BMI Misclassifies Over a Third of Adults, Study Reveals Health Measurement Flaws
More than one in three adults may be incorrectly categorised by BMI, the standard test utilised by medical professionals to determine if individuals maintain a healthy weight, potentially leaving millions with an inaccurate perception of their health status. The score is derived from a straightforward formula based on height and weight, but emerging research indicates it may be considerably less reliable than widely assumed.
Mounting Evidence Against BMI Reliability
The findings contribute to growing evidence that BMI is not fit for purpose and can provide a misleading depiction of a person's true body fat levels and associated health risks. Researchers from the University of Modena and Reggio Emilia, the University of Verona, and Beirut University compared BMI classifications with dual-energy X-ray absorptiometry, recognised as the gold-standard method for measuring body fat.
The study, published in the journal Nutrients and scheduled for presentation at this year's European Congress on Obesity in Istanbul, analysed 1,351 adults aged between 18 and 98, predominantly women and all of White European descent. Based on BMI, 1.4 per cent were underweight, 58 per cent a healthy weight, 26 per cent overweight, and 14 per cent obese, figures broadly consistent with population estimates in Italy's Veneto region.
Significant Discrepancies in Classification
However, DXA scans revealed a markedly different narrative. More than a third of individuals classified as obese by BMI were actually only overweight. Under the BMI system, a score of 18.5 to 25 is considered healthy, 25 to 29 denotes overweight, and 30-plus indicates obesity, the stage at which risks of illness escalate dramatically.
Among those labelled overweight, more than half were misclassified, with most actually being a healthy weight and a smaller proportion truly obese. Even within the healthy weight category, BMI was incorrect in approximately one in five cases. The most substantial discrepancy was observed in people classed as underweight, with two-thirds found to be a healthy weight based on body fat measurements.
Overall, DXA suggested slightly lower rates of excess weight, with 23 per cent overweight and 13 per cent obese. Professor Marwan El Ghoch, who led the study, stated, "Over a third of adults are misclassified when relying on BMI, which overestimates underweight, overweight and obesity compared with actual body fat measurements."
Limitations and Calls for Broader Measures
Professor Chiara Milanese, co-author, added, "Even when BMI and DXA find similar overall rates, the people identified are not always the same. This shows BMI can misrepresent weight in adults of different ages and genders." A key limitation of BMI is its inability to distinguish between fat and muscle, meaning individuals with high muscle mass may be labelled overweight, while others with higher body fat but less muscle could fall within the "healthy" range.
The researchers advocate for BMI to be used alongside other measures, such as waist-to-height ratio or skinfold tests, to provide a more accurate health assessment. They also called for further studies to evaluate whether similar misclassification occurs in more diverse global populations.
Implications for Public Health Policy
These findings resonate with calls from 58 international experts for a broader definition of obesity that includes measures like waist circumference and fat distribution, rather than relying solely on BMI. Separate analysis of over 300,000 adults in the United States suggests that adopting these additional measures could increase the number of people classified as obese by nearly 60 per cent.
Applied to the United Kingdom, this would raise the figure from around 13 million to nearly 21 million people. According to the latest Health Survey for England, approximately 64 per cent of adults are classed as overweight or obese based on BMI alone, a statistic that underpins national policy and NHS guidance.
Obesity is linked to serious health risks including type 2 diabetes, heart disease, and breathing problems, costing the NHS more than £11 billion annually, alongside wider losses from reduced productivity and welfare spending. Researchers emphasise that even when standard markers of metabolic health appear normal, carrying excess body fat is associated with a significantly higher risk of serious disease.
Maintaining a healthy weight remains one of the most crucial steps for protecting long-term heart, liver, and kidney health, they concluded, highlighting the urgent need for more precise health assessment tools.
