New research presented at the European Association for the Study of Diabetes (EASD) annual meeting in Vienna, Austria, suggests that it is possible to be “fat but fit.” The Danish study, which examined tens of thousands of individuals, found that those with a Body Mass Index (BMI) in the overweight category—and even some classified as obese—were not more likely to die within a five-year follow-up period compared to those with a BMI of 22.5-<25.0 kg/m2, the upper end of the normal weight range.
Conversely, individuals with a BMI in the middle and lower parts of the normal weight range (18.5 to <22.5 kg/m2) were found to have a higher mortality risk, as were those categorized as underweight. This study challenges the conventional wisdom that associates higher BMI with increased mortality.
Exploring the Complex Relationship Between BMI and Mortality
Dr. Sigrid Bjerge Gribsholt, leading the research at the Steno Diabetes Center Aarhus, Aarhus University Hospital, emphasized the dual challenges of underweight and obesity as global health issues. “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,” she stated.
The study, conducted by Dr. Gribsholt, Professor Jens Meldgaard Bruun, and colleagues, analyzed health data from 85,761 individuals (81.4% female, median age 66.4 years) to examine the relationship between BMI and mortality. The findings revealed that 7,555 participants (8%) died during the follow-up period. Notably, individuals in the underweight category were nearly three times more likely to die compared to those with a BMI at the top of the healthy range (22.5 to <25.0 kg/m2).
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.
Rethinking Healthy Weight Ranges
Interestingly, the study found higher mortality rates even among BMIs considered healthy. Those with a BMI of 18.5 to <20.0 kg/m2, at the lower end of the healthy weight range, were twice as likely to have died as those in the reference population. Similarly, individuals with a BMI of 20.0 to <22.5 kg/m2 were 27% more likely to die than the reference group.
On the contrary, participants with a BMI in the overweight range (25 to <30 kg/m2) and those in the lower part of the obese range (30.0 to <35.0 kg/m2) showed no increased mortality risk compared to the reference population, a phenomenon sometimes described as being metabolically healthy or “fat but fit.”
However, those with a BMI of 35 to <40.0 kg/m2 did face a 23% increased risk of death. The study’s results were adjusted for factors such as sex, comorbidity level, and education level, and similar patterns emerged across different ages, sexes, and education levels.
Understanding the Implications
The researchers were surprised to discover that BMI was not linked to higher mortality up to a BMI of 35 kg/m2, and even a BMI of 35 to <40 kg/m2 was only slightly associated with increased risk. Dr. Gribsholt suggested reverse causation as a potential explanation: “Some people may lose weight due to 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.”
Moreover, the study’s data, derived from individuals undergoing health scans, could not entirely rule out this possibility. Dr. Gribsholt also noted that people with higher BMI who live longer might possess certain protective traits influencing the results.
Professor Bruun emphasized that BMI isn’t the sole indicator of unhealthy fat levels. “Other important factors include how the fat is distributed. Visceral fat—metabolically active fat stored deep within the abdomen—secretes compounds that adversely affect metabolic health,” he explained.
“An individual with a BMI of 35 and an apple-shaped body may have type 2 diabetes or high blood pressure, while another with the same BMI may be free of these problems due to different fat distribution,” said Professor Bruun.
He concluded that obesity treatment should be personalized, considering factors like fat distribution and the presence of conditions such as type 2 diabetes when setting target weights.
This research underscores the complexity of the relationship between BMI and mortality, suggesting that a one-size-fits-all approach to weight management may not be effective. As the medical community continues to explore the nuances of metabolic health, the findings from this study could pave the way for more individualized and effective health strategies.
