BMI Doesn’t Tell the Whole Story After 65. Waist Size May Fill In the Gaps.

New research suggests that weight, waist circumference, strength and physical function should be considered together when evaluating your health.

For decades, body mass index has been the standard shorthand for determining whether someone’s weight falls into a “healthy” range. But after age 65, BMI alone may conceal as much as it reveals.

A new longitudinal study led by University of Rhode Island researchers found that BMI and waist circumference provide different—and complementary—information about an older adult’s mortality risk.

The researchers analyzed 14 years of data, collected from 2011 through 2024, from 6,905 adults ages 65 and older participating in the National Health and Aging Trends Study. Unlike research based on a single snapshot, the team examined repeated measurements of both BMI and waist circumference as participants aged.

The study was published in the Journal of the American Geriatrics Society in August 2026.

“What does body weight tell us about health?” said study co-author Bryan Blissmer, a University of Rhode Island professor of kinesiology. “Older adults can lose muscle and accumulate more fat around their middle without much change on the scale. That makes weight alone difficult to interpret.”

Two measurements, two kinds of information

BMI is calculated from a person’s weight and height. It is inexpensive and useful for identifying patterns across large groups, but it cannot distinguish muscle from fat or show where fat is stored.

Waist circumference offers another perspective because it provides information about abdominal fat. That fat can surround internal organs and has been associated with cardiovascular disease, diabetes and other health problems.

Waist size has limitations, too. It does not reveal how much muscle a person has, whether they are adequately nourished or how well they can perform everyday activities. That is why the researchers say the two measurements should be considered together.

When BMI was examined independently, older adults classified as overweight had a lower mortality rate than those in the normal-BMI category. This finding reflects the frequently debated “obesity paradox,” in which some older people with higher BMIs appear to survive longer.

But waist circumference revealed an important complication.

After accounting for BMI, a high waist circumference was associated with approximately 24% higher mortality among both men and women. People with a normal BMI but a large waist also had higher mortality than people whose BMI was normal and whose waist was smaller.

In other words, landing in the “normal” range on a BMI chart did not necessarily mean a person faced lower risk.

Being underweight was associated with substantially higher mortality as well. In older adults, low weight or unexplained weight loss may signal muscle loss, poor nutrition, illness or frailty.

Don’t try to gain—or rapidly lose—weight

The findings should not be interpreted as proof that excess body fat is protective or as encouragement to gain weight. The observational study identified associations; it did not establish that carrying extra weight causes people to live longer.

Instead, the takeaway is that the number on the bathroom scale is only one part of a much larger health picture.

For adults over 65, a more useful conversation with a healthcare professional may include waist circumference, recent weight changes, strength, mobility, nutrition, chronic conditions and the ability to handle daily activities. Unintentional weight loss deserves particular attention, not automatic congratulations.

The URI team plans additional research into how changes in BMI and waist circumference relate to frailty, physical function and healthy aging.

“Ultimately,” Blissmer said, “we want to understand both how long people live and how well they maintain their independence and quality of life.”