A new study is complicating the long-standing use of body mass index as a measure of health in older adults, finding that waist circumference may offer a more reliable signal of early death risk than the widely used BMI formula.
The research, published Aug. 19 in the Journal of the American Geriatrics Society, found that among adults 65 and older, having a larger waist circumference was linked to a higher risk of early death. But being classified as overweight by BMI was, counterintuitively, associated with a lower risk of early death in the same population — a seemingly contradictory finding that has prompted closer examination from medical experts.
What the study found
Researchers analyzed data from nearly 7,000 adults age 65 and older who were tracked as part of a large, nationally representative study funded by the U.S. National Institutes of Health. Participants were followed for up to 14 years, allowing scientists to examine how changes in BMI and waist circumference correlated with mortality risk over time.
Compared with men whose BMI fell in the normal range, men classified as overweight, with a BMI between 25 and 29.9, had a 46% lower risk of death. Men with class I or II obesity, defined as a BMI between 30 and just under 40, also showed a lower risk of death relative to the normal-BMI group.
Among women, those classified as overweight had a 27% lower risk of death compared with those in the normal BMI range. However, class I or II obesity was not associated with a statistically significant difference in mortality risk for women, and for both men and women with class III obesity, defined as a BMI of 40 or higher, there was no statistically significant difference in mortality risk compared with the normal-BMI group.
The pattern shifted considerably when researchers instead examined waist circumference. Men with a waist circumference greater than 40 inches and women with one greater than 35 inches had a 24% higher risk of early death compared with those below those thresholds — and that elevated risk held up even after researchers statistically accounted for participants’ BMI.
Making sense of the contradiction
The findings touch on what researchers sometimes call the “obesity paradox“: the puzzling pattern in which obesity is strongly linked to increased risk of conditions such as heart disease and type 2 diabetes, yet appears associated with lower mortality risk in some older populations.
Dr. Leana Wen, CNN’s wellness expert, an emergency physician and clinical associate professor at George Washington University who previously served as Baltimore’s health commissioner, cautioned against reading the results as evidence that gaining weight is protective in older age. The study’s authors explicitly caution against interpreting the findings as evidence that excess body fat is biologically beneficial, Wen said.
Wen pointed to several possible explanations. One is reverse causation: serious illnesses such as cancer and heart disease can cause people to lose weight, which could make lower body weight appear linked to higher mortality risk when an underlying illness is actually driving both outcomes. A related factor is survivorship bias — people who reach older age despite having obesity may differ in meaningful ways from those who developed obesity-related disease and died earlier in life. Wen also noted that BMI itself may simply be an imperfect measure of body composition.
Why BMI can be misleading with age
BMI is calculated using only height and weight, without accounting for what actually makes up that weight. Two people of identical height and weight could have very different body compositions — one with substantial muscle mass and relatively little fat, the other with less muscle and more fat — yet register an identical BMI.
That limitation becomes more pronounced with age, according to Wen. Older adults tend to lose muscle mass over time, and can also lose height and experience shifts in where fat is stored on their bodies. As a result, a higher BMI in an older adult could actually reflect greater preservation of lean muscle mass, while someone with a BMI in the “normal” range might have lost substantial muscle while retaining excess body fat — a combination that can leave a person more vulnerable despite an outwardly reassuring number.
Why waist size matters
Fat that accumulates around the waist and deep within the abdomen, often referred to as visceral fat, is associated with inflammation, insulin resistance and other metabolic changes that raise the risk of cardiovascular disease and diabetes. Waist circumference offers a simple way to estimate that visceral fat, and Wen said many experts now advocate using waist circumference, waist-to-hip ratio, or direct body fat measurements alongside BMI when assessing whether someone has overweight or obesity.
Measuring waist circumference requires no specialized equipment, allowing people to track it at home using a simple tape measure placed around the abdomen, between the bottom of the ribs and the top of the hip bones, kept level and snug without compressing the skin.
The risks of being underweight
The study also identified a particularly elevated mortality risk among older adults who were underweight, with men and women in that category facing nearly double the risk of early death compared with those in the normal BMI range. Wen said this finding aligns with known risks tied to frailty, since older adults depend on muscle strength to perform daily activities, maintain mobility and reduce their risk of falls. Someone losing weight due to muscle loss could become more vulnerable even as their BMI moves toward a range conventionally considered healthier. Still, Wen cautioned that being underweight may often be a consequence of poor health rather than its cause, since chronic disease can itself drive unintentional weight and muscle loss.
What older adults should focus on instead
Rather than fixating on a specific weight or BMI target, Wen recommended that older adults prioritize overall health and physical function — including the ability to perform everyday tasks independently, maintain muscle strength, and stay physically active through a mix of aerobic exercise and strength training, along with adequate protein intake.
She also advised paying attention to changes over time rather than single measurements: an expanding waistline can signal rising abdominal fat even when overall weight and BMI remain largely unchanged, while unexplained weight loss should prompt a conversation with a healthcare provider. Wen recommended that older adults discuss with their doctors what a healthy weight looks like for them individually, incorporating not just BMI but body composition, fat distribution, physical function, and metrics such as blood pressure, blood sugar and cholesterol.
This story touches on health and mortality-related research; readers experiencing concerns about their own weight, health or mental wellbeing are encouraged to speak with a licensed healthcare provider.
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