Study Findings on BMI and Mortality
Recent research presented at the annual meeting of the European Association for the Study of Diabetes (EASD) in Vienna, Austria (September 15-19) suggests that individuals can be classified as “fat but fit.”
Research Overview
This study, which examined tens of thousands of participants in Denmark, found that individuals categorized as having a Body Mass Index (BMI) in the overweight range— and even some with obesity—were not statistically more likely to die over a five-year follow-up than those with a BMI between 22.5 to <25.0 kg/m2, which falls at the upper limit of the normal weight range.
Unexpected Mortality Outcomes
Interestingly, individuals with a BMI in the middle and lower segments of the normal weight range (18.5 to <22.5 kg/m2) exhibited a greater propensity for mortality, as did those categorized as underweight.
Expert Insights
“Both underweight and obesity represent significant global health challenges,” states Sigrid Bjerge Gribsholt from the Steno Diabetes Center Aarhus, Aarhus University Hospital, Denmark, who led the research. “Obesity can disrupt metabolic processes, compromise the immune system, and contribute to various diseases, including type 2 diabetes, cardiovascular diseases, and up to 15 different forms of cancer. Conversely, underweight status is often associated with malnutrition, weakened immunity, and nutrient deficiencies.”
Changing Perspectives on BMI
Dr. Gribsholt further elucidates that the consensus on the BMI range correlated with the lowest mortality has evolved. Initially estimated between 20 to 25, it appears to be shifting upwards, potentially due to advancements in medical treatments and improvements in overall health.
Data Analysis
The research involved health data from 85,761 individuals (81.4% female, median age at baseline 66.4 years). Out of these, 7,555 participants (8%) passed away during the follow-up period. The analysis indicated that individuals classified as underweight were nearly three times (2.73 times) more likely to die than those falling within the higher end of the healthy range (22.5 to <25.0 kg/m2).
Mortality Risks Associated with BMI
Similarly, individuals with a BMI of 40 kg/m2 or higher (designated as severely obese) had more than double the mortality risk (2.1 times) compared to the reference population. Notably, higher mortality rates were also observed among those classified within healthy BMI ranges.
Individuals with a BMI between 18.5 and <20.0 kg/m2 were twice as likely to die compared to those in the reference category, while those with a BMI from 20.0 to <22.5 kg/m2 had a 27% increased mortality risk.
Exploration of “Metabolically Healthy” Obesity
Conversely, individuals with a BMI in the overweight range (25 to <30 kg/m2) and those on the lower end of the obese spectrum (30.0 to <35.0 kg/m2) did not exhibit a higher mortality risk than the reference population—a phenomenon often termed “metabolically healthy” or “fat but fit.” However, those with a BMI between 35 to <40.0 kg/m2 did show a 23% increased risk of mortality.
Consideration of Variables in Mortality
All findings were adjusted for variables such as sex, comorbidity status, and education level. A similar trend was noted across various age groups, sexes, and educational backgrounds. The researchers expressed surprise at the lack of mortality association with BMI up to 35 kg/m2, as well as the minimal risk associated with a BMI from 35 to <40 kg/m2.
Potential Explanations for Findings
Dr. Gribsholt suggests that reverse causation might play a role; some individuals may lose weight due to underlying health conditions, causing a misleading correlation between higher BMI and a lower risk of mortality. Additionally, individuals with higher BMI who enjoy longer lifespans may possess particular traits that confer protective effects.
Beyond BMI: The Importance of Fat Distribution
Regardless of these findings, it is crucial to note that BMI is not the sole indicator of unhealthy fat levels. As Professor Bruun points out, fat distribution plays a significant role in metabolic health. For instance, visceral fat—located deep within the abdomen—can have adverse health effects compared to subcutaneous fat located in the hips and thighs. Therefore, it is essential to personalize obesity treatment strategies and consider factors like fat distribution and existing health conditions, such as type 2 diabetes, when determining ideal weight targets.
Key Health Takeaway
While BMI remains a useful metric for evaluating body weight status, emerging research indicates that individuals may maintain health despite being overweight or obese, emphasizing the need to consider additional factors such as fat distribution and overall metabolic health in evaluating risks associated with weight.
