Higher body weight is often linked to greater risk of chronic disease, and carrying excess weight in midlife has also been associated with a higher risk of developing dementia later on.
But once dementia develops, the relationship between weight and health may look very different.
In a new observational study of more than 4,500 U.S. adults who developed dementia, people who were overweight or had obesity around the time of diagnosis had a lower risk of death during follow-up than those in the normal BMI range.
The study does not show that extra body fat helps people live longer or that people with dementia should try to gain weight. Instead, the findings may reflect how closely weight, nutrition and overall health are connected as dementia progresses.
“Because this finding is somewhat counterintuitive, an important question is whether it could simply be an artifact of bias,” said corresponding author Yuan Zhang, an assistant professor of sociomedical sciences at Columbia University's Mailman School of Public Health. “Our analyses suggest that the major sources of bias we examined do not explain the survival advantage associated with higher weight.”
The study, published in the Journal of Alzheimer's Disease, used data from the Health and Retirement Study, a long-running study of older U.S. adults. Researchers followed 4,523 people who did not have dementia when they entered the study but later developed it.
Compared with people in the normal BMI range, those classified as overweight had a 16% lower risk of death. People with class 1 obesity had a 27% lower risk, while those with class 2 or 3 obesity had a 22% lower risk.
One possible explanation has less to do with excess weight being protective and more to do with what happens as dementia develops.
People with dementia often lose weight, sometimes beginning years before diagnosis. That means someone who falls into the normal-weight range at diagnosis may have already lost a significant amount of weight as their health declined.
In that situation, lower weight may be a sign of disease progression rather than better health.
The researchers specifically looked at whether weight loss before diagnosis or other statistical quirks could explain the results. Those additional analyses did not appear to account for the association, although the study still cannot prove that higher body weight itself improves survival.
“This doesn’t mean that patients with dementia should attempt to become overweight or obese, as our study was not designed to test that question,” said Virginia Chang, an associate professor of social and behavioral sciences at NYU School of Global Public Health and the study's first author. “But our findings do suggest that weight status may serve as a useful prognostic signal for survival at the time of dementia diagnosis. They also support the notion that nutrition is a key factor in the health and survival of people with dementia.”
Nutrition can become increasingly difficult as dementia progresses. People may lose their appetite, forget to eat, have difficulty feeding themselves or experience other changes that make it harder to get enough food.
“Nutrition can be a big problem for people with dementia,” Chang said. “They may experience wasting and an inability to eat, which can be due to changes in the brain’s central drive, leading to loss of appetite and initiative, increased energy expenditure, and difficulties feeding themselves.”
Having more energy reserves or muscle may help some people better withstand those changes, the researchers suggest, but this study could not determine why people at higher weights lived longer.
BMI also offers only a rough picture of health. It is based on height and weight and cannot distinguish between muscle and body fat or show how a person's weight has changed over time.
The findings help illustrate why weight can mean different things at different stages of life. Higher weight in midlife may still raise the risk of developing dementia and other chronic diseases. But after dementia develops, unintentional weight loss and declining nutrition can become serious concerns of their own.
The study does not identify an ideal weight for someone with dementia. It does suggest, however, that changes in weight and eating may provide important clues about a person's overall health as the disease progresses.
The research was supported by the National Institutes of Health.