About the study

For the study, researchers from Aarhus University Hospital and the Steno Diabetes Center in Denmark analyzed health data from 85,761 adults. More than 80% of participants were women, and the median age at the start of the study was 66. Over five years of follow-up, 7,555 people — 8% of the group —  died.

The researchers compared the risk of death across body mass index (BMI) categories, a measure of weight relative to height. A BMI below 18.5 is considered underweight, 18.5 to under 25 is normal, 25 to under 30 is overweight, and 30 or higher is obese.

Findings showed that being underweight was linked to the highest risk of death. Participants with a BMI under 18.5 were nearly three times more likely to die than those with a BMI in the upper-normal range of 22.5 to under 25. People with BMIs in this upper-normal range were considered the reference population in the study.

Even those who technically fell into the lower end of the "healthy" category faced elevated risks. Individuals with BMIs between 18.5 and 20 were about twice as likely to die, and those with BMIs between 20 and 22.5 faced a 27% higher risk compared to the reference group.

By contrast, participants who were overweight or mildly obese — with BMIs of 25 to under 35 — were no more likely to die than those in the reference group. This outcome supports what some researchers call the "fat but fit" phenomenon — the idea that carrying extra weight does not always shorten life expectancy.

Commentary

According to the researchers, the findings highlight risks on both sides of the spectrum. "Both underweight and obesity are major global health challenges," said lead researcher Sigrid Bjerge Gribsholt of Aarhus University Hospital.

Gribsholt explained that obesity can disrupt metabolism, weaken immunity, and raise the risk of type 2 diabetes, cardiovascular disease, and certain cancers. By contrast, being underweight is tied to malnutrition, weaker immune defenses, and nutrient deficiencies.

Gribsholt noted that the BMI range linked to the lowest mortality may have shifted upward over time, thanks in part to medical advances and better general health. "It was once thought to be 20 to 25 but it may be shifting upward," she said.

The research team also cautioned that part of the elevated risk among thin participants may be due to "reverse causation," meaning that these individuals may have lost weight because of underlying illness, rather than low weight itself. Since the study data came from people undergoing medical scans, researchers could not fully rule out this explanation.

Still, the findings highlight that higher BMI may not always be as dangerous as once thought. "It is also possible that people with higher BMI who live longer — most of the people we studied were elderly —may have certain protective traits that influence the results," Gribsholt said.

The researchers also emphasized that BMI alone is not a perfect measure of health. Fat distribution plays an especially important role. Visceral fat — stored deep within the abdomen around organs such as the liver and intestines — is more metabolically harmful than fat stored in the hips or thighs.

"An individual who has a BMI of 35 and is apple-shaped—the excess fat is around their abdomen—may have type 2 diabetes or high blood pressure," explained Jens Meldgaard Bruun, a professor from the Steno Diabetes Center. "[W]hile another individual with the same BMI may be free of these problems because the excess fat is on their hips, buttocks and thighs."

This complexity has led many researchers to argue for a more personalized approach to weight management. Instead of relying on BMI alone, factors such as fat distribution, age, sex, and coexisting conditions should be considered when setting health goals.

"It is clear that the treatment of obesity should be personalized to take into account factors such as fat distribution and the presence of conditions such as type 2 diabetes when setting a target weight," Bruun said.

Advisory Board's weight management resources

For more resources on current treatments and approaches to weight management and prevention, Advisory Board offers the following:

• This expert insight outlines the five biggest questions about weight-management drugs and their answers.
• Similarly, this expert insight addresses what headlines get wrong about weight-management drugs and what healthcare leaders should know instead.
• Radio Advisory's Rachel Woods has also covered GLP-1 drugs on the podcast, discussing the potential future of these drugs and how they could help — or hurt — health systems' finances. 
• Other useful resources include this expert insight on the five catalysts that will impact the future of obesity care and this research on four key elements of comprehensive obesity care.
• Our weight management and obesity care resource library can also help leaders understand the current care landscape, manage innovations, and prepare for transformations in care.