Just about everyone agrees that taking carbohydrates, particularly sugar, during exercise increases endurance in both humans and animals. Eating sugar preserves stored muscle sugar called glycogen. However we have to find a new explanation because recent data show that taking sugar during exercise does not preserve muscle glycogen (Sports Medicine, September 2010). The NEW most likely explanation is that during prolonged, intense exercise, you become exhausted because you cannot keep up with your requirements for oxygen. This interferes with the sodium/potassium pumps, inside cell membranes, that pump potassium into cells and sodium out of cells.

Your brain sends electrical messages along nerves to tell your muscles to contract. When the electrical message that travels along nerves reaches its connection with muscles, other electrical messages travel along muscles to cause them to contract. The electricity comes from your cells' ability to keep sodium outside cells and potassium inside cells. This is done by "pumps" in the cell membranes.

During intense exercise, how fast you can move is limited by how long it takes to get oxygen into muscles. Anything that reduces your requirements for oxygen will help you to move faster. Sugar and other carbohydrates require less oxygen than fat and protein to supply energy to your "pumps", so sugar is a very efficient source of energy for the sodium/potassium pumps during exercise. When the sodium/potassium pumps lose their efficiency from lack of oxygen, potassium leaks from cells and you can't get enough electrical current to contract your muscles with the force you need to compete. So your muscles weaken and you have to slow down.

If you want to compete in sports that last more than 45 minutes, you will probably be faster and have greater endurance if you take in sugar while you exercise. Taking caffeine with sugar during prolonged exercise increases endurance even more. We drink sugared, caffeinated soft drinks when we race, and avoid them when we are not racing. When muscles contract, they remove sugar so rapidly from the bloodstream that you do not get a high rise in blood sugar. However when muscles are not contracting, you lose this benefit and can develop a high rise in blood sugar that can damage all of the cells in your body.