BMI is a useful screening tool at the population level, but it’s a surprisingly poor measure of any individual person’s health. The formula, which divides your weight in kilograms by your height in meters squared, tells you nothing about how much of your weight is fat, where that fat sits on your body, or what’s happening metabolically. In 2023, the American Medical Association formally acknowledged these limitations and recommended that BMI never be used as a sole measure in clinical settings.
What BMI Actually Measures
BMI was never designed to assess individual health. A Belgian mathematician named Adolphe Quetelet developed the formula in the 19th century as part of a project to define the “average man” using statistical distributions across large groups. It was a population-level tool from the start, meant to describe trends in body size, not diagnose anything in a single person.
The standard categories still used today classify a BMI under 18.5 as underweight, 18.5 to 24.9 as healthy weight, 25 to 29.9 as overweight, and 30 or above as obese (with further classes at 35 and 40). These cutoffs were based primarily on data from non-Hispanic white populations, which creates problems when applied broadly, a point the AMA now explicitly recognizes.
Where BMI Gets It Wrong
The core problem is that BMI can’t tell the difference between muscle and fat. Muscle and bone are denser than fat, so a muscular person with low body fat can easily land in the “overweight” or even “obese” category. The reverse is also true: older adults who have lost muscle and bone density may register a “normal” BMI while carrying excess fat that puts their health at risk.
This isn’t a rare edge case. A study presented at the North American Menopause Society found that 18.5% of women with a normal BMI actually had excess body fat when measured with a DEXA scan, which uses low-dose X-rays to directly measure fat, muscle, and bone. Nearly one in five women who looked fine by BMI standards were carrying hidden risk.
BMI also loses accuracy across different ethnic groups. Asian populations develop metabolic complications like type 2 diabetes at significantly lower BMIs than white populations. China recommends an overweight cutoff of 24 (not 25), and India uses 23. Even with these adjusted thresholds, Asian Americans remain 30 to 50% more likely to have diabetes than non-Hispanic white counterparts at the same BMI. Japan defines obesity starting at a BMI of 25, five full points lower than the international standard of 30. Using a single universal cutoff misses real risk in millions of people.
Fat Location Matters More Than Total Weight
One of BMI’s biggest blind spots is that it treats all fat the same. It can’t distinguish between fat stored just under the skin (which is relatively harmless) and fat packed around internal organs in the abdomen (which is strongly linked to heart disease, diabetes, and early death). Research consistently shows that the position of fat deposits matters more than the total amount of fat in the body.
A large study published in The BMJ illustrated this dramatically. People with a normal BMI but a large waist circumference had worse long-term survival than people classified as overweight or obese who did not carry weight around their midsection. Among the study’s participants, those with central obesity faced significantly higher cardiovascular death rates regardless of their BMI category. Overweight individuals with central obesity had an 11% higher mortality risk than overweight individuals without it. For obese individuals, having central obesity raised mortality risk by 27%.
Central obesity was strikingly common in the study, affecting over 53% of participants. That means more than half the people studied had a fat distribution pattern that BMI alone would never flag.
Better Ways to Assess Body Composition
Waist circumference is the simplest upgrade from BMI. Measuring around your midsection at the level of your navel gives a rough proxy for visceral fat. A waist-to-hip ratio above 0.85 in women or 0.90 in men indicates central obesity. This single measurement, which takes about five seconds with a tape measure, outperforms BMI as a predictor of cardiovascular death.
The visceral adiposity index combines waist circumference with other simple measurements to estimate abdominal fat distribution and function. Research in older adults suggests it is a more specific and sensitive tool than BMI for predicting obesity-related heart and metabolic diseases. As people age, central fat accumulation and loss of muscle mass become far more important than overall weight in shaping health outcomes.
DEXA scans provide the most detailed picture, directly measuring fat mass, lean mass, and bone density throughout the body. They’re the gold standard for body composition, but they require specialized equipment and aren’t part of routine checkups. For most people, combining BMI with a waist measurement and basic metabolic bloodwork gives a much more complete picture than BMI alone.
What the AMA Now Recommends
The AMA’s 2023 policy change reflected a growing consensus that BMI has been over-relied upon in medicine. The organization acknowledged that BMI is significantly correlated with fat mass in general populations but “loses predictability when applied on the individual level.” It also noted the formula’s historical use for racist exclusion and the fact that its data foundation doesn’t represent diverse populations.
The new policy recommends using BMI only alongside other valid measures: visceral fat assessment, waist circumference, body composition analysis, relative fat mass, and genetic or metabolic factors. It also states that BMI should not be used as the sole criterion to deny insurance reimbursement, a practice that has historically blocked people from accessing treatment based on a number that may not reflect their actual health status.
The practical takeaway is straightforward. BMI can give you a rough starting point, especially if your number falls well within the healthy range or well into the obese range. But for the large number of people in the gray zones, or for anyone in a demographic group where the standard cutoffs don’t apply well, BMI alone can lead to both false reassurance and false alarm. A tape measure around your waist may tell you more about your health than a scale ever will.