Being too thin can be deadlier than being overweight, Danish study reveals

Danes say "don't overdo it"

European Association for the Study of Diabetes

New research from Denmark challenges long-held assumptions about body weight and health, revealing that being overweight—or even moderately obese—does not necessarily increase the risk of death compared to those at the upper end of the "normal" BMI range. In fact, those who are underweight or at the lower end of the so-called healthy spectrum faced higher risks.

It is possible to be "fat but fit," new research being presented at the annual meeting of the European Association for the Study of Diabetes (EASD) in Vienna, Austria (September 15-19) suggests.

The study of tens of thousands of people in Denmark found that those with a BMI in the overweight category - and even some of those living with obesity - were no more likely to die during the five years of follow-up than those with a BMI of 22.5-<25.0 kg/m², which is at the top end of the normal weight range.

Individuals with a BMI in the middle and lower parts of the normal weight range 18.5 to <22.5kg/m², were also more likely to die. As were individuals with a BMI in the underweight range.

"Both underweight and obesity are major global health challenges," says Sigrid Bjerge Gribsholt, of the Steno Diabetes Center Aarhus, Aarhus University Hospital, Aarhus, Denmark, who led the research. "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.

"There are conflicting findings about the BMI range linked to 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."

To provide some clarity, Dr Gribsholt, Professor Jens Meldgaard Bruun, also of the Steno Diabetes Center Aarhus, and colleagues used health data to examine the relationship between BMI and mortality in 85,761 individuals (81.4% female, median age at baseline 66.4 years).

BMI is a measure of weight to height and a score of 18.5 to <25 kg/m² is generally considered to be of normal weight. A BMI of <18.5 kg/m² is categorized as underweight, 25 to <30 kg/m² is considered overweight and a BMI of 30 kg/m² is described as obesity.

7,555 (8%) of the participants died during follow-up. The analysis found that individuals in the underweight category were almost three times more likely (2.73 times) to have died than individuals with a BMI towards the top of the healthy range (22.5 to <25.0 kg/m², the reference population).

Similarly individuals with BMI of 40 kg/m² and above (categorized as severe obesity) were more than twice as likely (2.1 times) to have died compared with the reference population.

However, higher mortality rates were also found for BMIs that are considered healthy.

Individuals with a BMI of 18.5 to <20.0 kg/m², and so at the lower end of the healthy weight range, were twice as likely to have died as those in the reference population. Similarly, those with a 20.0 to <22.5 kg/m², and so in the middle of the healthy weight range, were 27% more likely to have died than the reference population.

By contrast, individuals with a BMI in the overweight range (25 to <30 kg/m²) and those with a BMI at the lower part of the obese range (30.0 to <35.0 kg/m²) were no more likely to have died than the those in the reference population - a phenomenon sometimes referred to as being metabolically healthy or "fat but fit."

Those with a BMI of 35 to <40.0 kg/m² did have an increased risk of death of 23%.

All of the results were adjusted for sex, comorbidity level and education level.

A similar pattern was obtained when the researchers looked at the relationship between BMI and obesity in participants of different ages, sexes and levels of education.

The researchers were surprised to find that BMI was not associated with a higher mortality up to a BMI of 35 kg/m² and that even a BMI 35 to <40 kg/m² was only associated with a slightly increased risk.

Dr Gribsholt says: "One possible reason for the results is reverse causation: 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.

"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.

"Still, in line with earlier research, we found that people who are in the underweight range face a much higher risk of death."

Whatever the explanation, BMI isn't the only indicator that an individual is carrying unhealthy levels of fat, says Professor Bruun.

He explains: "Other important factors include how the fat is distributed. 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, 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, while another individual with the same BMI may free of these problems because the excess fat is on their hips, buttocks 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."