New Insights on Body Weight and Mortality: Rethinking the Traditional BMI Range

A comprehensive Danish study involving over 85,000 adults has shed new light on the complex relationship between body weight and health. The research indicates that being slightly overweight might not compromise longevity, while being very underweight could significantly increase the risk of early death.

The study, presented at the European Association for the Study of Diabetes annual meeting, found that individuals with a BMI below 18.5 were nearly three times more likely to die prematurely compared to those with a BMI in the middle to upper end of the "healthy" range. Conversely, those with BMIs between 25 and 35 — commonly classified as "overweight" or "obese" — did not show a higher risk of mortality than the reference group. Only individuals with BMIs of 40 or higher faced a substantially increased risk, more than doubling the chances of early death.

Interestingly, the findings revealed a U-shaped curve when plotting BMI against mortality, suggesting that both ends of the BMI spectrum—underweight and severe obesity—are associated with higher health risks. This challenges the common notion that only higher BMIs are problematic, emphasizing that being underweight is also detrimental, especially in older populations.

The data also highlighted that a BMI at the lower end of the 'healthy' range (18.5-19.9) doubles the risk of early death, with even slightly higher BMIs (20-22.4) associated with a 27% increased risk. Meanwhile, carrying extra weight within the 25-35 BMI range did not necessarily translate into increased mortality risk, though BMIs of 40 or above did significantly raise the risk.

Physiological reasons for these findings include the body's need for fat reserves, which are essential in coping with illnesses or treatments like chemotherapy. Patients with more fat stored at the outset can draw on these reserves, aiding recovery, whereas those with very low fat may quickly exhaust their reserves, impairing their recovery ability.

Moreover, unintentional weight loss often signals underlying health issues, such as cancer or diabetes, which can cause weight reduction before diagnosis. Therefore, a low BMI might sometimes be an indicator of existing illness rather than a direct cause of increased mortality.

While headlines like "Being too thin can be deadlier than being overweight" grab attention, this research aligns with longstanding knowledge that food deprivation leads the body into a catabolic state, prioritizing brain function at the expense of other systems like immunity. The study also considers that the participants’ health scans may have revealed health problems influencing weight loss, which could confound results.

The findings advocate for a nuanced understanding of body weight and health, suggesting that the traditional BMI ranges might need revision. Modern medical progress, which helps manage obesity-related conditions, could mean that a BMI between 22.5 and 30 now presents the lowest mortality risk, at least within the Danish demographic.

However, BMI remains a blunt instrument, unable to account for factors like diet, lifestyle, or fat distribution, and may not be accurate across different ethnic or racial groups. It was originally designed using data from a small, homogeneous sample and often doesn’t consider individual variations.

The authors emphasize that better tools—such as blood tests, imaging, and detailed lifestyle assessments—could eventually replace BMI for assessing health. Until then, the current findings remind us that both extremes of weight are risky, and the key takeaway is that BMI alone does not define health.

Source: https://medicalxpress.com/news/2025-09-overweight-healthy-latest-reveals.html