5 Minutes
Study and key findings
A large, preliminary Danish study tracking more than 85,000 adults reports that very low body mass index (BMI) is linked with substantially higher risk of premature death. Presented as a conference paper at the Annual Meeting of the European Association for the Study of Diabetes, the analysis shows a U-shaped relationship between BMI and mortality: the lowest and highest BMI categories had the greatest risk, while a mid-range group had the lowest.
Using a BMI of 22.5–24.9 as the reference, researchers found that people with BMI below 18.5 had almost three times the risk of early death. Participants on the lower end of the conventional “healthy” range also showed elevated risk: BMI 18.5–19.9 roughly doubled mortality risk, and BMI 20.0–22.4 carried an approximately 27% higher risk compared with the reference group. At the higher end, people with BMI between 25 and 35 did not show a consistent increase in mortality; only very high obesity (BMI ≥40) was associated with a marked rise in risk (about 2.1 times).
These results are not yet peer reviewed and should be interpreted cautiously. The dataset was derived from individuals who had undergone body imaging for clinical reasons, which may introduce selection bias. Investigators also note the potential for reverse causation: illness-induced weight loss may explain some of the association between low BMI and mortality. Still, the pattern supports prior evidence that being very thin is a health risk and that the relationship between body weight and longevity is complex.
Scientific background and limitations
BMI — a ratio of weight to squared height — remains widely used in clinical practice and public health because it is simple to calculate. But BMI is a blunt instrument: it does not distinguish fat from muscle, account for fat distribution (visceral versus subcutaneous), or capture differences related to age, sex, ethnicity, diet or physical activity. The metric’s standard cutoffs were developed using historical data from a limited sample of European men; many experts argue these thresholds are not universally appropriate.
The Danish team suggests the BMI range associated with the lowest mortality in their sample may be broader and slightly higher than traditional “healthy” values — possibly BMI 22.5–30. This hypothesis aligns with other studies that report a so-called “obesity paradox,” where moderately higher BMI correlates with comparable or even improved outcomes in some settings. However, because the study participants had clinical imaging, the sample may over-represent people with suspected health issues, and other confounders (smoking, socioeconomic status, unintentional weight loss) could influence results.
Physiological mechanisms and clinical implications
Low body mass can reflect chronic undernutrition, muscle wasting (sarcopenia), or active disease. In starvation or prolonged illness the body enters a catabolic state, prioritizing energy for critical organs such as the brain while immune function and tissue repair suffer. For patients undergoing cancer therapy, for example, pre-existing fat and muscle reserves can determine resilience to treatment-related weight loss and influence recovery. Unintentional weight loss is also a recognized early sign of diseases such as malignancy or type 1 diabetes, so a low BMI can be a marker rather than a cause of increased mortality.
At the population level, these findings argue for more nuanced use of BMI in clinical decisions. Reliance on BMI thresholds for access to treatments or surgical eligibility should be balanced with other measures: body composition analysis, functional assessments, blood biomarkers, and clinical history. Advances in managing obesity-related conditions—better medications, improved cardiovascular care and diabetes management—may also shift the distribution of risk associated with body weight.
.avif)
Expert Insight
"These results reinforce that context matters," says Dr. Emma Solberg, a fictional clinical epidemiologist and science communicator. "BMI can flag risk at the population level, but clinicians need complementary tools — muscle mass measurement, frailty scores and relevant lab tests — to determine individual risk. Importantly, unexplained weight loss at any BMI requires prompt evaluation."
Conclusion
The Danish analysis adds to a growing body of evidence that the relationship between BMI and mortality is not linear. Very low BMI is associated with substantially higher risk of premature death, while moderate overweight may not increase mortality in the same way. The study underscores limitations of BMI as a sole measure of health and highlights the need for richer clinical assessment and further research — ideally peer-reviewed, representative, and adjusted for confounders — before changing population guidance. For clinicians and public-health professionals, the practical message is balanced: monitor unexplained weight loss, assess body composition and function, and treat BMI as one piece of a broader clinical picture.
Source: sciencealert
Leave a Comment