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The sugar-rush myth stems from what is known as the oral glucose tolerance test, which measures blood glucose levels in people who have fasted overnight and are then given a super-sweet glucose drink. This test, which is designed to help diagnose diabetes, does produce a rapid spike in blood glucose, often followed by an overcompensating crash.

In the real world, though, nobody eats pure glucose, and most people's bodies are pretty good at regulating blood glucose levels. Even when you consume a carbohydrate-loaded meal, it takes time for all the carbohydrates to be broken down into glucose, and a sharp spike in blood glucose after a meal is rare.

Unless you have type 1 diabetes, a rise in blood sugar levels prompts the pancreas to secrete insulin. The liver responds to the insulin by converting excess glucose into a starchy material called glycogen, which it stores for future use. When blood sugar levels fall again, the liver breaks down its stored glycogen into glucose, which it dribbles back into the bloodstream as needed. When functioning properly, this regulatory system controls blood glucose levels pretty tightly, so your intake of carbohydrate is not directly linked to what the liver is releasing back into the bloodstream.

There's also a second checkpoint between the carbohydrate in the food you eat and the glucose taken up by the brain. Research in the mid-1990s revealed the vital role of brain cells called astrocytes, which store glucose as glycogen and act as a buffer between glucose in the blood and that in the fluid that nourishes the brain. The astrocytes typically keep glucose levels in this fluid at just 20 to 30 per cent of blood levels. If the brain's neurons need more, they take it from the surrounding astrocytes. If they don't, the glycogen stays in storage. Only after a long period of demanding mental activity does the brain deplete the astrocytes, and then the blood glucose, to the point where it needs topping up with food.

Another myth is that because your brain subsists on glucose, it will perform better if you feed it glucose-producing carbohydrates before asking it to do anything important. Again, it's not actually that simple. Claude Messier, a neuroscientist at the University of Ottawa, Canada, gave rats a dose of the sugar-like compound 3-O-methylglucose, which can be thought of as a biologically useless form of glucose. At first glance, the brain cells mistake it for glucose, and happily scavenge it out of the bloodstream. But when they realise what it is, they spit it back out, like a vegetable-averse child rejecting a hidden pea. Metabolically, this should be a non-event: useless molecule in minus useless molecule out should equal no extra brainpower. Except that's not what happened to Messier's rats. Real glucose perked up their performance on memory tests, such as mazes, but they performed equally well on the metabolically inert 3-O-methylglucose.

"That's quite a conundrum," Messier says. "The only way I could figure it out is if the action of transporting the 3-O-methylglucose across the cell membrane triggered some signal in the brain that there was energy coming in. That got translated to promote memory." So much for the feed-the-brain theory. Fool-the-brain works too.

When it's not being tricked in experiments, though, your body has glucose regulation pretty much under control. So does it matter what you eat for breakfast? Well, yes, but not necessarily in the way your mother thought. Having reviewed nearly 100 studies Gibson is convinced that a small amount of carbohydrate can improve memory function on standardised laboratory tests, but it's a fairly tight response with the optimum dose the equivalent of about 25 grams of glucose - or 100 calories of carbohydrate. A banana, in other words, or a small bowl of breakfast cereal. But recent experiments have shown that the type of carbohydrate is more important than the amount you eat. The crucial factor is the food's glycaemic index (GI), a measure of how quickly it raises blood sugar compared with pure glucose. Low-GI foods are digested slowly, releasing sugar into the blood gradually, while high-GI foods release their sugars in a single, large hit (see "Brain food").

Gibson fed breakfasts with varying GIs to volunteers, while others went hungry. He then gave each group lists of words to memorise and tested them on how many they could remember, both immediately after seeing the words and after a short delay. The experiments revealed that, contrary to the idea of the sugar rush, the best memory boosters are foods that have a low GI - in this case, All-Bran. Surprisingly, the poorer brain fuels were those with a high GI, such as the Coco Pops (known as Cocoa Krispies in the US) used in Gibson's experiments, or cornflakes. "They both were better than no breakfast at all, but the performance was greater for the low-GI All-Bran than the high-GI Coco Pops," Gibson recalls (see Graphs).

It seems counter-intuitive: the brain needs glucose to perform, yet seems to prefer slow release over a short sharp hit. Gibson thinks the explanation for the test results lies in the hormone cortisol, which is produced in response to stress, as when anticipating an exam or, in this case, being examined by psychologists. Cortisol's natural function is to mobilise the body's resources for fight or flight. In small amounts it may enhance memory, but it doesn't take much to have the opposite effect. "If you have a lot of cortisol washing around, it tends to be correlated with impaired memory performance," Gibson says. "You need a bit, it seems, to perform well. But if you have too much, you perform badly."

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