It has long been known that memory declines with age, and that people who
are active - people who exercise more - have less cognitive decline.
Now we have a clearer picture of why. It’s about the sugar.
A 2008 Columbia University study showed that rising blood sugar levels,
a common occurrence with aging, affect a part of the brain critical to
making new memories. [1,2],
Researchers looked at measures that typically change during aging: blood
sugar and insulin levels rise, cholesterol levels rise, and obesity settles
in. Researchers looked specifically at the impact of those factors on
the hippocampus, a seahorse-shaped section of the brain that is critical
for memory and learning. Researchers found that of all these factors,
a rise in the blood glucose levels was the only one closely tied to decreasing
activity in a memory-critical part of the hippocampus called the dentate gyrus.
Using high-resolution brain imaging, researchers showed that rising blood
sugar levels selectively target the dentate gyrus. They mapped brain regions
in 240 elderly subjects. They found a correlation between elevated blood
glucose levels and reduced cerebral blood volume, or blood flow, in the
dentate gyrus. Reduced blood flow is an indication of reduced metabolic
activity and function in that region of the brain.
Researchers found the same association in aging rhesus monkeys and in mice.
“The paper identifies an etiology [cause] for normal age-related
memory decline,” said senior study author Dr. Scott Small, an associate
professor of neurology at the Taub Institute for Research on Alzheimer’s
Disease and the Aging Brain at Columbia University Medical Center in New
York City. “Elevations in blood glucose levels differentially target
the dentate gyrus part of the hippocampus implicated in aging and, as
we age, we develop a slight but gradually worsening difficulty in handling
blood sugar levels.”
Exercise helps to lower blood sugar levels, which is why older people who
are more active, have less cognitive difficulties. With exercise, the
muscles use up glucose in the bloodstream.
It’s not enough to deliver blood sugar to the brain efficiently.
The sugar must get into the cells where it can be used. If you have become
insulin resistant, your brain cells may not be efficient at taking up
the glucose that’s delivered. About 1 in 3 Americans have at insulin
resistance, which if it gets bad enough can lead to type II diabetes.
The findings suggest that maintaining blood sugar levels, even in the absence
of diabetes, helps maintain aspects of cognitive health.