Obesity in midlife is linked to a greater risk of dementia later in life for women, but poor diet and lack of exercise are not, new research suggests.
Researchers found that women who were obese at the start of the study had in the long term a 21% greater risk of dementia compared with women with a desirable BMI.
The study involved 1,137,000 women born in the UK between 1935 and 1950.
They had an average age of 56 and did not have dementia at the start of the study.
Study author Dr Sarah Floud, of the Nuffield Department of Population Health at the University of Oxford, said: “Some previous studies have suggested poor diet or a lack of exercise may increase a person’s risk of dementia.
“However, our study found these factors are not linked to the long-term risk of dementia. Short-term associations between these factors and dementia risk are likely to reflect changes in behaviour, such as eating poorly and being inactive, due to early symptoms of dementia.”
Participants were asked about their height, weight, diet and exercise at the start of the study.
A BMI between 20 and 25 was considered desirable for the study, and a BMI of 30 or higher was considered obese.
Those who reported exercising less than once a week were considered inactive, and women who exercised more often were considered active.
Their reported usual diet was used to calculate their calorie intake.
Scientists followed the women for an average of 18 years, and after 15 years from the start of the study, 18,695 women were diagnosed with dementia.
Among the obese women, 2.1%, or 3,948 of 177,991 women, were diagnosed with dementia.
This is compared with 1.6 % of women with a desirable BMI, or 7,248 of 434,923 women, who were diagnosed with the disease.
However, while they found that low calorie intake and inactivity were associated with a higher risk of dementia during the first 10 years of the study, after 15 years neither was strongly linked to dementia risk.
Dr Floud said: “Other studies have shown that people become inactive and lose weight up to a decade before they are diagnosed with dementia.
“The short-term links between dementia, inactivity and low calorie intake are likely to be the result of the earliest signs of the disease, before symptoms start to show.
“On the other hand, obesity in midlife was linked with dementia 15 or more years later.
“Obesity is a well-established risk factor for cerebrovascular disease. Cerebrovascular disease contributes to dementia later in life.”
The study was published in the online issue of Neurology, the medical journal of the American Academy of Neurology.
Dr Sara Imarisio, head of research at charity Alzheimer’s Research UK, said: “The diseases that cause dementia begin decades before memory problems start to show and this study also hints to factors like physical inactivity and weight loss in midlife as potential indicators of the condition.
“If we are to limit the growing impact of diseases like Alzheimer’s, it’s vital that research investigates the role of risk factors throughout life, and the best time for people to take action to reduce their risk.
“While this is a large study using NHS health records, at this stage we cannot generalise these findings to men or to the whole of the UK population.
“BMI can be a crude measure and is not necessarily a good indication of our general health but keeping tabs on the amount of body fat we carry is important for a healthy body and a healthy brain.”