Philip J. Goscienski, M.D.
Anyone who has run a marathon or even half that far has probably experienced a sudden letdown — a wave of fatigue and loss of energy that usually occurs after about two hours. Sometimes runners find it impossible to continue. In the most severe instances they might experience hallucinations. It's due to the depletion of glycogen, sometimes called animal starch. Glycogen consists of long chains of glucose, a form of sugar that is stored mainly in the liver and muscles.
Plant foods consist primarily of carbohydrates that are broken down into simple sugars like glucose and fructose. The latter is converted to glucose as well. Glucose that is not used immediately for energy is combined into long chains and stored as glycogen. When the capacity to store glycogen is full the body converts the excess glucose into fat.
Glucose is the primary fuel for prolonged physical activity. Most of us can accumulate about a pound of glycogen, enough to last for a couple of hours of running at a moderate pace. When the body runs out of glycogen it begins to burn fat for energy but that transition usually takes at least several minutes, often longer. Without rest or replenishment of glucose the race is usually over; only the best-conditioned athletes can make it past the wall.
Persons who go on a very low-carbohydrate diet such as Atkins quickly deplete their glycogen stores. Every ounce of glycogen used up requires about two ounces of water for processing that leaves the body via the kidneys. That's why low-carb dieters find such satisfaction in losing a couple of pounds during the first couple of days on the diet. They haven't lost an ounce of fat but they've gained some bragging rights.
Carbohydrate loading consists of eating plenty of foods like rice, pasta and potatoes in the 3 or 4 days prior to a long race. Sugary desserts and soft drinks have plenty of carbohydrates but have no other nutrients and are poor choices.
Runners who drink too little water during a race aggravate the problem by becoming dehydrated and losing precious minerals such as sodium and potassium. By taking advantage of sports drinks that contain glucose as well as minerals it's possible to avoid dehydration as well as collapse at the wall.
Casual exercisers will likely never hit the wall but it's a good idea to keep up with fluid intake at all times.
Philip J. Goscienski, M.D. is the author of Health Secrets of the Stone Age, Better Life Publishers 2005. Contact him at email@example.com.