General population study shows that BMI classification system wrongly identifies some people as having overweight or obesity
Use of gold standard dual-energy X-ray absorptiometry (DXA) to measure body fat shows BMI places many people in the wrong weight category
Research from Italy to be presented at this year’s European Congress on Obesity (ECO 2026, Istanbul, Turkey, 12-15 May) and published in the journal Nutrients shows 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 categorise 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 healthcare (i.e. general practitioners) and non-clinical (i.e. policy and health insurance) settings.
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, the person’s age and body fat percentage is used to decide their weight status category according to their level of adiposity.*
The study included 1351 adults of mixed gender 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 where White Caucasian (due to BMI variations in different ethnicities).
According to the WHO BMI system, among these participants there were 19 (1.4%) underweight individuals (BMI under 18.5), 787 (58.3%) 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-categorised 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 – three quarters of those misclassified fall into the normal weight category, while the other quarter should have been classified as having obesity.
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 analysed 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).
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 combining direct body composition or 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.”
Professor Marwan El Ghoch, University of Modena and Reggio Emilia, Modena, Italy. T) +39 0592055371 E) m.elghoch@unimore.it
Tony Kirby in the ECO Media Centre. T) +44 7834 385827 E) tony.kirby@tonykirby.com
The authors confirm no conflicts of interest.
This press release is based on oral presentation 0360 at the European Congress on Obesity (ECO) in Istanbul, Turkey, 12-15 May. The material has been peer reviewed by the congress selection committee. The full paper has been published in the journal Nutrients. As such, the full paper is provided in place of the abstract.
Notes to editors:
The following thresholds are used in DXA – body fat values to give weight status:
For males:
18–39 years BF% < 8% (underweight); ≥8% (normal weight); ≥21% (overweight); ≥26% (obesity); 40–59 years BF% < 11% (underweight); ≥11% (normal weight); ≥23% (overweight); ≥29% (obesity) 60–98 years BF% < 13% (underweight); ≥13% (normal weight); ≥25% (overweight); ≥31% (obesity).
For females:
18–39 years BF% < 21% (underweight); ≥21% (normal weight); ≥33% (overweight); ≥39% (obesity); 40–59 years BF% < 23% (underweight); ≥23% (normal weight); ≥35% (overweight); ≥41% (obesity); 60–98 years BF% < 26% (underweight); ≥26% (normal weight); ≥36% (overweight); ≥41% (obesity).
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Journal
Nutrients
Article Title
The WHO BMI System MisclassifiesWeight Status in Adults from the General Population in North Italy: A DXA-Based Assessment Study (18–98 Years)
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