To paraphrase the 17th century French philosopher, René Descartes, “we think; therefore, we eat.” At least, that’s the conclusion of researchers from the University of Alabama, Birmingham, in a fascinating new study about why rigorous mental activity increases hunger.
Cells in the brain are very different than cells in other parts of the body, which can store energy as glycogen or fat. For example, the tissues and muscles of the legs can store energy in various forms. They also have low energy requirements when immobile and not active. Doctors can even clamp off blood flow to the legs (below the renal arteries) for two hours during surgical procedures without causing ill effects.
Brain cells are completely different. They require a constant supply of energy in the form of blood glucose and oxygen. In fact, one-third of the body’s blood flow goes to the brain to meet these constant, high metabolic requirements.
Plus, the brain has a limited capacity to store energy. In fact, its cells can’t store energy as glycogen or fat — despite the temptation to label some thinkers as “fatheads.”
That demand explains why the brain can only go without blood, glucose and oxygen for about two minutes. (It’s not long. But certainly more than the two seconds typically shown in Hollywood crime dramas, and as commonly noted by real forensic pathologists.) After two minutes without blood flow to the brain, you lapse into unconsciousness, coma — and ultimately — brain death.
Plus, as the new study suggests, when the brain gets a mental workout, it senses that it may soon require more calories to keep going. So — apparently — it stimulates hunger, even when there has been little physical activity to account for caloric expenditure.
This process may partly account for the weight gain so commonly seen in college students after they first arrive on campuses. (At least at campuses where professors still require independent thinking and students still spend time studying.)
Exercise lessens post-study food binges
Scientists at the University of Alabama at Birmingham, together with another institution, experimented with exercise to try to counter post-study food binges on the college campus.
Dr. Gary Hunter, an exercise physiologist who oversaw the study, noted that strenuous physical activity increases the amounts of glucose and lactase circulating in the blood. (Lactase is a metabolic byproduct of intense muscle contractions when muscle activity temporarily exceeds the available oxygen, and they switch to “anaerobic” metabolism, without oxygen.)
Physical activity also increases blood flow to the head and brain. Because the brain uses glucose and lactate as fuel, researchers wondered whether the increased flow of fuel-enriched blood during physical exercise would feed an exhausted brain and reduce hunger.
Students ate less after exercise
For the study, researchers recruited 38 healthy college students to the U of Alabama exercise lab to determine their physical fitness and metabolic rates. They also reported their favorite foods, with pizza at the top of the list.
(Note: pizza was also on the top of the list way back in the mid-1980s when I did my first studies on carotenoids at the National Cancer Institute with the USDA on University of Maryland students. That study first documented the role of lycopene and lutein in food composition, human nutrition, and human metabolism. The students’ lycopene levels were off the charts due to their fondness for foods like pizza with tomato paste and foods consumed with tomato ketchup.)
For the new study, students sat quietly for 35 minutes before eating as much pizza as they wanted to establish a baseline level of “normal” consumption. At a later date, the students returned and spent 20 minutes answering selected questions from college and grad school entrance exams, which had been used to induce mental fatigue and hunger in other studies.
Next, half the students sat quietly for 15 minutes before being offered pizza. The rest of the students spent those same 15 minutes performing physical exercises on a treadmill. They ran for two minutes then walked for two minutes and repeated it five times. This sort of brief but intensive routine prompts release of glucose and lactate into the blood. Following exercise, these students were also offered pizza.
By and large, the exercise group did not overeat and on average consumed about 25 calories fewer than they had during the baseline study. Plus, when factoring in the calories expended by running, the exercise group consumed about 200 fewer total calories after their brain workouts, compared to the resting, non-exercise group.
The non-exercise group consumed about 100 more calories than they did at baseline.
So now you know that “The Thinker” — the famous sculpture by 19th century artist Auguste Rodin — is actually doing real work. To avoid overeating after periods of heavy thinking, go for a walk or engage in some other light physical activity. You may find you’re less hungry the rest of the day.
- “Exercise following Mental Work Prevented Overeating.” Med Sci Sports Exerc. 2016 Sep; 48(9):1803-9