by European Association for the Study of Obesity

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Research from Italy to be presented at this year'sEuropean Congress on Obesity(ECO 2026, Istanbul, Türkiye, 12–15 May) andpublishedin the journalNutrientsshows that when the gold standard technique of dual-energy X-ray absorptiometry (DXA) is used to measure body fat in the general population, it shows that the traditional WHO body mass index (BMI) classification system misidentifies significant numbers of people as having overweight or obesity.

In the past few years there has been a lot of criticism of the BMI system due to its inability to accurately capture body fat percentage or distribution, in order to correctly categorize weight status based on adiposity, explains Professor Marwan El Ghoch, of the Department of Biomedical, Metabolic and Neural Sciences, University of Modena and Reggio Emilia, Modena, Italy. He adds that despite these concerns, BMI as a weight classification system continues to be used in the general population in primary health care (i.e. general practitioners) and non-clinical (i.e. policy and health insurance) settings.

How the study was designed

In this new study, Professor El Ghoch and researchers from the University of Verona in Italy and Beirut University in Lebanon set out to determine the validity of the BMI classification system, specifically regarding its ability to identify correctly those with overweight and obesity, in a sample of the general population who had all had their body fat measured using DXA. With DXA, a person's age andbody fat percentageare used to decide their weight status category according to their level of adiposity.

The study included 1,351 adults of mixed genders, aged between 18 and 98 years (60% female), all of whom were referred to the Department of Neurosciences, Biomedicine and Movement Sciences, University of Verona, Verona, Italy. All the participants in this study were white Caucasian (due to BMI variations in different ethnicities).

What BMI showed versus DXA

According to the WHO BMI system, among these participants there were 19 (1.4%) underweight individuals (BMI under 18.5), 787 (58.3%) with normal weight (BMI 18.5-25), 354 (26.2%) with overweight (BMI 25-30), and 191 (14.1%) with obesity (BMI over 30). The overall prevalence of approximately 41% for overweight and obesity combined is consistent with the local population in the Veneto region of Italy. Participants were then re-categorized according to adiposity based on body fat percentage (BF%) measured by DXA.

DXA revealed that more than one-third (34%) of those with obesity defined by BMI had been misclassified and should be in the overweight category. For those with an overweight BMI, DXA showed that more than half—53%—had been misclassified, and three-quarters of those misclassified fell into the normal weight category, while the other quarter should have been classified as having obesity.

Misclassification patterns across groups

BMI and DXA had better agreement when considering those with a normal weight BMI (18.5 to 25), with DXA agreeing in 78% of cases. But 22% of those with normal weight were given a different category with DXA (9.7% underweight, 11.4% overweight and 0.8% obesity). Finally, despite the small absolute numbers, the biggest BMI-DXA disagreement was found in the underweight group—two-thirds (13 of 19; 68.4%) in the underweight category defined by BMI (under 18.5) were in the wrong category when analyzed by DXA, and should have been classified as having normal weight.

With all the correct and misclassifications combined, the DXA analysis found that the prevalence of overweight and obesity across the cohort was at around 37% overall (23.4% overweight, and 13.2% obesity, compared to 26.2% and 14.1% with BMI).

Implications and next research steps

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)."

Study 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."

Accordingly, the authors conclude, "Public health guidelines in Italy need to be revised to consider combiningdirect body compositionor their surrogate measures such as skinfold measurement or body circumference—such as the waist-to-height ratio—with BMI while assessing weight status in the general population. We believe a similar level of misclassification can be expected in white Caucasian populations in other countries in Europe and worldwide.

"However, to confirm this, and if a similar effect exists in other ethnicities, future research should extend the aim of our analysis to other countries across Europe and globally, as well as seeing if such misclassification occurs in people of other ethnicities."

Publication details Chiara Milanese et al, The WHO BMI System MisclassifiesWeight Status in Adults from the General Population in North Italy: A DXA-Based Assessment Study (18–98 Years), Nutrients (2026). DOI: 10.3390/nu17132162 Journal information: Nutrients