Measures of Obesity: Which is Best?

Body Mass Index (BMI) is the most commonly used measure to screen for and diagnose obesity, but is it the best tool to do so?

Determined by dividing weight in kilograms by height in meters, squared (kg/m²), a BMI >30 kg/m² is considered obese. While there is a strong correlation between BMI and body fatness, there are other variables to consider. First, women tend to have more body fat than men, so the same BMI diagnosis likely means different levels of fatness. Similarly, athletes have less fat than non-athletes and some races have more or less body fat than others. This could lead to overestimating (or underestimating) weight-related or other health conditions. Body mass also does not differentiate between fat and lean muscle (or fluids or bones), nor does it account for fat distribution. As such, BMI should always be considered in the context of a clinical assessment.

An alternate — or better yet — complementary measure is body fat percentage. This assessment distinguishes between body fat and lean muscle and identifies different baseline measures for men (>25 percent) and women (>30 percent) to account for varying levels of body fat. However, as with BMI, body fat percentage does not recognize fat distribution, which can be indicative of potential health issues.

To address the fat distribution gap, waist circumference is a useful determinant of visceral adiposity. This screening measure is important because abdominal obesity is strongly associated with insulin resistance, type 2 diabetes and cardiovascular disease. To that end, waist circumference is a much more reliable indicator of health issues as compared to BMI, and it might help explain to patients why those who have a large waist but are not otherwise overweight are still at risk.

Waist-to-hip ratio is another method to predict risk based on fat distribution. This calculation accounts for the difference between apple- and pear-shaped bodies. Both waist circumference and waist-to-hip ratio have different classifications for men and women, and although both measures are considered equally reliable, waist circumference often is favored because it is easier to obtain and interpret.

Regardless of how obesity is measured, weight loss is recommended. Programs, such as one that utilizes meal replacements, offer quick, measurable results in weight, body fat, waist circumference, and waist-to-hip ratio to help reduce the health risks associated with obesity.

The long and short of it is that a single measure is not effective as a diagnostic tool because it rarely gives the full picture. To help patients fully understand, and perhaps accept their obesity, it can be helpful to provide various measures and a clear description of how each is calculated, what it means, and why it is important.