Body mass index (BMI) has long been used as a quick measure of health. But new research from Denmark suggests it doesn’t tell the whole story.
Researchers studied more than 85,000 people and found that those in the overweight range, and even some with mild obesity, were no more likely to die during five years of follow-up than people at the top end of the “normal” BMI range. By contrast, people who were underweight or at the leanest end of the normal range faced higher risks of death. Severe obesity also carried a clear increase in risk.
“Both underweight and obesity are major global health challenges,” said lead researcher Sigrid Bjerge Gribsholt of the Steno Diabetes Center Aarhus.
She noted that while obesity can disrupt metabolism and lead to conditions like type 2 diabetes, being underweight is often tied to malnutrition and weakened immunity.
The team notes that illness may also play a role. Some people appear leaner because of underlying disease, which can drive up mortality risk.
“Since our data came from people who were having scans for health reasons, we cannot completely rule this out,” Gribsholt said.
Another factor is how fat is distributed in the body. Co-author Jens Meldgaard Bruun pointed out that “visceral fat — fat that is very metabolically active and stored deep within the abdomen, wrapped around the organs — secretes compounds that adversely affect metabolic health.”
Two people with the same BMI may have very different risks depending on whether fat is carried around the waist or stored in hips and thighs.
“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 added.
The findings underscore that BMI is a blunt tool: while very low and very high numbers do signal higher risks, being slightly above the “normal” range may not be harmful when other markers of health are in good shape.
This research was presented at the European Association for the Study of Diabetes (EASD) annual meeting. Sigrid Bjerge Gribsholt received a lecture fee and conference support from Novo Nordisk Denmark AS.