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A large study that followed more than one million women for nearly two decades has found that while obesity in mid-life is linked to a greater risk of dementia in later life, poor diet, and lack of exercise are not. The study is published in the online issue of Neurology, the medical journal of the American Academy of Neurology on 18 December.

'Some previous studies have suggested poor diet or a lack of exercise may increase a person’s risk of dementia,' said study author Sarah Floud, '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.'

The study involved one of every four women born in the United Kingdom between 1935 to 1950 - 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.

Body Mass Index or BMI is a measure of a person’s body size based on their height and weight. 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 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% greater risk of dementia compared to women with a desirable BMI. Among the obese women, 2.1% or 3,948 of 177,991 were diagnosed with dementia, compared to 1.6% (7,248 of 434,923) of 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 follow-up, these associations weakened substantially and after 15 years of follow-up neither was strongly linked to dementia risk.

'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 and inactivity and low calorie intake are likely to be the result of the earliest signs of the disease, before symptoms start to show' said Dr Floud. She 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, and cerebrovascular disease contributes to dementia later in life.'

One limitation of the study was that it only looked at women, so the results may not be the same for men.