"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," said said study author Sarah Floud from the University of Oxford in the UK.
"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," Floud said.
The study, published in the journal Neurolog, involved one of every four women born in the United Kingdom between 1935 to 1950, or nearly 1,137,000 women.
They had an average age of 56 and did not have dementia at the start of the study.
Participants were asked about their height, weight, diet and exercise at the start of the study.
For the study, Body Mass Index (BMI) between 20 and 25 was considered desirable, and a BMI of 30 or higher was considered obese.
Women who reported exercising less than once per week were considered inactive. Those who exercised more often were considered active. Women's reported usual diet was used to calculate their calorie intake.
Researchers then followed the women for an average of 18 years. After 15 years from the start of the study, 18,695 women were diagnosed with dementia.
Researchers adjusted for age, education, smoking and many other factors.
They found that women who were obese at the start of the study had, in the long-term, a 21-per cent greater risk of dementia compared to women with a desirable BMI.
Among the obese women, 2.1 per cent, or 3,948 of 177,991 women, were diagnosed with dementia.
This is compared to 1.6 per cent of women with desirable BMI, or 7,248 of 434,923 women, who were diagnosed with the disease.
However, while low calorie intake and inactivity were associated with a higher risk of dementia during the first 10 years of the study, these associations weakened substantially, and after 15 years, neither was strongly linked to dementia risk.
"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," Floud said.
Floud continued, "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 limitation of the study was that it looked at women only, so the results may not be the same for men.
( With inputs from IANS )