Research using data from a large U.S. health survey has disrupted conventional thinking about body mass index (BMI) and its role in predicting all-cause mortality. The research suggests that BMI alone does not necessarily have a strong relationship with a person’s risk of premature death.
The researchers based their BMI calculations on data from the 1999-2018 National Health Interview Survey. All-cause mortality data was based on the U.S. National Death Index. The authors note that similar studies in the past had relied on data from the 1970s, focusing primarily on non-Hispanic white adults. Changes in the U.S. population since then in BMI distribution, life expectancy, ethnic and racial diversity, and healthcare make the new research relevant to current discussions of weight and health.
BMI and Mortality: A Complex Relationship
Traditionally, BMI has been considered a key indicator of health, but the study, published in PLOS One, reveals a more nuanced picture. The findings show that for people with a BMI of 25.0 to 29.9 (in the overweight category), this measure alone may not necessarily indicate an increased mortality risk independent of other risk factors. However, mortality risk increased by 21% to 108% for adults with a BMI of 30 or greater, indicating obesity. Among older adults (age 65 and older), the researchers reported no significant rise in mortality at BMI levels between 22.5 and 34.9. In younger adults (age 20 to 64), there was no increased mortality risk at BMIs of 22.5 to 27.4.
The authors suggest that, for older adults, higher muscle mass could be reflected in the higher BMIs, which would explain why mortality risk did not increase for those with BMIs up to 35. “People with higher BMIs may be paradoxically healthier because of sustained muscle mass and bone density,” said study author Aayush Visaria, a postdoctoral research fellow at Rutgers Center for Pharmacoepidemiology and Treatment Science in New Jersey.
Limitations of BMI as a Health Measure
Although BMI is easy to calculate and widely used, it overlooks important factors such as body composition, fat distribution, and metabolic health. This can lead to misinterpretations. For instance, athletes often have high BMIs because they have increased muscle mass. Fat distribution around the waist, meanwhile, is known to increase the risk of disease but is not reflected in a person’s BMI.
“[BMI] does not distinguish between muscle mass and fat mass, and some individuals like bodybuilders may have a high BMI because of more muscle mass,” said Dagfinn Aune, a research associate at the School of Public Health at Imperial College London, who was not involved in the study.
The authors also point out that people in the overweight category may be better able to recover from critical illness or severe infection than leaner individuals. Furthermore, among people who develop hypertension or diabetes, those with lower BMIs may have more aggressive or treatment-resistant disease, while people with higher BMIs may be able to combat disease with weight loss.
Exploring Alternative Predictors
To better predict all-cause mortality, the researchers recommend supplementing BMI with other measures like weight history, waist circumference, and waist-to-hip ratio. By incorporating multiple measures and evaluating cardiometabolic health parameters such as blood pressure and cholesterol levels, physicians can better identify people at risk and tailor interventions accordingly.
“I think the real message of this study is that overweight as defined by BMI is not an appropriate measurement tool, at least for all-cause mortality,” said Visaria. “Overweight as a medical condition is still important but will likely need to be diagnosed taking into account body composition and body fat distribution,” he added.
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