Danish study finds underweight linked to higher short-term mortality; mild obesity shows no added risk
Researchers followed 85,761 mostly older adults for five years and found those with BMI below 18.5 had higher death rates than people who were overweight or mildly obese; authors caution reverse causation and selection bias.
A Danish cohort study presented this month found that people classified as underweight faced substantially higher mortality over five years than those in the upper range of a healthy body-mass index (BMI), while participants who were overweight or mildly obese showed no greater risk of death than the reference group.
Researchers followed 85,761 individuals for a median five-year period, during which 8% of participants (7,555) died. The study population was predominantly female (81.4%) and had a median age of 66.4 years at baseline.
Compared with the reference BMI range of 22.5 to 25, people with BMI at or below 18.5 were 2.7 times more likely to have died during follow-up. Participants with BMI between 18.5 and 20.0 had twice the risk of death, and those in the 20.0 to 22.5 range had a 27% higher risk. By contrast, participants with BMIs of 25 to 30 (overweight) and 30 to 35 (lower-end obesity) did not show an increased mortality risk relative to the 22.5–25 group. Individuals in the 35 to 40 range (class 2 obesity) had a 23% increased risk of death.
Lead author Dr. Sigrid Bjerge Gribsholt of Aarhus University Hospital cautioned against interpreting the associations as proof that higher body weight is protective. She said reverse causation is one plausible explanation: some participants may have lost weight because of underlying illness, and the study drew on data from people undergoing scans for health reasons, which could bias results. "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 it is possible that older people with higher BMI who lived longer may possess traits that mitigate risk.
The investigators plan to present the findings at the annual meeting of the European Association for the Study of Diabetes in Vienna. The results were highlighted in media reports following the release of the research summary; initial coverage cited ScienceDaily.
The Danish analysis arrives amid a body of research that complicates simple associations between body weight and health. A recent study published in the European Heart Journal found that visceral fat — fat stored deep around internal organs — was associated with signs of accelerated cardiovascular ageing, and that body shape may be an important marker of risk. That study reported that men who carry excess abdominal fat are more likely to show age-related changes in the heart and blood vessels than people without such fat, while women who store fat around the hips and thighs sometimes show lower cardiovascular risk.
Public-health and economic perspectives remain concerned about obesity. The UK government has estimated that obesity costs the National Health Service about £6.5 billion a year and is the second largest preventable cause of cancer. Separate analyses have put per-patient health-care costs and the broader economic burden of rising obesity at substantially higher levels, with some reports estimating costs to the nation in the tens of billions annually.
The study’s authors and external experts say the findings underline limitations of BMI as a sole measure of health and the importance of considering age, underlying illness and body composition. Observational data cannot establish causation, and the study’s median age of 66.4 and predominance of female participants may limit generalisability to younger or more sex-balanced populations. Further research that includes measures of muscle mass, fat distribution and longitudinal weight change will be needed to clarify the mechanisms behind the observed mortality differences and to inform clinical and public-health guidance.