New Study Of Over 1K Adults Find That BMI Can Be Highly Inaccurate
For decades, BMI has been treated as a go-to measure of health, but its limitations have been greatly debated for just as long. A new study just confirmed what many of us—physicians, health policy experts, and medical historians included—have long suspected: BMI gets it wrong for a lot of people.
When researchers compared standard BMI categories to more precise body fat measurements using DXA scans, they found that over one-third of adults were placed in the wrong weight category.
The findings, published in the journal Nutrients1, add to a growing body of evidence that BMI alone is an incomplete measure of health—something mindbodygreen has long emphasized.
What the research found
The study compared the results of two measurements.
- First, researchers calculated the BMI as categorized by the World Health Organization2.
- Then they reassessed the participants using body fat percentage (BF%) measured by DXA (dual-energy X-ray absorptiometry). DXA scans are considered the gold standard for body composition analysis.
Here’s a breakdown of the participants
- 1,351 adults between the ages of 18 and 98
- 60% female participants
- All were white caucasian, which is important to note given the known differences in BMI across ethnic groups
The misclassification rates were striking.
- Among individuals classified as obese by BMI, 34% were actually in the overweight category based on DXA results.
- Over half (53%) of those labeled overweight by BMI were placed in the wrong category—and about three-quarters of them were actually within the normal weight range.
- On the flip side, 68% of those classified as underweight by BMI were reassigned to normal weight when analyzed using DXA.
- The measurements were most accurate for the “normal” range, with both methods aligning in 78% of cases.
Taken together, the findings highlight a clear gap between how we currently define “healthy weight” and what’s actually happening inside the body.
Why BMI falls short
The issue comes down to what BMI actually measures—and what it doesn't. BMI is simply a ratio of height to weight. It doesn't account for muscle mass, fat distribution, or metabolic health, all of which play a critical role in long-term disease risk.
That means someone with significant muscle mass could be labeled "overweight" despite having healthy body fat levels. Meanwhile, someone with higher body fat (especially visceral fat, which tends to be much more dangerous for long-term disease risk), but lower weight might be missed entirely.
The researchers conclude that public health guidelines should be updated to include additional tools alongside BMI, such as direct measures of body composition or simpler alternatives like skinfold measurements or waist-to-height ratio.
Related read: An oncologist explains why BMI is an unhelpful (& potentially harmful) measure of women's health
Body composition is a more accurate reflection of long-term health
Body composition refers to your levels of body fat and lean body mass3. In its broadest sense, body composition components are muscle, fat, and bone.
But you see, muscle is more dense and compact than fat. Meaning, muscle mass occupies less space than fat in the body. Therefore, someone gaining muscle could experience comparable weight gain as someone who's increasing their fat percentage, so the difference lies in body composition.
Likewise, weight loss could be a positive thing (if you're making healthy, intentional lifestyle choices to lose fat) or a negative thing (if you're unintentionally losing muscle, due to inactivity, inadequate protein intake, specific health issues, and so on.).
This bares out in research too. Studies continue to show us that body composition—not weight alone—might be the key indicator for disease risk.
- A recent study found that body composition had a stronger link to cognitive decline. Individuals with higher levels of fat stored in organs like the pancreas or those with relatively low muscle mass alongside higher internal fat showed signs of accelerated brain aging.
- In fact, a study published in the American Academy of Neurology shows that where you store fat can increase your likelihood of developing Alzheimer's and Parkinson's disease. Conversely, they also found that stronger folks were less likely to develop these conditions.
- And according to research published in the Journal of the American Heart Association, body composition is strongly linked to cardiovascular events and all-cause mortality.
The through-line here is clear: it’s not just weight. It’s what weight is made of and where weight is stored, that shapes long-term health outcomes.
Related read: Experts explain why body recomposition is more important than weight loss
How to support a healthier body composition
You don’t have full control over where your body stores fat—genetics and hormones play a role. Many women tend to carry more fat in the hips and thighs during their reproductive years, while abdominal storage becomes more common after menopause and in men.
That said, body composition is highly modifiable. With the right inputs, you can shift the ratio of muscle to fat at any stage of life.
- Prioritize strength training. As you increase lean mass, your body becomes more metabolically active, meaning you burn more energy at rest.
- Dial in your protein intake. Protein provides the raw materials your body needs to build and repair muscle. It also has a higher thermic effect than fats or carbohydrates, meaning your body expends more energy digesting it, and it helps with satiety.
- Increase baseline daily movement. Your workouts matter, but so does everything else you do throughout the day. Non-exercise activity—like walking, standing, carrying, and general movement—plays a major role in overall energy expenditure and metabolic health.
A good guideline is to think less about weight loss, and more about what you’re building. By focusing on strength, you’re shifting the focus to muscle mass, which is a greater indicator of health than any number on a scale.
Related read: How much protein do women need every day? Read our deep dive into the science
The takeaway
If you've ever felt like BMI doesn't tell your whole story, this research validates that instinct. Don't let a single number define your health. Instead, focus on how you feel, your energy levels, and markers that actually reflect what's happening inside your body—not just a ratio of height to weight.
And if you’re able and have access, consider getting a DXA scan to get a more accurate read of your overall body composition.