Being underweight in middle and old age carries an increased risk of dementia, according to a large observational study that followed almost 2 million adults for two decades.

Compared with people of a healthy weight, underweight people (BMI <20 kg/m²) had a 34% higher risk of dementia, with the incidence of dementia falling for every increasing BMI category.

For example obese people with a BMI of more than 40 kg/m² had a 29% lower dementia risk than people with a healthy weight.

“Our findings contradict previous suggestions that obese people in mid-life have a higher subsequent risk of dementia”, the researchers wrote in The Lancet Diabetes & Endocrinology.

“The reasons for and public health consequences of these findings need further investigation,” they added.