Being overweight in middle age increases a person's risk of developing dementia, US scientists believe.
Obese women were 200% more likely to have dementia
Obese people in their 40s are 74% more likely to develop dementia compared to those of normal weight, a US National Institutes of Health team found.
The lifetime dementia risk in those who were overweight was 35% higher, their study of more than 10,000 US men and women over three decades revealed.
The findings are published online at bmj.com.
The authors warned that the present epidemic of obesity might lead to a boom of dementia in the future.
For the study, obesity was defined as a body mass index 30 or above and overweight as a BMI of between 25 and 29.9.
A normal BMI lied anywhere between 18.6 and 24.9 and is calculated by dividing a person's weight in kilograms by the square of their height in metres.
All of the participants underwent detailed health checks from 1964 to 1973 when they were aged 40 to 45 and were monitored until 1994 to see whether any had developed dementia.
Overall, 713 (7%) of the participants developed dementia.
Body mass index
BMI predicted dementia more strongly among women than men. For example, obese women were 200% more likely to have dementia than women of normal weight, while obese men had a 30% increase in risk.
Both men and women with the highest skinfold measurements - another indicator of obesity - had a 60-70% greater risk of dementia compared to those with the lowest measurements.
But a person's ethnicity did not appear to be important.
The researchers believe that obesity might increase dementia risk either through a direct effect on the brain or by its association with medical conditions that are also known to increase dementia risk, such as cardiovascular disease and diabetes.
However, when they looked at whether the individuals had diabetes or cardiovascular disease, neither appeared to alter the dementia risk.
A Swedish study recently found that the higher a woman's BMI, the greater the chance they would experience brain tissue loss, which is one of the first indications a person is going to develop dementia.
Dr Rachel Whimer and colleagues, who conducted the latest study, said: "If the results can be confirmed, perhaps treatment of obesity might reduce the risk of dementia."
Professor Clive Ballard, of the Alzheimer's Society, said: "The findings are consistent with previous studies showing that risk factors for heart disease, such as high blood pressure and high cholesterol, are also risk factors for dementia.
"And it is likely that the association between obesity and dementia is explained by the increased frequency of these other risks in people who are substantially overweight.
"Given that the number of people with dementia is already increasing dramatically as our population ages, it will be extremely important to minimize additional preventable risk factors such as obesity."
He said evidence showed leading a healthy lifestyle could help to reduce the risks of dementia.