BMI is often debated as not being a useful tool. However, oftentimes these discussions fail to understand exactly what BMI is and how best to utilize it as a tool.
It is important to understand that BMI does not discriminate between fat mass and lean mass and does not directly address adiposity (i.e., body fat percentage). However, it is a tool based on large sample sizes and is primarily used as a risk prediction tool over large numbers of people. As such, on a population level, BMI can approximate levels of adiposity to a useful degree. Furthermore, it is a good tool for estimating risk for specific outcomes (e.g., mortality, cardiovascular disease, diabetes).
However, on an individual level, BMI can be less accurate, and it is best used in conjunction with other data (e.g., body composition tests) in the full assessment of an individual’s weight status. Individuals who carry a substantially high level of lean mass (e.g., bodybuilders or powerlifters) may fall into the overweight or obese category based solely on BMI but may have body fat percentages in the single digits.
Conversely, people who carry a minimal amount of lean mass (e.g., highly sedentary people) may fall into the normal/healthy category based solely on BMI but may have levels of body fat that would be considered overweight or obese