The skinnier you are in middle age, the more likely you are to get dementia in your old age, according to British researchers who sound baffled by their own findings. In the largest study of its kind, the researchers looked at up to 20 years of medical records from almost 2 million people, average age 55, and found that people classed as underweight or on the low side of normal were 34% more likely to develop dementia than people considered to be at a normal weight. Contradictory to earlier—but much smaller—studies, the researchers found that the risk continued to decrease as weight increased, with overweight people 18% less likely to develop dementia, and obese people 29% less likely, the BBC reports. "The reasons for and public health consequences of these findings need further investigation," the researchers write in the Lancet.
"We haven't been able to find an explanation," the lead researcher tells the Guardian. "We are left with this finding which overshadows all the previous studies put together. The question is whether there is another explanation for it. In epidemiology, you are always left with the question of whether there is another factor." He stresses that the study should not be used as an excuse for overeating, because even if obesity does have a protective effect, "you may not live long enough to benefit." In a statement, an Alzheimer's Society spokesman says that while the study shows the need for more research, people are still encouraged to keep their brains healthy by not smoking, exercising regularly, and eating a healthy, balanced diet. (Other researchers say this diet cuts the Alzheimer's risk by 53%.)