A new study suggests that body weight alone may not tell the full story about health in older adults.
Researchers found that older people with a higher body mass index, or BMI, sometimes had a lower risk of death, while carrying more fat around the waist was linked with a higher risk.
The findings may sound surprising because excess weight is usually linked with health problems such as type 2 diabetes, heart disease and high blood pressure. However, the relationship between weight and health can change as people get older.
Researchers analyzed information from 6,905 adults in the United States who were age 65 or older. The data were nationally representative, and participants were followed between 2011 and 2024.
The study was published in the Journal of the American Geriatrics Society. It was conducted by researchers including Bryan Blissmer, Ph.D., of the University of Rhode Island.
BMI is a simple number calculated from a person’s height and weight. Doctors have used it for many years to place adults into categories such as underweight, normal weight, overweight and obesity.
But BMI has an important weakness. It cannot tell the difference between fat and muscle, and it does not show where fat is stored in the body.
This matters particularly in later life. People often lose muscle as they age, and two older adults with the same BMI may have very different amounts of muscle and body fat.
Waist circumference provides another piece of information. A larger waist can be a sign that a person has more fat around the abdomen and internal organs.
This type of deep abdominal fat is often considered more harmful than fat stored in some other parts of the body. It is linked with inflammation, problems controlling blood sugar and a higher risk of heart and blood vessel disease.
In the new analysis, researchers examined BMI and waist size separately and then looked at the two measures together. They wanted to see how each measure was related to the risk of dying during the study period.
Among men, being overweight was associated with a 46% lower risk of death compared with being in the normal BMI range. Men with class I or class II obesity, meaning a BMI from 30 to below 40, had a 51% lower risk.
Women showed a broadly similar pattern. These findings add to earlier research suggesting that a moderately higher BMI in older age does not always carry the same meaning that it does in younger adults.
Waist size told a different story. Among men, having a high waist circumference was associated with a 24% higher risk of death after researchers accounted for BMI.
Similar patterns were seen in women. This suggests that knowing where body fat is stored may provide useful information that a person’s weight and height cannot provide on their own.
When the researchers considered BMI and waist size at the same time, overweight men continued to have a lower risk of death regardless of their waist measurement. Some men with class I or II obesity also had a lower risk, particularly when their waist circumference was elevated.
At the other end of the scale, being underweight was associated with a higher risk of death in both men and women. Having a normal BMI but a large waist was also linked with higher risk.
One possible explanation for the underweight finding is that low body weight in older age can sometimes reflect loss of muscle, poor nutrition, frailty or an existing illness. These problems can make older adults less able to recover from infections, injuries and other health stresses.
The study does not mean that gaining weight will help older people live longer. Because this was an observational study, it can show links between body measurements and death risk, but it cannot prove that a particular BMI directly caused people to live longer.
There are other reasons to be cautious. BMI remains useful for population studies, and obesity can still increase the risk of diabetes, joint problems, sleep apnea and cardiovascular disease even when an association with lower overall mortality appears in a particular group.
The strongest message from the research is that BMI should not necessarily be used alone when assessing older adults. Waist size, muscle mass, physical fitness, medical conditions, nutrition and changes in weight over time can all provide important clues about health.
Study co-author Bryan Blissmer said BMI and waist circumference offer complementary information. Looking at both measures may help doctors make a more informed assessment of health risks in people over 65.
Overall, the study challenges the idea that a single number on the scale can define healthy aging. Its findings are useful because they come from a large national sample followed for many years, but they should not be interpreted as proof that obesity is protective.
A more practical conclusion is that body composition and fat location become especially important in later life. Future research should examine whether adding measures of muscle strength and physical function can further improve predictions of health and survival in older adults.
Source: University of Rhode Island.
