A new study shows that sugar may not be so sweet for the brain – and may lead to memory problems.

Researchers from four universities report in the Annals of Neurology that people who absorb glucose more slowly than those who metabolize it quickly are more forgetful and are more likely to have a faulty dentate gyrus, a pocket in the hippocampus section of the brain. The hippocampus is involved with learning and memory formation. 

The findings were based on glucose testing, memory evaluations and fMRI scans of the brains of 240 healthy people ages 65 and older without dementia, and applied even in those without diabetes, which is characterized by an inability to readily convert sugar into energy.

Glucose metabolism naturally slows with age, and memory begins to decline in our 30s, says co-author Scott Small, an associate professor of neurology at Columbia University Medical Center in New York. The new study suggests a possible association between the two, because elevated blood sugar appears to damage the dentate gyrus, Small says.

The dentate gyrus's exact function is unknown. But it's one of several circuits in the hippocampus that, if disrupted, impairs memory, such as a person's ability to learn the names of new people or to remember where they parked their car.

The possible connection between its dysfunction and poor glucose regulation may explain earlier observations that exercise benefits the dentate gyrus, Small says. Until now, scientists believed that physical activity reduced the risk of age-related memory loss by allowing glucose to be absorbed more quickly into muscle cells, but were not sure why. This indicates, Small says, that the dentate gyrus could be the missing link.