The marathon was not part of the ancient Olympics but was introduced as a special event in the 1896 games. Seventeen participants from five nations started the race but only nine finished. One American entered but did not finish. The original race was from Marathon to Athens, approximately 25 miles in length. Both men and women’s marathon will be held this weekend in Paris and participants will face the usual challenges: fatigue, heat, dehydration and a depletion of fuel.

For many years physiologists were aware that physical exercise primarily uses a combination of fat and carbohydrates as fuels. If you’re one of those people who shy away from eating carbohydrates, the marathon is not for you. And forget it if you’re into keto. A high protein diet will not get you to the 26-mile mark. And even if you make it, you may end up like Dorando Pietri who entered the stadium in the 1908 marathon, staggered and went the wrong way. What was he thinking? He wasn’t. Blood glucose is the fuel of the brain, and when you run out, your thinking is muddled.

We know where fat is; just look in the mirror, but where’s the carbohydrate? About 500 calories of carbohydrate is stored in the liver and some is floating around in the blood as glucose, but the vast majority is stored in muscle as glycogen. We now know that muscle glycogen is the primary carbohydrate utilized during long term moderately vigorous exercise. In the latter half of the 20th century, researchers in Sweden began using the biopsy needle to analyze muscle tissue. The Swedes must have had some willing subjects because being a subject in a muscle biopsy study is not for the faint of heart. The needle looks like a large nail and an incision must be with a scalpel just to enter the muscle. Once in the muscle, a small piece of tissue is removed and analyzed. Among other things, the biopsies eventually revealed the amount of glycogen in the muscle.

One early finding was that long term aerobic exercise primarily uses the slow twitch fibers; highly trained fast twitch muscles are unnecessary. Researchers determined this by measuring the glycogen in the muscle before and after a lengthy exercise session. The other important finding was that muscle glycogen is limited; it runs out. Consider the fact that as an athlete is running the marathon, they are slowly depleting their glycogen. Glycogen is not mobile; we cannot take glycogen from the arm and send it to the legs where it is needed. If it’s not in the leg, it is not available.

One might say, “What about fat, surely you don’t run out?” True, even thin runners store thousands of calories of fat. They don’t run out. Furthermore, fat is mobile. When exercise begins the body releases fat (from anywhere) into the bloodstream. When blood goes into the leg muscle, the fat is immediately taken in and metabolized. Slow twitch muscles love fat. But fat provides less energy when metabolized. Bottom line — when glycogen depletes, the athlete must slow down.

But here’s the good news; Swedish researchers found that we can increase the amount of glycogen in the muscle by manipulating diet and exercise prior to the race. One simple way is to eat a high carbohydrate diet (60-70% carbohydrate) for three days prior to the race along with rest. Some marathoners will go on a relatively hard run four days or so prior to the race, depleting their glycogen, and then eat a high carbohydrate diet. Many protocols exist and the athletes have surely tested them, but all protocols involve rest and diet.

During exercise the liver releases glucose into the blood. If muscle glycogen is limited, the muscle competes with the brain for glucose. Marathoners must pay close attention to their physical status during a marathon, monitoring themselves to determine how fast they can run. If glucose to the brain is limited, decision making is compromised. Not only does a good stock of muscle glycogen allows us to maintain running velocity, it helps us make good decisions. It’s interesting to note that the winner is always clear thinking, while those in the back are occasionally confused.

Jim Johnson is a retired professor of exercise and sport science after teaching 52 years at Smith College and Washington University in St. Louis. He comments about sport, exercise, and sports medicine. He can be reached at jjohnson@smith.edu