'Obesity paradox': Study finds common health metric could be misleading for older people

A person's Body Mass Index (BMI) may not tell the full story about their risk of dying in older age, with new research suggesting waist circumference can identify risks that a normal BMI misses.

The study of 6,905 adults in the United States aged 65 and over found that being overweight was associated with a lower mortality hazard than having a normal BMI, particularly among men.

But a larger waist circumference was associated with a higher mortality hazard in both men and women.

The researchers say the findings do not mean obesity is protective. Instead, they highlight the limitations of using BMI as the sole indicator of a person's health.

BMI, or body mass index, is a measure calculated from a person's height and weight. It is commonly used to classify people as underweight, normal weight, overweight or obese, but it does not distinguish between muscle and fat.

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"BMI and waist circumference provide complementary information about health risk in older adults," co-author Dr Bryan Blissmer from the University of Rhode Island said.

"Our findings suggest that considering both measures together, rather than relying on BMI alone, may improve mortality risk assessment and support more informed clinical decision-making."

In the study, a BMI below 18.5 was considered underweight, 18.5 to 24.9 was considered normal weight, 25 to 29.9 was considered overweight, and 30 or above was considered obese.

Obesity was divided into three classes: Class I, BMI 30 to 34.9; Class II, 35 to 39.9; and Class III, 40 or above.

However, for their analysis, the researchers combined Class I and Class II into a single category.

The researchers analysed 14 years of data from the US National Health and Aging Trends Study, following 6,905 people from 2011 to 2024.

They used a statistical measure called a hazard ratio to compare the likelihood of death occurring over the follow-up period between different groups.

A hazard ratio of 1 indicates no difference from the comparison group. A value below 1 indicates a lower hazard of death, while a value above 1 indicates a higher hazard.