The type of substrate (fuel) and the rate at which it is utilized during exercise is largely dependent on the intensity and duration of the exercise. During strenuous exercise there is an obligatory demand for carbohydrate (CHO) oxidation that must be met; fat oxidation cannot substitute. In contrast, there is an increase in fat oxidation during prolonged moderate intensity exercise as carbohydrate fuels are depleted. Fats are also more dominant in long duration exercise since fat oxidation requires more oxygen than does carbohydrate oxidation.

Fuel Selection During Exercise

Again, fats are the primary source of fuel for muscle during low-intensity exercise (70% of VO2max). Proteins contribute less than 2% of the substrate used during exercise of less than one hour's duration but may increase up to 5-15% during the final minutes of exercise lasting 3-5 hours.

At about 40% of VO2max there is a ?crossover point? where carbohydrate becomes more dominant than fat in supplying fuel to the muscle. This shift to CHO metabolism is caused by two factors: the recruitment of fast twitch fibers (which are better equipped to metabolize CHOs) and increasing levels of epinephrine in the blood (which contribute to glycogen breakdown). This shift can also be seen when examining the contribution of fat oxidation at various exercise intensities. At 20%, 50%, 80%, and 100% of VO2max, fat oxidation yields 175 kcal/min, 250 kcal/min, 200 kcal/min, and 25 kcal/min, respectively. As such, someone looking to burn more fat during exercise would want to hover in the 50% to 80% VO2max range.

As the duration of exercise increases, there is a greater contribution from fat and less from carbohydrate. This is the result of increased levels of blood lipase, an enzyme that helps break down fat, during low-intensity longer duration exercise.

Muscle Glycogen Utilization

At the onset of most types of exercise, and for the entire duration of very strenuous exercise, muscle glycogen is the primary carbohydrate fuel for muscular work. The intensity of exercise determines the rate at which muscle glycogen is used as a fuel. The heavier the exercise, the faster glycogen is broken down. Furthermore, studies have shown that glycogen is depleted faster from fast-twitch fibers, especially during interval type work .

Historically, it has been thought that increased epinephrine levels in the blood were responsible for the initiation of glycogenolysis (glycogen breakdown). However, other research has shown that the breakdown is also triggered within the muscle itself as a result of increased Calcium ions .

Maintenance of Normal Blood Glucose Levels During Exercise

As glycogen, and thus glucose, is the primary fuel sources during most exercise, it is needs to be remembered that glucose plays a primary role in supplying our vital organs with energy. As such, the body has many systems in place to maintain adequate levels of glucose in the blood during times of inadequate carbohydrate intake (starvation/fasting) and accelerated glucose removal from the blood (exercise). Blood glucose concentration is maintained through four different processes:

? Mobilization of glucose from liver glycogen stores

? Mobilization of fats to spare blood glucose

? Synthesis of new glucose in the liver from amino acids, lactic acid, and glycerol

? Blocking of glucose entry into the cell to force the substitution of fat as a fuel.

These processes are controlled by several ?slow? and ?fasting? acting hormones such as thyroxine, cortisol, growth hormone, epinephrine/norepinephrine and insulin/glucagon. Maintaining normal blood glucose concentrations is a major task when you consider that the liver may only have 80g of glucose before exercise begins, and the rate of blood glucose oxidation approaches 1g/min in heavy exercise or in prolonged (>3 hours) moderate intensity exercise.

Carbohydrate Replenishment and Diets

This is one of the main reasons why it is recommended to replenish glucose and glycogen levels during activities such as soccer games, marathons, triathlons, 3-5 set tennis match, and so forth. Each of these sporting activities require moderate to high levels of exertion for extended periods of time and thus fuel replenish, mainly via sports drinks, is essential in maintaining physical performance and skill execution.

The effect of carbohydrate intake and diet composition on performance has been heavily studied. The vast majority of studies show that a high carbohydrate diet is essential for those who engage in intense or long duration exercise. For instance, one study had trained subjects run a 30 km race twice, once following a high carbohydrate (CHO) diet and the other time after a mixed diet. The initial muscle glycogen level was 3.5g/100g of muscle following the CHO diet and 1.7g/100g of muscle during following the mixed diet. The best performance of all subjects occurred during the high CHO diet. Although the starting pace was not faster, the additional CHO allowed them to maintain the pace for a longer period of time .

Bear in mind that substrates and hormones can interact and alter certain processes. This is commonly seen when a high glycemix index CHO food is ingested just prior to exercise. In this case, the resulting spike in blood glucose elicits a rise in insulin levels. As a result, fat mobilization is reduced (due to insulin's anabolic effect on fats) forcing the muscle to use additional muscle glycogen. This is not desired if you are looking to sustain a given exercise intensity for prolonged periods of time.

Putting It All Together

The common misconception is that to lose weight you must stay in your ?fat burning zone?. Eventhough there is some truth to this statement, it is inaccurate in the sense that losing weight is about burning as many calories as possible. As such, exercising at a higher intensity for moderate durations would be most effective in burning calories. It is important to remember that a calorie is a calorie regardless of its source and since 1 lb of fat is equivalent to 3500 calories, calorie-reducing exercise regimens are most suitable for those looking to lose weight.

Such exercise programs can take the form of interval work, where a bout of high-intensity exercise is combined with a bout of low-intensity. This type of training of has been shown to elevate the body's metabolism for up to 16 hours post-exercise. It also fosters the maintenance of lean body mass while stimulating the release of lipase ? a good thing since maintaining muscle and losing fat should be the goal. Having said this, I would also recommend implementing longer duration low-intensity exercise as well for it's aforementioned benefits on fat oxidation. However, ensure to keep the intensity in the 50-80% VO2max range for best results!

References:

Essen et al. (1978). Glycogen depletion of different fiber types in human skeletal muscle during intermittent and continuous exercise. Acta Physiologica Scandinavia, 103: 446-55.

Hultman et al. (1967). Physiological role of muscle glycogen in man with special reference to exercise. In Circulation Research XX and XXI, ed. C.B. Chapman, 1-99 and 1-114. New York: The American Heart Association.

Karlsson, J. & Saltin, B. (1971). Diet, muscle glycogen and endurance performance. Journal of Applied Phsyiology, 31: 203-206.