BMI: what it is and what it is not

Over the past decade, the global syndemic of over and under nutrition has become one of the most overt, public health problems and concerns that we, as health practitioners, are faced with daily. The controversial term “BMI” has been typically force-fed, as an appropriate diagnostic tool to tell someone that they are overweight or obese or even underweight. There is no wrong in using the BMI as a quick reference tool to determine whether you are in a healthy weight range for your height, but what about other confounders like age, sex, ethnicity, medical background and family history? It is therefore important to understand what the BMI measures, and what it does not, to appropriately interpret those findings.

BMI is calculated as weight in kilograms divided by your height multiplied by your height, giving an answer indicative of overall excess weight. You may even remember using a large paper based chart and finding your weight on one side, height on the other and then arriving at this final number. A normal BMI is between 18.5 to 25, and a person with a BMI below 18 is considered underweight and a BMI above 25 is considered overweight or obese.

Although BMI is regularly used in the healthcare setting today, it does have some limitations that are important to be aware of when interpreting the results.

BMI is unable to distinguish between fat free mass and muscle mass. In the case of many athletes, such as rugby players, wrestlers, bodybuilders and powerlifters, their BMI is commonly overestimated because of their unusually high levels of muscle mass. Looking at their BMI alone will therefore suggest that they are at the same health risk as someone who is inactive and extremely overweight. The BMI is also not a useful tool for pregnant women for the same reason.

The categories of BMI classifications have also been shown to overestimate the health risk in people from ethnic minority backgrounds. Multiple research studies have found that Asian populations have a higher risk of metabolic disturbances at lower BMI compared to Whites of the same BMI. This raises the debate about whether genetic compositions may be the root cause of these differences in BMI risk and if a greater emphasis should be made on ethnicity specific BMI classifications. On the other hand, BMI may underestimate metabolic risk of the elderly and people with a physical disability who have age-related muscle deficits and muscle wasting.

Overall, it is important to be aware that BMI is not accurate enough to use for the sole purpose of diagnostic value, however it can be a rather simple screening tool to assess for weight problems in children and adults. Remember when interpreting the BMI that it cannot differentiate between body fat and muscle mass. In saying that, it may be useful to use measures such as waist circumference and waist-to-height ratio given they are able to accurately measure abdominal body fat, which is commonly associated with a higher metabolic risk.