg/m2.
When measuring the BMI in children, both age and gender are taken into account and the 3rd to 97th percentiles for height and weight are documented for use in this assessment. However, at puberty BMI may become inaccurate again, due to the inclusion of weight from bone and muscle growth, both of which will weigh more than fat. It is more likely that BMI is accurate in definitely obese children than in those who are only overweight.
In conclusion, BMI is a surrogate indicator of body fat in adults and children, but is not a diagnostic tool. Its value comes in monitoring weight status in populations and as a screening tool for individuals who are at risk of having an abnormal weight.
1. Department for Health and Human Services, Center for Disease Control and Prevention, Body Mass Index: Considerations for Practitioners,