Fat but fit? Study challenges understanding of weight and mortality risk

Obesity researchers are suggesting it is possible to be “fat but fit,” based on a study of tens of thousands of Danish participants. The study found that overweight participants — and some with obesity — did not have a higher mortality likelihood during the five years of follow-up compared to those at the top end of the normal weight range. 

Additionally, the paper reveals that individuals with a body mass index (BMI) in the middle and lower parts of the normal weight range, 18.5 to <22.5 kg/m2, were more likely to die in the follow-up period. The same was said for underweight individuals.

“Both underweight and obesity are major global health challenges,” says lead researcher Sigrid Gribsholt, of the Steno Diabetes Center Aarhus, Aarhus University Hospital, Denmark.

“Obesity may disrupt the body’s metabolism, weaken the immune system, and lead to diseases like type 2 diabetes, cardiovascular diseases, and up to 15 different cancers, while underweight is tied to malnutrition, weakened immunity, and nutrient deficiencies.”

Gribsholt highlights that there are conflicting findings about which BMI range is linked to the lowest mortality. “It was once thought to be 20 to 25, but it may be shifting upward over time owing to medical advances and improvements in general health.”

A BMI score of 18.5 to <25 kg/m2 is typically considered to be normal weight. A BMI of <18.5 kg/m2 is classified as underweight, 25 to <30 kg/m2 is considered overweight, and a BMI of 30 kg/m2 is defined as obesity.

BMI and mortality

The research team examined health data to study the relationship between BMI and mortality in 85,761 individuals — 81.4% were female, with a median age at baseline of 66.4 years.

The analysis found 7,555 (8%) of the participants died during follow-up. Underweight individuals were almost three times more likely to have died than those in the reference population who were at the top end of the healthy weight range (BMI between 22.5 and <25.0 kg/m2).

Similarly, individuals with a BMI of 40 kg/m2 and above (severe obesity) were more than twice as likely to have died compared to the reference population.

However, the researchers observed higher mortality rates among BMIs that are considered “healthy.”

People at the lower end of the healthy weight range (BMI of 18.5 to <20.0 kg/m2) were twice as likely to have died as those in the reference population.

Meanwhile, those in the middle of the healthy weight range (BMI of 20 to <22.5 kg/m2) were 27% more likely to have died than the reference population.

In contrast, overweight individuals and those at the lower part of the obesity BMI range (30 to <35.0 kg/m2) were “no more likely” to have died than those in the reference population — the phenomenon sometimes referred to as being metabolically healthy, or “fat but fit.”

Reverse causation

All of the results were adjusted for sex, comorbidity level, and education level. Overall, those with a BMI of 35 to <40 kg/m2 had an increased risk of death of 23%.

The researchers noticed a similar pattern when studying the relationship between BMI and obesity in participants of different ages, sexes, and levels of education.

Surprisingly, the team found that a BMI up to 35 kg/m2 was not linked to a higher mortality.

Even a BMI of 35 to <40 kg/m2 was only associated with “a slightly increased risk.”

Gribsholt assumes that one possible reason for the results is reverse causation, meaning that some people may lose weight because of an underlying illness. “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.”

“Since our data came from people who were having scans for health reasons, we cannot completely rule this out.”

She suggests it is also possible that people with higher BMI who live longer may have certain protective traits that influence the results. “Still, in line with earlier research, we found that people who are in the underweight range face a much higher risk of death.”

Regardless of the explanation, Bruun underscores that BMI isn’t the only indicator that an individual is carrying unhealthy levels of fat.

“Other important factors include how the fat is distributed,” he explains. “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.”

As a result, he posits that an individual with a BMI of 35 who is apple-shaped — having excess fat around their abdomen — may have type 2 diabetes or high blood pressure. Another individual with the same BMI might be free of these problems because the excess fat is on their hips, buttocks, and thighs.

Generally, excessive amounts of visceral fat are linked to faster aging in the heart and blood vessels. However, previous research suggests that this hidden fat surrounding organs may potentially slow aging in women’s hearts when formed around the 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 concludes.

The researchers are presenting their new findings at the ongoing annual meeting of the European Association for the Study of Diabetes in Vienna, Austria (September 15–19).