A new long-term study shows that people who were obese or overweight in middle age had an increased risk of dementia later in life.

"Obesity and overweight in middle age as measured by body mass index [BMI] and skinfold thickness were strongly associated with risk of dementia in later life," write researchers in a study, which appears in BMJ Online First.

"If these results can be confirmed elsewhere, perhaps treatment of obesity might reduce the risk of dementia," write the researchers, who included Rachel Whitmer, PhD, a gerontological epidemiologist with Kaiser Permanente's research division in Oakland, Calif.

Trends May Intertwine in the Future

Dementia is usually seen in older adults. The most common form is Alzheimer's disease. Other types of dementia were also included in the study.

The incidence of dementia is expected to increase by 400 percent in the next 20 years as the population ages, according to the study. Meanwhile, obesity is also on the rise in the U.S. and around the world.

"Failure to contain the present epidemic of obesity may accentuate the expected age-related increase in dementia," write Whitmer and colleagues.

Long Look at Weight, Dementia

Whitmer's study included 10,276 people who were members of Kaiser Permanente Northern California Medical Group, a health insurance plan.

Between 1964 and 1973, the men and women got detailed health evaluations including BMI and skin-fold thickness at the shoulder and at the back of the arm.

At the time, participants were 40-45 years old. They were all still members of the health plan in 1994.

From January 1994 to April 2003, 713 participants (6.9%) were diagnosed with dementia. On average, they were about 74 years old when dementia was diagnosed.

Greater Dementia Risk Seen in Obese, Overweight People

People who were obese or overweight in middle age had a higher risk of late-life dementia than those of normal weight, according to the study.

Stroke, diabetes, heart disease, high blood pressure, and high cholesterol were taken into account. So were age, sex, race, and education level. Marital status, smoking, and alcohol use in midlife were also noted.

"Obesity in middle age increases the risk of future dementia independently of comorbid conditions," researchers write.

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 percent 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 percent 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 percent 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 percent increase in risk," write Whitmer and colleagues.

"Overweight women were 55 percent more likely to have dementia than women of normal weight, while overweight men had a nonsignificant 16 percent 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.

Next Steps

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.

By Miranda Hitti, reviewed by Brunilda Nazario, MD

SOURCES: Whitmer, R. BMJ Online First, April 29, 2005. News release, BMJ.