According to a large study of older Americans, higher body weight at the time of a dementia diagnosis may be linked to longer survival, according . Researchers found that people who were overweight or obese when diagnosed had a significantly lower risk of death than those in the normal weight range, even after accounting for other health conditions and lifestyle factors. The findings, published in the Journal of Alzheimer’s Disease, contrast with evidence that obesity during midlife increases the likelihood of developing dementia. Although the researchers could not determine why higher weight was associated with better survival, their analysis suggests that the relationship cannot easily be explained by several common sources of statistical bias.
The survival differences varied by weight category. People who were overweight (body mass index of 25 to 29.9) had a 16 percent lower risk of death than those at a normal weight. Those with class 1 obesity (BMI of 30 to 34.9) had a 27 percent lower risk, while individuals with class 2 or 3 obesity (BMI equal to or greater than 35) had a 22 percent lower risk. To investigate the relationship, researchers analysed 4,523 participants in the Health and Retirement Study, a nationally representative study of U.S. adults ages 51 and older conducted by the University of Michigan. All participants were free of dementia when they entered the study but developed the condition later. Data collected between 1998 and 2022 allowed the team to examine weight changes before diagnosis and track survival afterward.
One complication in dementia research is that weight loss often begins long before the condition is formally identified. Earlier research by Virginia Chang and Yuan Zhang found that weight loss can begin up to a decade before diagnosis and accelerate in the years immediately preceding it. As a result, some people classified as normal weight when diagnosed may have previously been heavier and experienced substantial weight loss as their disease progressed. Other potential biases could also distort the relationship. Studying only people who develop dementia can introduce misleading statistical associations between weight and mortality. Survivor bias is another concern. Obesity increases the risk of dying from conditions such as cardiovascular disease; people with obesity who survive long enough to develop dementia may be healthier than others who were initially in the same weight category.
The researchers conducted additional analyses to examine these possible distortions but found little evidence that the major biases they evaluated accounted for the observed survival differences. “Because this finding is somewhat counterintuitive, an important question is whether it could simply be an artifact of bias. Our analyses suggest that the major sources of bias we examined do not explain the survival advantage associated with higher weight,” Yuan Zhang, an assistant professor of Sociomedical Sciences at Columbia University’s Mailman School of Public Health, said. One possible biological explanation involves the body’s ability to withstand nutritional decline. Dementia can disrupt brain processes that regulate hunger and eating behavior, while also making it increasingly difficult for people to prepare food or feed themselves.
Some individuals experience higher energy expenditure, reduced appetite, and progressive loss of muscle and body tissue. The scientists observed that people with greater fat stores or muscle mass may have more reserves to draw upon when eating becomes difficult. These reserves could potentially help them tolerate periods of inadequate nutrition, although the study did not establish that this mechanism explains the survival advantage. “Nutrition can be a big problem for people with dementia. “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,” Chang said.
