Attention skinny people—you may get dementia. That’s the finding from a study of more than 2 million people that looked at the relationship between body mass index (BMI) in middle age and dementia later in life. This study has gotten a lot of press because it found, essentially, that the thinner you are, the greater your dementia risk…which goes against what most people might think and implies that there’s no need to worry about staying trim.

Before you break out the cheesecake to celebrate, though, let’s take a closer look.


The researchers used a health-care database from a network of primary-care practices in the United Kingdom to find height/weight measurements on people aged 40 and older. They followed these people until they either left the network, were diagnosed with any form of dementia or died.

The result was more than a little surprising: Dementia risk decreased with each bump up in BMI. People who were underweight (BMI 20 or lower in this study, or under 139 pounds for a 5’10” tall person) had a 34% higher risk than average of developing dementia over 20 years than average—maybe not so surprising, since being underweight is obviously not a healthy thing to be. But then the higher that people went up on the weight spectrum, the lower their risk for dementia became, and that seemed downright odd. At the far end of the weight scale, those who were morbidly obese in midlife, with a BMI of 40 or higher (279 pounds for a 5’10” person) had a 29% lower risk for dementia than average. Crazy, huh? As one headline put it, “Being Fat Lowers Dementia Risk In Middle And Old Age, Contradicting Everything We Thought We Knew.”

To help us figure this out, we interviewed Deborah Gustafson, PhD, a professor of neurology at SUNY Downstate Medical Center in Brooklyn, New York and the University of Gothenburg in Sweden. Dr. Gustafson, who was not involved with the UK study, has done extensive research on the risk factors for cognitive decline and was among the first to study the relationship between body weight and dementia.

Regarding the new study, she was, like us…skeptical.


Like all observational studies, the UK weight study has some inherent limitations. While these kinds of studies are valuable for pointing researchers toward associations, they can’t show cause and effect. Beyond this general limitation, though, Dr. Gustafson had specific concerns about this particular study’s methods…

• It included mixed age groups. You had to be 40 or older to be included, but there was no upper limit. Some people were 80 years old when their baseline info was recorded. (No matter how you slice it, being 80 isn’t middle-aged.) So it’s hard to draw conclusions from this data about how weight in middle age affects dementia risk when you’re older. “During midlife, a person normally gains weight,” explains Dr. Gustafson. “At around 65 or 70 years old, a person typically loses skeletal muscle and gains fat, but overall, BMI tends to decrease.” Mixing up data from these two very different stages of life, she says, means “the study is not going to work.”

• It likely missed many cases of dementia. Patients with dementia were identified only through a review of medical records in this study, but people may come into the health-care system with more acute illnesses that mask dementia, so this approach misses many late-onset cases of dementia. A better approach would be to conduct thorough evaluations among a representative sample, which can take hours, followed by discussion among more than one expert to confirm the diagnoses. That’s what Dr. Gustafson has done in her studies. “It’s expensive to conduct a study with time-consuming evaluations, which is why it can’t be done on 2 million people,” she admits. But it’s more accurate.

• It didn’t distinguish between different types of dementia.That’s another red flag, says Dr. Gustafson. Certain hereditary forms of dementia tend to strike earlier in life. “Early-onset dementia is a different beast, so it would have helped if the researchers had separated dementias diagnosed before age 65 from those diagnosed after age 65,” she says. Late-onset dementia may be influenced by being overweight, while early-onset dementia is more likely to be hereditary and influenced by specific genes.


To get a clearer understanding of the relationship between weight and dementia, Dr. Gustafson recently reviewed a decade of published research and published her findings in the Journal of Alzheimer’s Disease. Here are her recommendations…

• Watch your weight in midlife. “Studies investigating the association between midlife BMI and risk for dementia demonstrated generally an increased risk among overweight and obese adults,” she wrote. One reason may be that excess weight increases your risk for high blood pressure, high cholesterol levels and diabetes, says Dr. Gustafson, adding, “All of these factors have been shown to increase the risk for dementia.”

• Being too skinny over the age of 70 increases your risk. Being underweight (BMI 18.5 or lower) is associated with increased dementia risk. “That’s quite thin, such as 5’ 4” and 107 pounds,” she notes. No one is sure why, but there may be metabolic abnormalities that keep people underweight that also contribute to dementia risk. In some cases, the dementia process may begin decades before clinical symptoms and lead to a lower body weight.

• A little extra weight in later life may be protective. There is a consistent finding in the medical literature that over age 70, having a BMI in the “overweight” range (25 to 29.9) is protective. Some but not all studies find that even being obese (BMI 30 to 34.9) protects, too. Says Dr. Gustafson: “If you’re a little heavier going into late life, you may be less likely to develop dementia.” While no one is sure why, “It may be that fat tissue produces hormones that are protective for the brain.”

We know it’s not quite as much fun as a headline that says being fat is a good thing for your brain and memory. That would be nice for people who are heavy. But the real story appears to be that a healthy lifestyle that helps keep weight in the normal range throughout your middle years and into your 70s is good for your brain, too. Once you get into your 70s, a little extra weight may be fine.

At any age, however, a healthy diet and exercise is important for body—and mind. Says Gustafson, “All of the things that we have been promoting for a long time—eating right and getting physical exercise—are actually relevant for dementia, too.” For the latest dementia-preventive recipes, see the Bottom Line article, Can These Four Recipes Prevent Alzheimer’s Disease?. And learn more in the Guide to Alzheimer’s Disease: What Everyone Should Know.