“So, it is possible to identify some of these cognitive effects in individuals susceptible to dementia and Alzheimer’s disease three decades before they become clinically ill,” Dr. Wolf said.
Modifiable Risk Factors
Although some dementia risk factors, such as parental dementia, cannot be controlled, other risks may be modified through preventive interventions. Elevated plasma homocysteine is another strong, independent risk factor for dementia and Alzheimer’s disease, which Dr. Wolf noted accounts for a twofold increase in dementia risk. Dietary changes may help to lower homocysteine levels, particularly eating more foods rich in folate (B9), B6, and B12, though evidence supporting this is lacking.
Plasma docosahexaenoic acid (DHA) levels are also associated with dementia risk, with one study showing individuals with the lowest levels of DHA develop dementia at about twice the rate of those with higher DHA levels. A dietary questionnaire included in this study found that those with the highest plasma DHA levels ate 2.9 servings of fish a week. Another study found that red cell membrane levels of DHA—considered to be “the most accurate way to measure fatty acids,” Dr. Wolf said—are correlated with visual memory and brain volume. The study reported that persons with the lowest levels of DHA had brain volumes that were equal to those of people two years older and scores on visual memory tests that were equal to those of people three years older.
Obesity and its associated metabolic changes, such as diabetes, also have a significant negative impact on brain volume and performance on neuropsychologic tests. Visceral fat, insulin resistance, fasting glucose, glucose tolerance, hemoglobin A1c, and elevated fasting insulin levels correlated with six years of structural brain aging. These risks may be minimized through diet and exercise.
“The association with fasting insulin levels is present even in nondiabetics,” Dr. Wolf noted. “So there is a subclinical level of metabolic deregulation, which is influencing the brain structure and presumably function even in nondiabetics.”
Biomarkers measuring visceral fat, a highly metabolically active fat (measured as BMI, waist circumference, waist-to-hip ratio, and abdominal fat on CT scans), are an important predictor of vascular and cognitive consequences of obesity, Dr. Wolf said.
“An interesting relationship [was found] between waist-to-hip ratio and the presence or absence of hypertension and performance of delayed recall and a reduction in recall,” Dr. Wolf said. “Persons at the highest levels of waist-to-hip ratio who were also hypertensive had the poorest performance on visual reproductions. Waist-to-hip ratio was also related to temporal horn volume.”
— Rebecca K. Abma