For years, scientific research suggested that obesity is linked to dementia, but a new study involving two million people in the U.K. suggests that the opposite may be true: Those who are underweight—or even on the low side of normal–may be the most vulnerable.
The study, published yesterday in the journal Lancet Diabetes and Endocrinology, analyzed nearly 2 million individuals who had their body-mass index recorded between 1992 and 2007 and followed their health for years. The data only stopped flowing if the patient transferred out of practice, the practice shut down, or the patient developed dementia or died (whichever occurred first).
Together with his team, lead author Dr. Nawab Qizilbash of Oxon Epidemiology, a medical consultancy in the U.K., found that patients with a BMI under 20 were 34% more likely to develop dementia as they aged than others who had a BMI of 20 to 25, which is considered the target healthy range. This under-20 group includes people who are medically deemed average weight since the official threshold for low BMI is 18.5 or under.
Besides apparently contradicting the link between obesity and dementia, the study suggests that obesity may actually protect against it: very obese people with a BMI of over 40 were 29% less likely to get dementia later on than those of normal weight. Of course, middle aged-people shouldn’t use this as an excuse to binge eat and give up on exercise—the authors emphasize that obesity can cause many other immediate health complications.
The study is also purely data-driven, so the researchers admit there may be confounding factors at play and that explaining the results could prove complicated, especially since BMI is not always a reliable indicator of overall health. Here’s Sarah Boseley, writing for The Guardian:
Many issues “related to diet, exercise, frailty, genetic factors, and weight change” could play a part, says the paper. There have been small studies that suggest a deficiency of vitamin E or vitamin D may play a part in dementia, but these are purely speculative, said Qizilbash.
But the study “opens up an avenue to look at the protective effects on dementia of diet, vitamins, weight change as well as frailty and potentially genetic influences”.
What’s important, though, is that if researchers can understand the underlying biochemical circumstances that prevents overweight people from getting dementia, they may be able to devise new treatments that could apply to anyone.