It is possible to be “fat but fit,” new research presented at the annual meeting of the European Association for the Study of Diabetes (EASD) held in Vienna, Austria (15–19 September) 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, 18.5–22.5kg/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 [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, 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 18.5–25 kg/m2 is generally considered to be of normal weight. A BMI of <18.5 kg/m2 is categorized as underweight, 25–30 kg/m2 is considered overweight and a BMI of 30 kg/m2 is described as obesity.
A total of 7,555 (8%) of the participants died during follow-up. The study revealed that individuals classified as underweight were nearly three times more likely (2.73 times) to have died compared to individuals with a BMI in the upper range of healthy (22.5–25.0 kg/m2, the reference group).
Similarly, individuals with a BMI of 40 kg/m2 or higher (classified as severely obese) were over twice as likely (2.1 times) to have died compared to the reference group.
However, increased mortality rates were also observed in individuals with BMIs within the healthy range. Those with a BMI of 18.5–20.0 kg/m2, at the lower end of healthy weight, were twice as likely to have died compared to the reference group. Similarly, individuals with a BMI of 20.0–22.5 kg/m2, in the middle of the healthy weight range, were 27% more likely to have died than the reference population.
Conversely, individuals with BMIs in the overweight range (25–30 kg/m2) and those at the lower end of the obese range (30.0–35.0 kg/m2) did not show an increased risk of death compared to the reference group, a concept known as being metabolically healthy or “fat but fit.”
Individuals with a BMI of 35–40.0 kg/m2 did show a 23% increased risk of death. These findings were adjusted for sex, comorbidity level, and education level.
Similar trends were observed when analyzing the relationship between BMI and mortality across different age groups, genders, and education levels.
Researchers were surprised to find that BMI was not significantly associated with higher mortality rates up to a BMI of 35 kg/m2, with only a slightly increased risk observed for BMIs of 35–40 kg/m2.
Dr. Gribsholt suggested that reverse causation, where weight loss is a result of underlying illness, could explain the results. He also noted that individuals with higher BMIs who live longer may have certain protective traits influencing the outcomes.
Regardless, the study confirms that individuals in the underweight category face a significantly higher risk of death. However, BMI alone may not be the sole indicator of unhealthy fat levels, as distribution of fat also plays a crucial role, according to Professor Bruun.
