New research from Italy suggests that the widely used body mass index (BMI) system may be misclassifying many people. The findings, which will be presented at the European Congress on Obesity (ECO 2026, Istanbul, Turkey, May 12-15) and published in Nutrients, show that when body fat is measured using dual-energy X-ray absorptiometry (DXA), considered the gold standard method, BMI often incorrectly labels individuals as overweight or obese.
BMI has faced increasing criticism in recent years because it does not directly measure body fat or account for how fat is distributed in the body. Professor Marwan El Ghoch of the Department of Biomedical, Metabolic and Neural Sciences at the University of Modena and Reggio Emilia explains that this limitation makes it difficult for BMI to accurately reflect adiposity. Despite these concerns, BMI remains widely used in both clinical care (i.e. general practitioners) and non-clinical settings (i.e. policy and health insurance).
Study Uses DXA to Reevaluate BMI Classifications
To better understand BMI’s reliability, researchers from the University of Verona and Beirut University examined a general population group whose body fat had been measured using DXA. This method classifies weight status based on age and body fat percentage, providing a more direct assessment of adiposity.
The study involved 1351 adults between the ages of 18 and 98 years, with 60% female participants. All individuals were referred to the Department of Neurosciences, Biomedicine and Movement Sciences at the University of Verona. The participants were all White Caucasian, reflecting known differences in BMI across ethnic groups.
Using standard WHO BMI categories, 19 participants (1.4%) were classified as underweight (BMI under 18.5), 787 (58.3%) as normal weight (BMI 18.5-25), 354 (26.2%) as overweight (BMI 25-30), and 191 (14.1%) as obese (BMI over 30). This resulted in a combined overweight and obesity rate of about 41%, consistent with data from the Veneto region of Italy. Researchers then reassessed participants using body fat percentage (BF%) measured by DXA.
Significant Misclassification Found With BMI
The comparison revealed notable differences. Among individuals classified as obese by BMI, more than one-third (34%) were actually in the overweight category based on DXA results. Misclassification was even more pronounced among those labeled overweight by BMI, where over half (53%) were placed in the wrong category. Of these, about three quarters were actually within the normal weight range, while the remaining quarter met criteria for obesity.
BMI and DXA showed better agreement among individuals in the normal weight range (BMI 18.5 to 25), with both methods aligning in 78% of cases. However, 22% of those individuals were assigned a different category when evaluated with DXA (9.7% underweight, 11.4% overweight and 0.8% obesity).
The largest discrepancy appeared in the underweight group. Two-thirds (13 of 19; 68.4%) of those classified as underweight by BMI (under 18.5) were reassigned to a different category when analyzed using DXA, and should have been considered normal weight.
Overall, DXA results showed a combined overweight and obesity prevalence of about 37% (23.4% overweight and 13.2% obesity), compared to 41% when using BMI.
Experts Urge Changes to Weight Assessment Guidelines
Professor El Ghoch, who led the study, says: “Our main finding highlights the fact that a large proportion of individuals, exceeding one-third of adults among the Italian general population, is misclassified and placed in an incorrect weight status category, when relying on the traditional WHO BMI classification resulting in an overestimation of the prevalence of underweight, overweight, and obesity when compared to the classification based on body fat percentage as measured by the gold standard technique of dual-energy X-ray absorptiometry (DXA).”
Co-author Professor Chiara Milanese of the University of Verona adds: “Another key finding of our study is that, even though both systems identify a similar overall prevalence of overweight and obesity, we are talking in some cases about different people — or in other words the individuals identified by DXA are not all the same as those from BMI classification. This is due to the disagreement between WHO BMI and DXA-derived BF% classification systems in determining weight status in the general population among body weight ranges and age groups of both genders.”
The researchers conclude that public health guidelines in Italy should be updated to include additional tools alongside BMI. These may include direct measures of body composition or simpler alternatives such as skinfold measurements or body circumference indicators like the waist-to-height ratio. They also suggest that similar misclassification patterns are likely in White Caucasian populations in other countries in Europe and Worldwide. However, further research is needed to determine whether these findings apply across different ethnic groups and regions.
