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More on Age and Blood Sugar Affect Memory, Study Finds

Age and Blood Sugar Affect Memory, Study Finds

New York (Jul 16, 2009)

Pensive looking female senior citizen

Normal changes in how our bodies regulate the amount of glucose, or sugar, in the blood as we grow older may contribute to age-related memory decline, a recent study by NewYork-Presbyterian Hospital neurologist Scott Small, MD and colleagues suggests. "As blood sugar levels increase with age, they appear to damage a specific region of the hippocampus, the brain structure involved in fixing new memories," Dr. Small said. He and his colleagues reported these findings in the December 2008 issue of the Annals of Neurology.

How Memory Works

The hippocampus is made up of several regions (the entorhinal cortex, dentate gyrus, CA1 subfield and subiculum) that work together as a circuit in learning new memories, according to Dr. Small – "the name of someone you met recently, where you put your keys, a new software program." Aging, diabetes, vascular diseases, and stroke all damage the hippocampus, but each targets a different region. Alzheimer's disease, for example, affects the entorhinal cortex, and stroke affects the CA1 subfield and the subiculum, while normal cognitive aging targets the dentate gyrus. "The early stages of Alzheimer's and normal aging look very similar using standard memory tests, though," said Dr. Small, "because they access the whole circuit. It's as though different parts of your computer are down, but the end result is the same, it's not working."

High Glucose and Poor Cognition

Using a special type of brain scan called high-resolution functional magnetic resonance imaging (fMRI), a form of MRI, Dr. Small and his colleagues mapped cerebral blood volume (CBV) in the different parts of the hippocampus. Since blood transports oxygen, CBV is a good indicator of the amount of oxygen found in brain tissues, and therefore of tissue health and function. To ensure the most accurate results the researchers studied three groups of subjects – older people, aging rhesus monkeys, and mice – and discovered the same link: as blood glucose levels rose with age, the blood volume in the dentate gyrus fell. In the human subjects they also found that those with more blood glucose did less well on memory tests.

Glucose Regulation Diminishes Over Time

"An increase in blood glucose is a natural and inevitable aspect of aging," Dr. Small said. "Starting in our 30's we gradually have a harder time absorbing sugars that we eat." Following a well-balanced meal, for example, a young person will get his blood glucose down much faster than an older person, he explained. As a result, our brains are exposed to higher and higher levels of blood glucose over time. "The reason for these increases," he said, "is that we gradually become less sensitive to insulin, the hormone that helps cells absorb fuel in the form of glucose from the bloodstream."

Any strategy that improves our ability to handle blood sugar, through behavioral changes such as exercise or through medication, is likely to have a positive impact on age-related memory loss, said Dr. Small. In a 2007 study Dr. Small and his group did find that physical exercise was beneficial to the dentate gyrus. "We didn't clearly understand why at the time, but we now think that as you exercise your muscles start expressing a glucose transporter, that makes them spongier to glucose," and that the resulting lower glucose levels are beneficial to the dentate gyrus, he said.

"Large-scale public health studies often produce conflicting results," Dr. Small said. "One day milk is good, then milk is bad, or eggs are good, eggs are bad. In all of the studies of aging and disease what always comes out as 'good' is exercise," he pointed out. "At this point, there is nothing else to recommend." Dr. Small added that he is working with colleagues to develop a number of compounds derived from food that might imitate the benefits of exercise, "but that's into the future."

Scott Small, MD is an Associate Attending Neurologist at NewYork-Presbyterian Hospital/Columbia University Medical Center, and an Associate Professor in Neurology at Columbia University College of Physicians and Surgeons.

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