BMI is a flawed measure with few practical alternatives
July 1, 2024
Source: Tomiyama, Int J Obesity, 2016 LINK This is from weight and laboratory data from the National Health and Nutrition Examination (NHANES)
Body Mass Index (BMI), which is used to assess whether people are underweight, normal weight, overweight, or obese, was not designed to support clinical decision-making. This measure is only modestly correlated with metabolic obesity. However, there are not great practical alternatives.
BMI was initially designed by a Belgian mathematician seeking to assess obesity in the overall population, not to judge the health of individuals. The measure does not include waist size, and abdominal obesity is higher risk than obesity around the hips. It is not adjusted for age. Athletes with large muscle mass are misidentified as obese, and Asian people can be obese with adverse metabolic health even with BMIs in the “normal” range. Those with normal BMIs but more belly fat measured by waist circumference have increased mortality. The American Medical Association recommended de-emphasizing BMI last year.
The ideal measure of obesity should be highly correlated with whether a person has fat in and around their organs. This is the type of fat which dramatically increases the risk of diabetes and cardiovascular, kidney and liver disease.
There are alternatives to BMI, but each has disadvantages. None can be assessed remotely, and many require highly trained examiners or expensive equipment:
Waist circumference assesses abdominal fat, a risk factor for cardiovascular disease and early death. Interobserver variation is high, but this is a widely used metric.
Waist hip ratio is recommended by the World Health Organization to assess obesity. Again, interobserver variation is high.
Skinfold thickness uses a caliper to estimate body fat composition. This metric requires a skilled examiner and there are differences among caliper manufacturers.
Bioelectrical impedance sends a weak electrical current through the body to estimate percentage body fat. This requires specialized equipment and training.
DEXA scan (dual energy x-ray absorptiometry) uses low dose radiation to measure body composition. There is a small amount of radiation exposure, and this requires specialized equipment and training.
Hydrostatic weighing submerges the body in fluid to calculate body density. This and air displacement are only done in research laboratories.
Air displacement plethysmography is like fluid submersion and estimates body density and fat percentage
3D body scanners use lasers and cameras to create a 3-D model of the body. This is an emerging technology that is not widely available.
Implications for employers:
Body mass index is likely here to stay as a proxy for obesity. It’s simple and inexpensive to calculate and requires little measurement skill or training and no specialized equipment.
Employers should be aware that BMI is by no means the “last word” on whether an individual is at higher risk for cardiometabolic complications.
Some with substantial abdominal fat and large waist circumferences might benefit from anti-obesity therapy even if their BMI is normal.
Others with substantial muscle mass can be incorrectly identified by BMI as obese, and weight loss would not improve their metabolic health.
The chart above demonstrates that restricting GLP-1 prescriptions to those with a higher BMI than 30 (or 27 with a metabolic comorbidity) would mean that a higher portion of those treated would be likely to be metabolically unhealthy and therefore gain the most benefit from these expensive medications. However, employers using a different cutoff for drug eligibility would lose substantial rebates associated with these medications.
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