Fast carbs are foods that quickly turn into glucose in the body, often before you even finish swallowing. They include sugar, of course, which is half glucose, but the main culprit in the Western diet is wheat flour, which is mostly amylopectin A, the type of starch that is most quickly digested into glucose by the enzymes in your saliva. Making the digestion much faster is the fact that the flour is ground extremely fine, providing enormous amounts of surface that is then exposed to the enzymes.
We have long had a nickname for the effects of fast carbs on the brain: food coma. While the name is meant to be somewhat tongue-in-cheek, the effect is real, as a recent study in the journal Physiology & Behavior shows.
The researchers tested the effects of glucose on simple reaction time, arithmetic, and an effect called Stroop interference, where, for example, printing the word “blue” in a color other than blue slows down thinking. In all three tests, subjects responded more slowly when given glucose or sucrose than when given the sucralose placebo. The effect was greater after a 10 hour fast (such as usually occurs just before breakfast).
The effect after fasting was a 15% slowdown in reaction time. Without fasting, the reaction time was faster, but the difference between glucose and placebo was greater, with glucose causing a 17% slowdown.
Other studies have shown that glucose induces sleep by activating sleep-promoting neurons in the brain, and that by raising insulin levels, glucose can make it easier for the amino acid tryptophan to enter the brain, where it is converted into the hormones serotonin and melatonin, both of which are involved in sleep. The tryptophan you get from that Thanksgiving turkey would not by itself make you sleepy. It is the insulin following the rolls, mashed potatoes, and desserts that allows it to get into the brain.