Having excess body fat in midlife is linked to a greater risk of dementia 15 or more years later, according to a study that followed more than one million women for nearly two decades.
The study involved one of every four women born in the UK between 1935 to 1950, or nearly 1,137,000 women, who had an average age of 56. According to the study, the participants did not have dementia at the beginning of the research when they were asked about their height, weight, diet and exercise.
While previous studies have suggested that poor diet or a lack of exercise may increase dementia risk in people, the researchers, including those from the University of Oxford in the UK, said these factors are not linked to long-term risk of the mental illness.
The scientists also measured the participants' BMI -- a measure of a person's body size based on their height and weight.
It is measured by dividing the weight of an individual in kilogrammes by the square of their height in metres.
For the study, a 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, and those who worked out more often were noted as active.
The participants' reported usual diet was used to calculate their calorie intake.
Following them for an average of 18 years, the researchers found that 18,695 women were diagnosed with dementia after 15 years from the start of the study.
According to the study, obesity during midlife is linked to dementia after 15 years, even after adjusting for age, education, smoking and many other factors.
It found that women who were obese at the start of the study, had a 21 per cent greater risk of dementia in the long term compared to women with a desirable BMI.
Among the women who were obese, the study reported that 2.1 per cent were diagnosed with dementia, compared to 1.6 per cent with a healthier BMI who were diagnosed with the disease.
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 researchers 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," said study co-author Sarah Floud from the University of Oxford.
However, the researchers said, obesity in midlife was linked with dementia only 15 or more years later.
One limitation mentioned by the researchers in the study was that it looked only at women, so the results may not be the same for men, they said.
The findings of the study was published in the journal Neurology.