Overweight and obesity do not always increase the risk of an early mortality

 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, suggesting that it is possible to be "fat but fit," according to a study presented at the annual meeting of the European Association for the Study of Diabetes (EASD) held in Vienna, Austria.

"Both underweight and obesity are major global health challenges," explained Dr 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 [being] 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, 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).

A total of 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 (22.5–25.0 kg/m2, the reference population). Similarly, individuals with a 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 18.5–20.0 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 BMI of 20.0–22.5 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 (25–30 kg/m2) and those with a BMI at the lower part of the obese range (30.0–35.0 kg/m2) were no more likely to have died than those in the reference population—a phenomenon sometimes referred to as being metabolically healthy or "fat but fit." Those with a BMI of 35–40.0 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 of 35–40 kg/m2 was only associated with a slightly increased risk.

"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,” added Gribsholt. "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 is not the only indicator that an individual is carrying unhealthy levels of fat, added Bruun: "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 be 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."