Study: Being underweight increases mortality risk more than being overweight (but there's a 'but')

It's often thought that being overweight poses the greatest health risk, but that turns out not to be the case. A new Danish study shows that thin people have a higher risk of death than those who are a little overweight.

For the Aarhus University Hospital study, the BMI of 85,000 people was examined. The group was reexamined five years later.

The study initially seemed unsurprising: people who were underweight or extremely overweight were more likely to die than those with a healthy BMI. However, underweight people had a significantly higher mortality risk than those who were a little overweight . Underweight people were almost three times more likely to die compared to people with a healthy BMI (between 22.5 and 25). Severe obesity (BMIs over 40) had a 2.1-fold higher risk.

After five years, the participants who were overweight (a BMI of between 25 and 30) and even some who were obese (a BMI between 30 and 35) did not appear to have died more often than people with a healthy BMI between 22.5 and 25. Strikingly enough, people with a low-normal BMI (18.5 to 22.5) and people who were underweight (a BMI lower than 18.5) had a higher risk of mortality.

The researchers were surprised by the outcome. "One possible explanation for the results is reverse causality: some people lose weight due to an underlying disease. In those cases, it's the disease, not the low weight itself, that increases the risk of death, which could give the impression that a higher BMI is protective," explains lead researcher Sigrid Bjerge Gribsholt. "Because our data came from people who didn't undergo scans for health reasons, we can't completely rule this out."

Gribsholt also doesn't rule out that people with a higher BMI who live longer have certain protective characteristics that influence the results. However, she says it's clear that underweight people have a much higher risk of death.