For some time, researchers have known that type 2 diabetes is linked to memory decline, presumably due to faulty blood sugar regulation. A new study suggests that blood sugar (glucose) spikes even in non-diabetic older people can cause age-related memory decline.
We form memories in the hippocampus of the brain in an area called the dentate gyrus. Researchers at the Columbia University Medical Center used high-resolution functional magnetic resonance imaging (fMRI) to look at blood flow in the dentate gyrus of elderly individuals. FMRI is a neuroimaging technique that can detect blood flow to areas in the brain or spinal cord. Blood flow in the brain is linked to mental activity. When blood glucose levels were elevated or spiked, there was reduced blood flow in the dentate gyrus. Experiments with aging rhesus monkeys and diabetic mice confirmed these results.
Glucose is the only energy source that the brain can use. Insulin controls the amount of glucose in the blood that gets into cells. It prompts cells to take up glucose. Glucose uptake is rapid in young healthy individuals. In type 2 diabetes, insulin signaling usually doesn’t work well. The cells don’t respond and so glucose builds up in the circulation. As we age, insulin signaling also becomes less efficient. As our bodies produce glucose after ingestion of a meal, for example, it takes longer for cells to take up glucose, generating blood sugar spikes.
The new studies suggest that memory decline may not only be a problem in the aging population, but also in young people with type 2 diabetes. Eight percent of the U.S. population or 23.6 million people have diabetes according to the American Diabetes Association. Most adults (90-95%) have type 2 diabetes, but the diagnosis of diabetes or pre-diabetes in children and young people is increasing at an alarming rate.
Glucose-linked memory decline can be ameliorated with physical activity. Previous studies (reviewed in the International Journal of Sports Medicine, 2000) have shown that exercise can counteract the insulin insensitivity in type 2 diabetes and increase insulin sensitivity in healthy people. More recently, a study (Obesity, 2007) found that physical activity increased insulin sensitivity even in overweight and obese children; an important finding in the light of the epidemic increase in overweight and obese children in the U.S. These results have been confirmed in diet-induced obese mice (American Journal of Physiology and Metabolism, 2008). Both insulin insensitivity and tissue inflammation were decreased by exercise. Direct evidence for improvement in memory and cognition with exercise comes from a study in Australia (Journal of the American Medical Association, 2008). In this study, physical activity (mostly walking for an average of 20 minutes/day) improved the memory in elderly adults with mild cognitive impairment. Although other factors may impact memory and cognitive function, exercise has both short and long term effects on glucose metabolism. In the Australian study the effects of physical activity lasted another 12 months after the study ended. Because exercise improves glucose-insulin regulation, it is a simple but effective way to slow memory decline and potentially delay the onset of dementia and Alzheimer disease.
Annals of Neurology 2008, 64: 698-706