For older adults, the number on the scale may not tell the whole story. A new study of nearly 7,000 people ages 65 and older found that a larger waist circumference was linked to a higher risk of early death, even after researchers accounted for BMI. At the same time, some older adults classified as overweight by BMI had a lower mortality risk than those in the normal BMI range. The findings highlight why body composition, muscle strength and abdominal fat may matter more with age than weight alone.
What the Study Found
Researchers followed nearly 7,000 adults ages 65 and older for up to 14 years, examining how BMI and waist circumference were associated with mortality. Among men, those classified as overweight, with a BMI between 25 and 29.9, had a 46% lower risk of death than men in the normal BMI range. Men with class I or II obesity also had a lower risk. Women classified as overweight had a 27% lower risk of death than women in the normal BMI range. Class I and II obesity were not associated with a statistically significant difference in mortality among women. For both men and women with class III obesity, defined as a BMI of 40 or higher, researchers found no statistically significant difference in mortality risk. The picture changed when researchers examined waist size. Men with a waist circumference above 40 inches and women above 35 inches had a 24% higher risk of early death than those below those thresholds. That association remained even after accounting for BMI.
Why BMI Can Be Misleading With Age
BMI is calculated using height and weight, making it useful for studying large populations. But it cannot distinguish between muscle and fat. Two people with the same height and weight can have very different body compositions. One may have more muscle and relatively little fat, while the other may have less muscle and more body fat. Age makes that distinction especially important. Older adults commonly lose muscle mass, may lose height and can experience changes in how and where their bodies store fat. That means an older person with a higher BMI may have more lean muscle than the number suggests. Someone with a normal BMI, meanwhile, could have lost substantial muscle while still carrying excess abdominal fat.
Why Belly Fat Matters
Fat stored deep inside the abdomen, known as visceral fat, surrounds internal organs and is associated with inflammation, insulin resistance and metabolic changes linked to cardiovascular disease and diabetes. Waist circumference offers a simple way to estimate abdominal fat without specialized equipment. Experts have advocated using waist circumference and other measures of body fat alongside BMI rather than relying on BMI alone. Other health indicators, including blood pressure, blood sugar, cholesterol and physical fitness, can provide additional context. To measure waist circumference, a tape should be placed around the abdomen between the bottom of the ribs and the top of the hip bones. It should remain level and snug without compressing the skin.
The “Obesity Paradox” Needs Caution
The study does not show that gaining body fat helps people live longer. Researchers caution that the findings demonstrate associations rather than proof that excess body fat protects against early death. Several factors could help explain the seemingly contradictory results. One possibility is reverse causation. Conditions such as cancer and heart disease can cause people to lose weight, making lower body weight appear more closely linked to mortality when an underlying illness is actually driving both. Survivorship bias may also play a role. Older adults who have lived with obesity may differ in important ways from people who developed obesity-related illnesses and died earlier. BMI itself is another limitation because it does not reveal how much of a person's weight comes from muscle or fat.
Why Being Underweight Can Be Concerning
The study found another striking result: men and women who were underweight had close to twice the risk of early death compared with people in the normal BMI range. Low weight can be associated with frailty, while loss of muscle can make everyday activities more difficult and increase the risk of falls. Unintentional weight loss can also be a warning sign of an underlying health problem. Chronic disease can cause both weight and muscle loss, meaning low weight may sometimes be a consequence of declining health rather than its cause.
Focus on Strength and Function, Not Just Weight
For older adults, maintaining physical function may be more useful than chasing a particular number on the scale. Regular physical activity can include aerobic exercise such as brisk walking, cycling or swimming, combined with strength training. Adequate nutrition and sufficient protein are also important for maintaining muscle. Changes over time can offer valuable clues. A growing waistline may indicate increasing abdominal fat even when overall weight has barely changed. Likewise, unexplained weight loss deserves attention and a conversation with a healthcare provider.
What Comes Next
The findings add to evidence that assessing health in older adults requires more than BMI alone. Waist circumference, body composition, muscle strength, physical function, blood pressure, blood sugar and cholesterol can provide a broader picture of health. For individuals, the right weight and health goals should be determined with a healthcare provider based on the full picture rather than the scale alone.
FAQ
Is belly fat a better predictor of longevity than BMI in older adults?
The study found that a larger waist circumference was associated with a higher risk of early death, even after researchers accounted for BMI. It suggests waist size may provide important health information that BMI misses.
What waist size was linked to higher mortality in the study?
Men with a waist circumference above 40 inches and women above 35 inches had a 24% higher risk of early death compared with people below those thresholds.
Does being overweight help older adults live longer?
No. The study found an association between certain BMI categories and lower mortality, but it did not show that gaining body fat increases longevity.
Why can BMI be misleading for older adults?
BMI uses only height and weight and cannot distinguish muscle from fat. Because older adults commonly lose muscle and experience changes in fat distribution, BMI may not accurately reflect their body composition.
What should older adults focus on besides BMI?
Older adults should consider waist circumference, muscle strength, physical activity, physical function, nutrition, blood pressure, blood sugar and cholesterol alongside weight and BMI.
