In a longitudinal population-based study conducted over a 27-year period and published last week in the online version of the British Medical Journal, researchers concluded that being overweight or obese in middle age considerably increases risk of dementia in later life.
According to the Centers for Disease Control and Prevention, 64 percent of adults age 20 years and over are overweight or obese.
A recent study into the prevalence of Alzheimer's disease in aging populations predicted that that the incidence of dementia is expected to increase 400 percent in the next 20 years.
The researchers examined data from health evaluations of 10,276 members of Kaiser Permanente's health care plan between 1964 and 1973. At the time of the evaluations the subjects were aged 40 to 45, and all were still members of the plan in 1994.
Their height and weight were measured according to standardized procedures, and they were categorized by body mass index (BMI) as obese (BMI of 30 or over), overweight (25.0 to 29.9), normal (18.6 to 24.9) or underweight (less than 18.9).
Subcapsular (shoulder) and tricep skinfold (back of upper arm) thickness was also measured using Lange skinfold calipers.
The researchers decided to investigate a correlation between the results of these examinations in middle age and later incidence of dementia between since, although a recent study have indicated that obesity in elderly women increases the risk of dementia, BMI tends to decrease with aging.
The initial onset of dementia may also affect appetite and cause weight loss, skewing further the temporal association between weight and dementia.
Between January 1994 and April 2003 dementia was diagnosed in 713 participants - 6.9 percent of the study group.
Obese women were seen to be twice as likely to develop dementia as normal women, while overweight women had a 55 percent increased chance.
Obese and overweight men fared better than their female counterparts, but the same pattern was still identified, with a 30 percent increase in risk amongst obese subjects and 16 percent in overweight subjects.
Overall, obese people had a 74 percent increased risk of dementia compared with normal people, and the overweight people a 35 percent increased risk. Men and women in the higher fifth of the distribution of subcapsular or tricep skinfold thickness were also seen to have a 72 percent and 60 percent increased chance or dementia, compared with those in the lower fifth.
The researchers did admit some limitations to the study, however: no information was collected on weight cycling, dieting, nutrition or mid-life measures of cognitive function which other studies have shown may be associated with dementia.
They also said that some measure of central obesity, such as waist circumference, would have been informative but was not measured.
Furthermore, since diagnoses of dementia were made during medical visits, cases amongst those who did not attend visits may have been missed. "If these results can be confirmed elsewhere, perhaps treatment of obesity might reduce the risk of dementia," wrote the researchers.
"Failure to contain the present epidemic of obesity may accentuate the expected age related increase in dementia."
Obesity is also linked to increased risk of heart disease, some cancers, high blood pressure and type 2 diabetes.