Middle-aged obese people are nearly 30% less likely to develop dementia than those who are a healthy body weight, according to a study. The same study found that people who are underweight are one third more likely to develop dementia than those who are at a healthy body weight.
These findings, from a very large British study, are surprising because previous research found a link between obesity and an increased risk of developing dementia.
The study looked at data from the Clinical Practice Research Datalink, a large database that holds the medical records of about 9% of the people in the United Kingdom. The researchers looked at data from nearly two million people with an average age of 55 at the start of the study. During an average of 9 years follow-up, more than 45,000 of these people developed dementia.
Those who were under a body mass index of 20 were 34% more likely to be diagnosed with dementia. The risk of dementia steadily went down as their body mass index increased. Those who were severely obese, having a body mass index of 40 or more, were 29% less likely to develop dementia.
These results stayed the same when the researchers took into account other factors that can increase the risk of dementia, including smoking and alcohol consumption. They also accounted for age at the time when dementia was diagnosed and what decade in which the people were born.
"The reasons why a high BMI might be associated with a reduced risk of dementia aren't clear, and further work is needed to understand why this might be the case," adds Dr Nawab Qizilbash, the study's lead author. "If increased weight in mid-life is protective against dementia, the reasons for this inverse association are unclear at present. Many different issues related to diet, exercise, frailty, genetic factors, and weight change could play a part."
Dementia is any of several conditions that cause a loss in memory and thinking skills, but the best known form of dementia is Alzheimer's.
The study was published in the journal The Lancet Diabetes and Endocrinology.