In the largest study ever to look at relationships between body weight and likelihood of developing dementia, researchers found that being too thin appears to carry the greatest risk.
Among nearly two million people in the U.K. observed over time, those who were underweight had the highest dementia risk in old age, and the risk continued to drop with rising body mass index (BMI) - a measure of weight relative to height. The observations contradict many previous, much smaller, studies that tied obesity to increased dementia risk. The analysis included two million people followed for up to two decades; more than 10 times the number of people analyzed in all previous studies.
For their analysis, published in Lancet Diabetes and Endocrinology, the researchers used a database of medical information about U.K. residents.
The data covered 1,958,191 people who were between 45 and 66 years old at the start of the observations, and free of dementia symptoms or diagnoses. People were followed for an average of nine years, and by the end of the study period some 45,000 people had been diagnosed with dementia.
The study team calculated the participants' starting BMIs and looked at risk of dementia in different weight categories. Compared with people of a healthy weight (BMI of 20-24.9), those who were underweight - with a BMI less than 20 - had a 34 percent higher risk of dementia.
The risk of dementia continued to fall as BMI increased, and very obese people (BMI over 40) had a 29 percent lower risk of dementia than healthy weight people.
Compared to people at the median BMI for the entire group, around 26-27, underweight people had a 64 percent higher risk of dementia.
The authors adjusted for some other possible factors that could influence dementia risk, such as age, sex, smoking, alcohol use, history of stroke and heart attack and use of blood pressure or statin drugs. They also examined rates of death in all weight categories and found that underweight people were at the highest risk of dying, followed by those who were very obese. People who were modestly overweight were slightly less likely than those in the healthy BMI range to die. And the lowest risk of death was at a BMI of 26.
The scientists cautioned that the study results should not change the current recommendations on attaining a healthy body weight since they also found that overweight and obese people had a higher risk of death.
There is more research needed in this area but if I was to draw one possible conclusion from these findings it is that losing muscle mass and becoming frail put you at increased risk of dementia, as well as many other chronic conditions. I have often said that the health challenge for under 50-year-olds is to not put on too much fat, while the challenge for over 50's is to not lose too much muscle.
I'd encourage you to find out about strength training. As well as saving your muscles, it could save your mind.
Article Author: David Beard, Calico Calico Exercise Physiologist & Healthy Aging Expert
