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Study Reveals Significant BMI Misclassification Compared to DXA for Weight Status

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BMI Misclassification Study

Research presented at the European Congress on Obesity (ECO 2026) and published in the journal Nutrients indicates that the traditional World Health Organization (WHO) Body Mass Index (BMI) classification system often misidentifies individuals' weight status when compared to Dual-energy X-ray Absorptiometry (DXA), a recognized standard for measuring body fat.

Professor Marwan El Ghoch of the University of Modena and Reggio Emilia noted that the BMI system has faced criticism for its inability to accurately reflect body fat percentage or distribution, yet it continues to be used in healthcare and policy settings.

Methodology

A study led by Professor El Ghoch and researchers from the University of Verona, Italy, and Beirut University, Lebanon, evaluated the validity of the BMI classification system. The study included 1,351 White Caucasian adults (60% female), aged 18 to 98 years, all of whom had their body fat measured using DXA. DXA determines weight status based on age and body fat percentage (BF%).

Key Findings

According to WHO BMI criteria, the study participants included 1.4% underweight, 58.3% normal weight, 26.2% overweight, and 14.1% with obesity. The combined prevalence of overweight and obesity was approximately 41%.

Upon re-categorization using DXA-measured body fat percentage, significant discrepancies emerged:

  • More than one-third (34%) of individuals classified as obese by BMI were reclassified to the overweight category by DXA.
  • Over half (53%) of those with an overweight BMI were misclassified. Among these, three-quarters moved to the normal weight category, and one-quarter were reclassified as having obesity.
  • For individuals with a normal weight BMI (18.5-25), DXA agreed in 78% of cases. However, 22% were reclassified (9.7% underweight, 11.4% overweight, and 0.8% obesity).
  • The most significant disagreement was found in the underweight group, where two-thirds (68.4%) of those classified as underweight by BMI were reclassified to normal weight by DXA.

Overall, DXA analysis found the prevalence of overweight and obesity across the cohort to be approximately 37% (23.4% overweight, 13.2% obesity), compared to 26.2% overweight and 14.1% obesity by BMI. The study indicates that BMI overestimates the prevalence of underweight, overweight, and obesity compared to DXA.

Conclusion and Recommendations

Professor El Ghoch concluded that a substantial proportion of adults in the Italian general population are misclassified by the traditional WHO BMI system.

Professor Chiara Milanese added that while both systems might identify a similar overall prevalence of overweight and obesity, they often identify different individuals due to classification disagreements across weight ranges and age groups.

The authors recommend that public health guidelines in Italy be revised to incorporate direct body composition measures or surrogate measures—such as skinfold measurements or waist-to-height ratio—alongside BMI when assessing weight status. They suggest similar misclassification rates may be observed in White Caucasian populations in other countries, and advocate for further research in diverse populations and ethnicities to confirm this.