New research from Denmark is challenging long-held assumptions about body weight and health, suggesting that being very thin may be significantly more dangerous than carrying extra pounds. The study indicates that the traditional 'healthy' Body Mass Index (BMI) range may not accurately reflect the lowest risk of mortality.
The U-Shaped Curve of Risk
A large study tracking more than 85,000 Danish adults has revealed a U-shaped relationship between BMI and the risk of death. This means individuals at both the very low and very high ends of the BMI spectrum face the greatest danger. The findings were presented as a conference paper at the Annual Meeting of the European Association for the Study of Diabetes and are awaiting peer review.
The data showed that being underweight carried the most severe risk. People with a BMI below 18.5 were nearly three times more likely to die prematurely compared to those in the reference group with a BMI between 22.5 and 24.9. Even those at the lower end of the 'healthy' range faced elevated risks, with a BMI between 18.5 and 19.9 doubling the likelihood of an early death.
Reassessing the 'Overweight' Category
Perhaps more surprisingly, the research found that carrying some extra weight did not consistently translate into a higher mortality risk. Participants with BMIs between 25 and 35 – typically categorised as 'overweight' or 'obese' – showed no significant increase in risk compared to the reference group.
Only individuals with a BMI of 40 or more saw their risk of death more than double. This adds weight to the evolving scientific discussion around the concept of being 'fat but fit' and questions whether the standard 'healthy' BMI range of 18.5 to 24.9 is still optimal.
Why Is Being Underweight So Risky?
The findings underscore that thinness is not inherently protective. The body requires energy reserves to cope with illness and stress. Fat reserves can be crucial during medical treatments like chemotherapy, where patients often lose weight. Someone with minimal reserves may deplete them quickly, hampering recovery.
Furthermore, unintentional weight loss is frequently a warning sign of serious underlying illness, such as cancer or type 1 diabetes. This means a low BMI can sometimes be a marker of disease rather than its cause. The study's authors acknowledge this 'reverse causation' as a possible explanation for their results, as all participants had undergone body scans for suspected health issues.
BMI: A Blunt Tool in a Diverse World
The research highlights the ongoing debate about BMI's limitations as a standalone health metric. Developed nearly 200 years ago from data on white European men, BMI fails to account for critical factors like muscle mass, fat distribution, diet, lifestyle, and ethnic differences in body composition.
Although the NHS adjusts BMI thresholds for diabetes risk in Asian and Black groups, critics argue the measure remains a poor representation of our diverse population. Its continued use in significant healthcare decisions – from access to surgery to fertility treatments – raises concerns about fairness and accuracy.
The Danish researchers suggest modern medical advances, which help manage obesity-related conditions, might have shifted the safest weight range higher. They propose a BMI between 22.5 and 30 may now carry the lowest mortality risk, at least within the studied population.
While these preliminary findings require further validation, the core message is clear: being very thin poses significant dangers, and carrying some extra weight may not shorten life. The ultimate lesson is not to swap one simplistic view for another, but to recognise that BMI alone is a fragile and incomplete measure of human health.
