As many people age, their blood sugar levels creep up. In extreme cases, this can lead to diabetes. But a new study suggests that even moderately elevated blood sugar levels can cause memory impairments and shrink the hippocampus--a part of the brain critical for storing new information.
Many older people develop a condition called reduced glucose tolerance--an impairment of the body's ability to move glucose from the blood into the cells that use it for energy. This raises the level of blood sugar, which can impair memory. Diabetics tend to score poorly on memory tests, for example, as do individuals with elevated blood sugar levels who are not yet diabetic. However, it was not known whether this loss of memory resulted from damage to the brain.
Antonio Convit, a psychiatrist at New York University, and his colleagues examined 30 healthy subjects between 53 and 86 years old. The team took magnetic resonance images of the subjects' brains, gave them a battery of cognitive tests, and performed a standard test that measured their ability to remove glucose from the blood. As expected, subjects with elevated blood sugar levels performed worse in memory tests. Additionally, their hippocampus was smaller. No other brain structures differed between the groups, Convit and his colleagues report online this week in the Proceedings of the National Academy of Sciences.
Although the cause of shrinkage isn't known, Convit points out that the hippocampus has long been known to be especially vulnerable to physiological stress. Reduced glucose tolerance could provide such stress by starving the hippocampus when it most needs fuel. "In the long run that may be causing the damage," Convit says.
That scenario makes sense to Carol Greenwood, a nutritionist at the University of Toronto, who thinks that the hippocampus may be in trouble even before diabetes sets in. It's a "slippery slide down the slope," she says. Neuroendocrinologist Bruce McEwen of The Rockefeller University in New York City agrees but is waiting for more evidence to nail it down. "At this point we don't know yet what the causal factors are."