Why BMI Can Be Misleading for Athletes and Older Adults
BMI can't distinguish muscle mass from fat mass — it only measures total weight relative to height. This means muscular athletes often land in the "overweight" category despite low body fat, while older adults who've lost muscle mass can show a "normal" BMI despite a higher actual body fat percentage.
These two failure modes — athletes scoring too high, older adults scoring artificially normal — are the most well-documented and consistently cited limitations of BMI, and both come from the exact same underlying mechanism.
The muscle-mass problem
Muscle tissue is denser than fat tissue — it weighs more per unit of volume. A muscular athlete can have significant lean mass and very low body fat, but because BMI only sees total weight relative to height, that extra muscle weight pushes the BMI number up as if it were excess fat, often landing in the "overweight" or even "obese" category despite being objectively lean and fit.
The reverse problem in older adults
Aging is commonly associated with gradual muscle loss (sarcopenia), even without significant weight change — someone can maintain the same total weight over decades while their body composition shifts toward a higher percentage of fat and a lower percentage of muscle. Because total weight stayed flat, their BMI can look unchanged and "normal" even as their actual health-relevant body composition has shifted.
What to check instead, or alongside BMI
Body fat percentage — measured through methods like the US Navy tape-measure method — gives a more direct read on body composition than weight-and-height alone. Waist circumference, strength and mobility measures, and standard lab results (blood pressure, cholesterol, blood sugar) all provide context BMI alone can't capture.