A new study from Italian researchers shows that BMI, one of the most widely used health metrics globally, is getting weight classifications wrong for more than one-third of adults. When researchers compared traditional BMI categories to precise body fat measurements using dual-energy X-ray absorptiometry (DXA), a gold-standard scanning method, they found significant mismatches that could affect how doctors assess health risk and how people understand their own weight status .

Why Is BMI So Unreliable for Weight Assessment?

Body mass index, or BMI, calculates weight status using only height and weight. It does not directly measure body fat or account for how fat is distributed throughout the body. This fundamental limitation means BMI can misclassify people with high muscle mass as overweight or obese, while missing others who have excess body fat despite a normal BMI reading .

Researchers from the University of Verona and Beirut University examined 1,351 adults between ages 18 and 98 to understand how often BMI gets it wrong. Using standard World Health Organization (WHO) BMI categories, they classified participants as underweight, normal weight, overweight, or obese. They then reassessed the same individuals using body fat percentage measured by DXA scans, which directly measures adiposity, or body fat levels .

What Did the Researchers Actually Find?

The comparison revealed striking differences across weight categories. Among people classified as obese by BMI, more than one-third (34%) were actually in the overweight category based on DXA results. The misclassification was even more dramatic among those labeled overweight by BMI: over half (53%) were placed in the wrong category entirely .

Of those 53% misclassified overweight individuals, approximately three-quarters were actually within the normal weight range, while the remaining quarter met criteria for obesity based on actual body fat measurements. The two methods showed better agreement only among people in the normal weight range, where both aligned in 78% of cases. However, even in this group, 22% received different classifications when evaluated with DXA .

The largest discrepancy appeared in the underweight group. Two-thirds of those classified as underweight by BMI (13 of 19 people, or 68.4%) were reassigned to a different category when analyzed using DXA and should have been considered normal weight instead .

How to Get a More Accurate Assessment of Your Weight Status

• Request body composition testing: Ask your healthcare provider about DXA scans or other direct body fat measurement methods that provide more accurate adiposity assessment than BMI alone.
• Discuss waist-to-height ratio: This simpler alternative measures waist circumference relative to height and may provide better health risk information than BMI for many people.
• Consider skinfold measurements: Healthcare providers can use calipers to measure skin thickness at multiple body sites, offering a more direct assessment of body fat distribution than BMI.

The overall prevalence of overweight and obesity was similar between the two methods: 41% using BMI versus 37% using DXA. However, this masks a critical problem. As researchers noted, the individuals identified as overweight or obese by DXA are not always the same people identified by BMI. This means public health interventions and clinical recommendations based on BMI may be targeting the wrong populations .

"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," explained Professor Marwan El Ghoch of the Department of Biomedical, Metabolic and Neural Sciences at the University of Modena and Reggio Emilia.

Professor Marwan El Ghoch, Department of Biomedical, Metabolic and Neural Sciences, University of Modena and Reggio Emilia

The research team also noted that even though both systems identify a similar overall prevalence of overweight and obesity, the individuals identified differ significantly across different body weight ranges and age groups. This discrepancy has real implications for how people receive health guidance and how health insurance companies assess risk .

What Changes Are Experts Recommending?

Researchers concluded that public health guidelines should be updated to include additional assessment tools alongside BMI. These may include direct measures of body composition, simpler alternatives such as skinfold measurements, or body circumference indicators like waist-to-height ratio. The study was presented at the European Congress on Obesity in May 2026 and published in the journal Nutrients .

The findings raise important questions about how weight assessment guidelines are applied globally. The study involved only White Caucasian participants, reflecting known differences in BMI accuracy across ethnic groups. Researchers suggest that similar misclassification patterns are likely in White Caucasian populations in other European countries and worldwide, though further research is needed to determine whether these findings apply across different ethnic groups and regions .

For individuals concerned about their weight status, these findings suggest that BMI alone should not be the sole basis for health decisions. Consulting with a healthcare provider about more comprehensive body composition assessment may provide clearer insight into actual health risk and whether weight management interventions are truly necessary.