There’s a bit of a mystery surrounding obesity. Even though it’s a risk factor for a whole host of health problems, it also seems to offer some mortality benefits. For example, obese individuals diagnosed with kidney disease, pulmonary embolisms, cardiovascular disorders, and more have a reduced risk of dying. Now this so-called “obesity paradox” has popped up in a new study of Alzheimer’s patients, as well.
Public health researchers made the discovery after examining health data dating from 1998 to 2022 from more than 4,500 Alzheimer’s-free people 51 and older enrolled in the Health and Retirement Study. They found that those who were obese at the time of their Alzheimer’s diagnosis were also less likely to die. Overweight individuals (with a BMI from 25 to 29.9) had a 16 percent lower risk, those with class 1 obesity (BMI of 30 to 34.9) had a 27 percent lower risk, and those with class 2 and 3 obesity (BMI equal to or greater than 35) had a 22 percent lower risk.
Read more: “Why Do More Women Than Men Develop Alzheimer’s?”
It’s not the first time obesity has been found to be protective against dementia. A recent review found that those with obesity before the age of 65 had a 17 percent higher risk of developing the disease, but those with obesity at 65 and older showed a 9 percent lower risk.
But why?
The obesity paradox has puzzled researchers since it was discovered over a quarter of a century ago, and possible explanations include statistical artifacts. “Because this finding is somewhat counterintuitive, an important question is whether it could simply be an artifact of bias,” study author Yuan Zhang of Columbia University explained in a statement.
It’s possible the findings arise from simple survivor bias. Because obesity is a risk factor for other adverse health conditions, the thinking goes, it’s possible only the healthiest live long enough to develop the disease. But if that were true with Alzheimer’s, the apparent survival benefits of obesity would also increase with age, and the researchers found that they actually went down as the participants got older.
Another statistical artifact could arise from the nature of Alzheimer’s itself. People with the disease often begin shedding pounds a decade before they’re diagnosed, which could put formerly obese participants into a normal weight category by that time. So the team compared people who remained in the normal weight class with those who remained in the obese class and found the same benefits.
“Our analyses suggest that the major sources of bias we examined don’t explain the survival advantage associated with higher weight,” Zhang said.
So why does obesity confer this rather large mortality advantage?
The researchers still aren’t sure, but it could have a physiological explanation. Because difficulty swallowing, loss of appetite, and subsequent malnutrition are hallmarks of Alzheimer’s, having greater energy reserves could boost the survival rates.
“This doesn’t mean that patients with dementia should attempt to become overweight or obese, as our study wasn’t designed to test that question,” study author Virginia Chang of New York University cautioned. “But our findings do suggest that weight status may serve as a useful prognostic signal for survival at the time of dementia diagnosis.” 
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