Tracking Obesity's Effects
For those with an obese BMI (30 or higher) in middle age, the risk of dementia in old age was 74% higher than for those with normal BMI. For those who were overweight (BMI of 25-29.9), late-life dementia risk was 35% higher than those with normal BMI, the researchers report.
The skin-fold tests also showed a similar pattern.
Men with the largest skin-fold measurements in midlife had a 72% higher risk of dementia in old age than those with the smallest skin-fold measurements. Middle-aged women with the biggest skin-fold readings had a 60% increase in their risk of late-life dementia compared with those with the skinniest shoulders and arms in midlife.
BMI-Dementia Risk Stronger in Women
"Body mass index was associated with dementia more strongly in women," say researchers.
"Obese women were twice as likely to have dementia as women of normal weight, while obese men had a nonsignificant 30% increase in risk," write Whitmer and colleagues.
"Overweight women were 55% more likely to have dementia than women of normal weight, while overweight men had a nonsignificant 16% increase in risk compared with men of normal weight," they continue.
The researchers say they aren't sure how to explain the gender gap.
They note that there were fewer obese and overweight men in the study, which could limit their ability to detect those effects. BMI also doesn't pinpoint where fat is distributed, and the researchers speculate that fat around the waistline could play a role in obese women's late-life dementia risk. But waist measurements weren't available to check that.
Underweight BMI (less that 18.5 for adults) wasn't tied to dementia risk in either sex, but few people fell into that category, the study shows.
More work on the topic is needed, say the researchers, noting their study's limitations.
The researchers didn't have any information on diet, nutrition, weight fluctuations, waist circumference, or midlife measures of mental function. They note that "other studies have shown that several different nutritional factors are associated with dementia."
However, Whitmer and colleagues say another recent prospective study "found that obesity in elderly women increases the risk of dementia."
Anyone seeking to reach a healthy weight should check in with their doctor before starting a diet or exercise program.