BMI is a useful starting point, but it’s not a good standalone indicator of health. It captures one dimension of your body, the ratio of weight to height, while ignoring several factors that matter more: where your fat sits, how much of your weight is muscle versus fat, and what’s actually happening with your blood sugar, blood pressure, and cholesterol. In 2023, the American Medical Association formally recognized these limitations and recommended that BMI be used alongside other measures rather than on its own.
What BMI Actually Measures
BMI divides your weight in kilograms by your height in meters squared. A Belgian mathematician named Adolphe Quetelet developed it in the 19th century as a way to estimate obesity trends across large populations, not to diagnose individuals. That distinction matters. A tool designed to describe thousands of people at once loses accuracy when applied to one person standing in a doctor’s office.
The CDC currently defines a healthy BMI as 18.5 to 24.9, overweight as 25 to 29.9, and obesity as 30 or above (with three classes of obesity ranging up to 40 and beyond). These categories are simple and easy to calculate, which is exactly why they became the default screening tool worldwide.
Where BMI Gets It Right
At the population level, BMI does correlate with health risks. Research from the American Heart Association shows a graded increase in cardiometabolic risk factors, including diabetes, high blood pressure, elevated triglycerides, and inflammatory markers, as BMI rises above 25. The higher the category, the greater the clustering of risk. For large-scale public health tracking, BMI remains a practical tool because it requires nothing more than a scale and a tape measure.
For people at the extremes, BMI also tends to be informative. A BMI of 40 almost always indicates excess body fat with real health consequences. A BMI of 16 signals serious underweight. The muddier territory is everything in between.
Why BMI Misses So Much
The core problem is that BMI treats all weight the same. Muscle and bone are denser than fat, so a muscular person can register as “overweight” or even “obese” while carrying very little body fat. Conversely, older adults who have lost muscle and bone density can show a normal BMI while carrying a higher percentage of body fat than their number suggests. Harvard Health notes that BMI overestimates fat in athletes and underestimates it in older people with low muscle mass.
BMI also can’t tell you where your fat is stored, and location turns out to be critical. About 90% of body fat in most people is subcutaneous, the soft layer just under the skin. The remaining 10% or so is visceral fat, packed around your liver, intestines, and other organs deep in the abdomen. Visceral fat is biologically active. It pumps out inflammatory compounds, raises insulin resistance, and drives cardiometabolic disease in ways subcutaneous fat does not. Two people with identical BMIs can have dramatically different amounts of visceral fat, and therefore dramatically different health risks.
Normal BMI Doesn’t Always Mean Healthy
Perhaps the most striking evidence against relying on BMI alone comes from research on people who are “metabolically obese” at a normal weight. A systematic review and meta-analysis covering more than 400,000 people with normal BMIs found that roughly 27% of them had metabolic disorders: things like insulin resistance, high blood pressure, or abnormal cholesterol. That’s about one in four people whose BMI says they’re fine but whose bloodwork tells a different story.
These individuals often carry excess visceral fat or have low muscle mass, neither of which BMI can detect. They may not get flagged for further screening, which means their risk goes unrecognized.
BMI Thresholds Don’t Fit Every Population
The standard BMI cutoffs were largely derived from studies of European populations. For people of Asian descent, health risks begin climbing at lower BMI values. A WHO consultation found that the threshold for observed risk in Asian populations ranges from a BMI of 22 to 25, and the high-risk cutoff falls between 26 and 31, both lower than the standard categories. Additional action points were proposed at BMIs of 23, 27.5, 32.5, and 37.5 to better reflect when intervention is warranted across different ethnic groups.
This means a person of South Asian or East Asian heritage with a BMI of 24, technically “healthy weight” by standard charts, may already face elevated risk for type 2 diabetes and cardiovascular disease. Using a single universal threshold obscures these differences.
The Paradox in Older Adults
BMI behaves differently at the other end of the age spectrum too. A large systematic review of older adults found that higher BMI is often associated with lower mortality, not higher. Among the studies examined, 25 reported a protective effect of higher BMI, 9 found no significant association, and only 1 identified it as a clear risk factor. Four observed a U-shaped curve, where both low and high BMIs increased risk.
This so-called “obesity paradox” likely reflects the fact that as people age, the bigger danger shifts from excess fat to the loss of muscle and bone. An older adult with a BMI of 27 may have protective reserves of lean mass and energy stores, while someone at 21 may be frail and losing muscle. BMI can’t distinguish between these two very different bodies, which is why clinicians working with older patients increasingly focus on grip strength, walking speed, and body composition rather than weight alone.
Better Ways to Assess Body Fat Risk
Waist circumference and waist-to-hip ratio capture something BMI cannot: where fat accumulates. Because these measurements correlate with visceral fat, they offer a more direct window into cardiometabolic risk. Research in older adults found that a higher waist-to-hip ratio was consistently identified as a risk factor for mortality across eight studies, even in cases where BMI appeared protective. Greater abdominal fat relative to hip fat signals increased inflammation and insulin resistance, which BMI simply does not reflect.
The AMA’s 2023 policy recommends pairing BMI with measures like visceral fat assessment, waist circumference, body composition analysis, relative fat mass, and genetic or metabolic factors. No single number captures health, but a combination of these tools gives a far more accurate picture than BMI on its own.
What This Means in Practice
If your BMI falls in the “healthy” range, that doesn’t guarantee your metabolic health is fine. And if it falls in the “overweight” range, that doesn’t automatically mean you’re at risk, especially if you’re physically active, carry more muscle, or your waist circumference is within a healthy range. A BMI of 27 in a 70-year-old with strong grip strength and good mobility means something completely different than the same number in a sedentary 45-year-old with a 40-inch waist.
BMI is best understood as a rough screening flag, not a diagnosis. It can prompt further evaluation, but it should never be the final word. If your doctor treats your BMI as the whole story, it’s reasonable to ask about waist circumference, blood sugar, blood pressure, and cholesterol, the markers that actually connect to disease risk in a way a height-weight ratio cannot.