Fuel for Your Engine

by Daniel Woronow, MD, September ’03

Running requires fuel, just as an engine requires coal or gasoline. The body’s two principal fuels are derived from either glucose or fat. Just as an engine must have oxygen to burn coal or gasoline, your muscles require oxygen for complete energy release from glucose or fat- related sources. Oxygen enters the blood through the lungs. It is then carried by other molecules within the body, and delivered to the muscles, by the cardiovascular system. Utilization of glucose can occur either with or without oxygen. Glucose utilization without oxygen is called anaerobic metabolism (“without air”). Glucose utilization with oxygen is called aerobic metabolism (“with air”), and this is much more efficient.

Anaerobic and Aerobic Metabolism

The initial phase of glucose utilization is anaerobic. Relatively small amounts of energy are produced. Pyruvate is the intermediate byproduct, and is converted to lactic acid if muscle oxygen supplies remain inadequate. Pyruvate is further metabolized aerobically, to the final end products of carbon dioxide and water, when muscle oxygen supplies are adequate. Most of the glucose energy release to the muscles occurs during this aerobic phase.

Exhaustion and “out of breath” sensations develop during anaerobic metabolism. Such intense levels of exertion cannot be sustained indefinitely. Oxygen must eventually be used to metabolize the toxic lactic acid byproducts, which accumulate during anaerobic metabolism. This is the so-called “oxygen debt,” which eventually must be “repaid” by the cardiovascular-respiratory system and liver. This is why fatigue and an increased breathing rate may be experienced for a substantial time, even after the completion of an endurance activity. The lactic acid accumulated during a marathon event may require as much as one day for repayment of the oxygen debt. Exercise training improves the body’s ability to supply oxygen to the muscles, and thus increases the ability to continue aerobic metabolism, which is more efficient than anaerobic metabolism.

Glucose and Glycogen

The preferred fuel of exercising muscles is glucose, though the muscles can also use fat sources for energy supply. The complete metabolism of glucose requires less oxygen consumption per carbon atom than that of fat metabolism. Therefore, glucose metabolism should require less breathing effort than fat metabolism. During endurance activities, glucose-based sources become depleted, and the body will rely more heavily on free fatty acid sources over the course of several hours. This may contribute to the need to breathe faster as the duration of the run increases. Physical training and conditioning enhance the ability of muscles to utilize fat sources, particularly the Type IIA or intermediate fast-twitch muscle fibers.

Most of the body’s glucose is stored in a chemical called glycogen, which is found in the muscles and liver. Energy resources from glycogen are not nearly as plentiful as those from fat. Most muscle glycogen is depleted after about two hours of sustained activity. Other glucose sources within the body subsequently become overwhelmed, and this leads to the sensation of “hitting the wall” during a long run.


The supply of glucose can be supplemented from carbohydrate, and therefore consuming a commercial sports carbohydrate product during sustained exercise can replenish glucose supplies to active muscle, and delay the need to draw on fat-based sources. One way of increasing the stores of carbohydrate is through “carbohydrate loading” before a race. There are several methods for doing this, but they generally consist of depleting glycogen stores with a prolonged workout about one week prior to a race. The carbohydrate intake is then steadily increased in the days leading to the race as the runner tapers off his mileage. There are both advantages and disadvantages to glycogen/carbohydrate loading.

Surprisingly, muscles require more energy when relaxing than when contracting. Muscles that are relatively depleted of nutrients may enter a phase of sustained contraction, which is manifest as muscle cramps.


Many exercise authorities have recommended that people who exercise should consume about 1.5 grams of protein per day for each kilogram of weight. Thus a 70 kg (154 lb) athletic person might ingest 105 grams of protein per day. This amount of protein may not be advisable for individuals with kidney, liver, or metabolic problems. Some sources of protein also include excessive amounts of fat and/or cholesterol.


A supply of water is essential in physical activity. Amazingly, a water loss of 5 percent reduces work capacity by 20-30 percent. A 5 percent water loss would be equivalent to a 4- or 5-pound water loss during an endurance activity. Additional dehydration can lead to circulatory difficulty, circulatory collapse, or heat stroke. During a marathon, runners may lose more than 11 pounds of water through perspiration. It may be helpful to weigh oneself before and after exercise so as to ensure that the body’s supply of water has been replenished.

Other Substances

The body is relatively good at conserving sodium (salt). During heavy training in hot temperatures, taking half a teaspoon of salt per liter of water may help replenish salt loss. Much has been written recently about hyponatremia, an often fatal consequence of salt loss. (See the article by Michele Burr).

Potassium losses in a healthy athlete are generally offset by a normal dietary intake. Magnesium losses are similarly low. Vitamins and minerals are often utilized at higher rates by endurance athletes, and hence it may be wise to take a daily multivitamin with minerals. More elaborate vitamin cocktails are probably unnecessary.

Daniel Woronow is a cardiologist, and runs regularly with MCRRC.