Danish study finds being underweight linked to higher mortality than overweight; 'fat but fit' phenomenon noted

A Danish analysis of more than 85,000 people found that being underweight was associated with substantially higher short-term mortality than being overweight or mildly obese, and that some people in higher body mass index (BMI) ranges showed no increased risk of death compared with those in the upper end of a healthy BMI.

Researchers followed 85,761 individuals for five years and recorded 7,555 deaths — about 8% of the cohort. The study, reported by ScienceDaily and led by Dr. Sigrid Bjerge Gribsholt of Aarhus University Hospital, used the BMI range 22.5 to 25 as the reference category and found that people in the underweight category (BMI of 18.5 or below) were 2.7 times more likely to die during follow-up than the reference group. Those with BMIs of 18.5 to 20.0 were about twice as likely to die, and individuals with BMIs of 20.0 to 22.5 were 27% more likely to die than the reference population.

By contrast, participants classified as overweight (BMI 25–30) and those in the lower obese range (BMI 30–35) showed no greater mortality than the 22.5–25 reference group. The investigators did find an elevated risk among people with BMI 35–40 (class 2 obesity), who had a 23% increased risk of death compared with the reference category.

The study population was predominantly female (81.4%) with a median age of 66.4 years at baseline. Gribsholt cautioned that reverse causation could influence the results: people may lose weight because of underlying illness, and the dataset was drawn from individuals who were undergoing scans for health reasons, which could skew findings. "In those cases, it is the illness not the low weight itself that increases the risk of death, which can make it look like having a higher BMI is protective," she said. She also noted that the predominance of older participants could mean survivors with higher BMI possess protective traits not captured in the analysis.

The investigators described the observation that some people with higher BMI do not show greater mortality as consistent with the concept of being metabolically healthy or "fat but fit," a phenomenon reported in earlier research. Their findings are slated for presentation at the annual meeting of the European Association for the Study of Diabetics in Vienna.

The Danish analysis adds to a body of literature questioning the adequacy of BMI as a sole measure of health. A recent study published in the European Heart Journal found that visceral fat — fat around internal organs — was linked to accelerated ageing of the heart and blood vessels, and that body shape may matter: men with an "apple" pattern of abdominal fat showed more rapid cardiac ageing, while women with greater gluteofemoral (hip and thigh) fat had indicators of healthier, younger hearts.

Public-health and health-system impacts of obesity remain substantial. The UK government has previously estimated that obesity costs the National Health Service about £6.5 billion a year. Other analyses have placed larger, broader economic costs that include lost productivity and long-term care.

Researchers behind the Danish study and outside experts said the findings reinforce that BMI is an imperfect proxy for individual health risk and that clinical assessment should consider other measures, including signs of underlying illness, body composition and fat distribution. The authors urged caution in interpreting the protective signal seen in some higher-BMI groups and called for further work to disentangle the effects of existing disease, age, fitness and fat distribution on mortality.