In most instances, people who are obese have higher rates of chronic disease, including heart disease, type 2 diabetes, some types of cancer, as well as shorter life spans than people with lower body mass index (BMI). But in an exception that has puzzled public health researchers, older adults with obesity have been found to have a lower risk of dementia than non-obese older people.

It’s a phenomenon called the “obesity paradox,” and a new study from the University of Minnesota School of Public Health (SPH) aims to address this mystery. To better understand the relationship between body fat and dementia risk, SPH researchers analyzed data on health information from over 5,000 people (average age of 75 years old) over a period of eight years. By controlling for several contributing factors—including lifestyle, demographics, and previous medical history— the researchers were able identify patterns between weight status and increased dementia risk.

The study, published in Neurology, found:

• People who experienced weight loss at older ages are more likely to subsequently develop dementia. Across all BMI categories, elevated dementia risk was observed only in people who had lost weight from midlife to late life.
• No evidence that individuals with obesity were less likely to develop dementia. In a tempering of the “obesity paradox,” risk of dementia did not vary across BMI categories for individuals who had maintained or gained weight.

“Maintaining a healthy body weight and avoiding obesity has traditionally been a core principle of public health messaging,” says Ethan Cannon, SPH doctoral student and lead author. “We wanted to explore the ‘obesity paradox’ by disentangling the effects of low BMI and weight loss on dementia risk. Our results do not suggest that obesity is protective against dementia, but instead highlight the potential of weight loss as a predictor of dementia risk.”

Future research on this topic may differentiate the associations of intentional and unintentional weight loss, and explore the role of weight loss in late-life on risk of other aging-related conditions.