Study finds being underweight may carry higher mortality risk than mild obesity, challenging BMI assumptions

A major Danish study tracked more than 85,000 adults over several years and found that being underweight was linked to a substantially higher risk of death than being at the high end of the normal BMI range. The research also suggested that people who are overweight or moderately obese did not have higher mortality than those in the upper-normal category. The findings were presented at the European Association for the Study of Diabetes meeting in Vienna last week and add nuance to the idea that health risk rises strictly with BMI. The study also reinforces that obesity and being underweight remain major global health challenges, but the relationship between body mass and health is complex.

Lead author Dr. Sigrid Bjerge Gribsholt said the results challenge the notion that being heavier automatically shortens life, noting that some individuals with excess weight who maintain good fitness and metabolic health may fare better than their leaner peers. The study showed the greatest risk at the population extremes: both severe obesity and being underweight. Gribsholt emphasized that obesity and underweight are major global health challenges, and that risk is not uniform across all BMI values.

The investigators said body fat distribution and underlying health status matter. Visceral fat—fat stored around the abdomen—appears to impair metabolic health, while identical BMI values can carry different risk depending on whether fat accumulates around the abdomen or on the hips and thighs. Professor Jens Meldgaard Bruun noted that someone with BMI 35 who is apple-shaped may have higher risks for type 2 diabetes or hypertension than another person with the same BMI who stores fat in the lower body.

The study’s authors argued that underweight status can reflect malnutrition or disease and may precede health decline, complicating interpretation of mortality risk. They also discussed the possibility that mild overweight could, in some contexts, offer energy reserves during illness. In Denmark, a culture that promotes cycling and an active lifestyle may contribute to these dynamics, the researchers said.

The team called for obesity treatment to be personalized, taking into account fat distribution, muscle mass, cholesterol and blood sugar, sex, and ethnicity, rather than relying on BMI alone. They caution that BMI should be interpreted alongside other measures and indicators in clinical practice. Dr. Akshaya Srikanth Bhagavathula, an epidemiologist at North Dakota State University who was not involved in the study, told Fox News Digital the findings align with other recent work but challenge current medical guidelines that assume health risk rises steadily with BMI. "The higher death rates in people with lower BMI likely occur because hidden illnesses cause the situation to appear backwards," Bhagavathula said. "Diseases like cancer or heart failure cause weight loss first, which makes low weight seem more dangerous than it actually is."

Obesity, meanwhile, tends to come before serious health problems, according to the World Health Organization. Mild excess weight may provide a "protective effect" during illness. Bhagavathula added that in older adults, age-related muscle loss increases vulnerability, such as to falls and infections, while mild excess weight may provide energy reserves during illness. He emphasized that BMI should be considered alongside waist-to-height ratios, muscle mass, cholesterol and blood sugar, sex and ethnicity. He urged that current health messaging should shift toward how the body processes food, encouraging physical activity and balanced eating rather than BMI alone.

Current health messaging often emphasizes weight loss, which can create stigma and lead to unnecessary medical treatment for people who function well at higher BMI. The Danish findings suggest a more nuanced approach, one that weighs body composition and metabolic health alongside BMI. Deirdre Bardolf is a Lifestyle writer with Fox News Digital.

The researchers presented their findings at the annual meeting of the European Association for the Study of Diabetes in Vienna and noted that more work is needed to translate these observations into clinical guidelines. They said future studies should refine risk assessment by considering fat distribution, muscle mass, and comorbidities, and should explore how these factors interact with age, sex, and ethnicity. While not reversing concerns about obesity, the work underscores that health risks associated with BMI are not uniform and that a personalized approach to weight management may be warranted.

Overall, the study adds to a growing body of literature that invites clinicians and policymakers to look beyond BMI alone. It highlights the role of visceral fat, muscle health, and other metabolic factors in shaping mortality risk and supports calls for more individualized targets in obesity treatment and prevention. As researchers continue to investigate these relationships, public health messages may shift toward supporting healthy body composition and metabolic wellness—through balanced nutrition, regular physical activity, and appropriate medical screening—rather than focusing exclusively on weight.