Sometimes, what your brain wants is not good for your body.
Donuts are a good example: It’s early morning and you’re driving to work after a nice breakfast of black coffee, two eggs—easy-over—with bacon. Yet, you’re still hungry, and you're having difficulty paying attention to the traffic. Why? Your brain is not cooperating because it is not satisfied with that breakfast. It lacked one critical ingredient that your brain urgently needs—sugar. You have been fasting since last night's dinner and your blood levels of sugar have fallen. From your brain’s perspective, though, sugar is indispensable. And it will do whatever is necessary to convince you to eat sugar as often as possible.
Your brain needs sugar (usually in the form of glucose) to function normally. The billions of neurons in your brain require a constant supply of sugar to maintain their ability to produce energy and communicate with other neurons. Your neurons can only tolerate a total deprivation of sugar for a few minutes before they begin to die. Therefore, as blood levels of sugar decrease with the passage of time since your last meal, you begin to experience a craving for something sweet. Essentially, the presence of sugar in your brain is considered normal, and its absence leads to the craving—and to the initiation of foraging behaviors, such as seeking out a vending machine for cupcakes or a candy bar. There is a reason donut shops and sugar-laden cereals are so popular: You can lay the blame on neurons within the feeding center of your hypothalamus. If your brain did not want those donuts so badly, the shops would not be so densely distributed along your route to work.
Once inside the brain, sugar is also used to produce a very important neurotransmitter chemical called acetylcholine. Acetylcholine allows you to learn and remember, to regulate your attention and mood, and to control how well you can move. Your brain makes acetylcholine from choline, which is obtained from the diet, specifically from acetyl groups that originate from the metabolism of sugar. We frequently obtain choline in our diet by eating lecithin, which is present in baked goods such as donuts and cupcakes and, often, in chocolate. Thus, a chocolate-covered donut first thing in the morning is going to provide your brain with everything it wants and needs to pay attention and learn new things. Sadly, the eggs and bacon you had for breakfast at home were completely insufficient for preparing your acetylcholine neurons to function. (Ironically, recent research suggests that eating too much sugar places your acetylcholine neurons at risk of death leading to the symptoms of dementia.)
As the day progresses, your acetylcholine neurons consume choline and sugar as you think and learn. Your active brain utilizes the equivalent of 10 donuts worth of sugar every day. Now, as evening arrives, you notice that you’re having trouble paying attention again; and you’re experiencing some mental slowing. What’s happening in your brain? While you were thinking and learning, another neurotransmitter was increasing in concentration and slowly but powerfully began turning off your acetylcholine neurons. This chemical is called adenosine, and it inhibits the function of acetylcholine neurons. The longer you are awake, the more persuasive its influence.
What's the cure? Coffee.
The caffeine in your coffee is able to prevent the actions of adenosine and release your acetylcholine neurons from their chemical shackles; your attentiveness improves and you are ready for anything—at least until the caffeine effect wears off.
Tomorrow morning, almost without doubt, you are going to crave coffee and donuts (or something like it) because it’s what your brain wants. However, before you stop at the donut shop, please go back and read the first sentence again. . .
© Gary L. Wenk, Ph.D. Author of Your Brain on Food, 2nd Edition, 2015 (Oxford University Press).