Much of Greenwood and Winocur's research has looked at the impact of diet on adult-onset or Type II diabetes. It's relatively well established that a high-fat diet increases the risk of coming down with diabetes, but that same diet may also be having a negative impact on thinking abilities. “What we know from our animal studies is that the chronic consumption of high fat diets is associated with cognitive deficits,” says Greenwood. “That is, diets and lifestyles that are consistent with an increased risk of diabetes development are also consistent with cognitive impairment.”

Greenwood and Winocur discovered that rats fed a low-fat diet (roughly 10 percent of their caloric intake) have better memory than those fed a high-fat diet (roughly 40 percent of caloric intake, which, remember was what most of us were taking in when these things were last comprehensively studied ten years ago).

In a study published in the March 2001 issue of the journal Neurobiology of Learning and Memory, Greenwood and Winocur have pinpointed what they believe is so detrimental about high-fat diets to cognition: glucose. Or rather, a lack of glucose.

Their hypothesis is that a high-fat diet impairs glucose metabolism in the brain. And why would that matter? Probably for two reasons.

“For memory processing there is a double need for glucose,” says Greenwood. “It is important for brain function because when the brain is performing memory tasks, glucose is needed to support the energy of the neuron. When brain cells are working, they're actively firing and that's a high energy-demanding process. So they need glucose for that.”

Also, she adds, “we know that the neurotransmitter that predominantly subserves a lot of memory function is acetylcholine, and glucose is necessary to produce the acetyl part of that.”

To test their hypothesis, Greenwood and Winocur administered glucose to rats already showing memory deficits from a high-fat diet. These rats showed substantial improvement on cognitive tasks after the glucose was administered. Particularly affected were tasks involving hippocampal function, an area of the brain responsible for long and short-term memory.