Is low BMI harmful in old age?

Despite the success of drugs like Ozempic (semaglutide) in promoting both weight loss and longer life, doubt remains about the role of body mass index (BMI) at older ages. Many studies have shown that low BMI is associated with higher mortality in older people.

To address this contradiction, CUNY SPH Professor Mary Schooling and colleagues published a study in the Journal of Epidemiology and Community Health on the effect of BMI on mortality in older people by age group.

In keeping with many observational studies in older people, the researchers found that low BMI was consistently associated with higher mortality. They also found that low BMI was more common in the older age group. This presents a conundrum: low BMI is both lethal and increasingly common with older age.

“The answer is that people with high BMI are less likely to survive to be recruited into a study at older ages,” says Dr. Schooling. “Observational studies of the effect high BMI on mortality at older ages are inevitably missing all those with high BMI who died before they could be recruited into the study.

“A comparison of people with high and low BMI at older ages is between a minority of people able to survive high BMI into old age and a larger proportion of people with low BMI who survive into old age, which is not a fair comparison.”

Such a biased comparison makes high BMI look consistently protective in old age, when in fact high BMI is harmful, she says.

The researchers say observational studies of associations in survivors should be taken with a grain of salt.

“The observed associations should be consistent with how the sample characteristics change with age,” says Schooling.

Despite the success of drugs like Ozempic (semaglutide) in promoting both weight loss and longer life, doubt remains about the role of body mass index (BMI) at older ages. Many studies have shown that low BMI is associated with higher mortality in older people.

To address this contradiction, CUNY SPH Professor Mary Schooling and colleagues published a study in the Journal of Epidemiology and Community Health on the effect of BMI on mortality in older people by age group.

In keeping with many observational studies in older people, the researchers found that low BMI was consistently associated with higher mortality. They also found that low BMI was more common in the older age group. This presents a conundrum: low BMI is both lethal and increasingly common with older age.

“The answer is that people with high BMI are less likely to survive to be recruited into a study at older ages,” says Dr. Schooling. “Observational studies of the effect high BMI on mortality at older ages are inevitably missing all those with high BMI who died before they could be recruited into the study.

“A comparison of people with high and low BMI at older ages is between a minority of people able to survive high BMI into old age and a larger proportion of people with low BMI who survive into old age, which is not a fair comparison.”

Such a biased comparison makes high BMI look consistently protective in old age, when in fact high BMI is harmful, she says.

The researchers say observational studies of associations in survivors should be taken with a grain of salt.

“The observed associations should be consistent with how the sample characteristics change with age,” says Schooling.