A study using a room calorimeter (picture, right) finds that exercising before breakfast increases fat oxidation during and in the 24 hours following the exercise bout.
Excessive post-exercise consumption, or EPOC, refers to an increased oxygen uptake in the minutes and hours following intense anaerobic or aerobic exercise. EPOC encompasses the term "oxygen debt", which is often used by athletes, but not exercise physiologists. EPOC refers to a variety of physiological functions that body performs following exercise. Immediately following a bout of exercise oxygen consumption remains noticeably elevated due to thermoregulation, an elevated heart rate, lactate oxidation and hormone. Oxygen consumption continues to be raised for up to 24 hours after the exercise bout as the body begins repairing structurally damaged muscle and replenishing lost glycogen.
The body prefers carbohydrate sources during intense exercise because they are more oxygen efficient. In other words, carbohydrates require less oxygen consumption per unit of energy than fats or protein energy sources. The body's carbohydrate sources are primarily located in the liver and muscle as glycogen. However, fat represents a far more abundant source of energy. When glycogen stores are depleted the body begins oxidizing fat for energy. Interestingly, elite endurance athletes begin fat oxidation much sooner during a bout of exercise. This is believed to be an attempt by the body to stave off complete depletion of carbohydrates.
The body's energy sources can be calculated by measuring inspired and expired gas composition. The ratio of CO2 expired to O2 consumed is called the respiratory quotient. The respiratory quotient is about 1.0 for carbohydrates, 0.8 for protein and 0.7 for fats. Protein breakdown results in nitrogen waste, which can be measured with the respiratory quotient to determine substrate mixture used for energy. Furthermore, total energy utilization can be measured used a room calorimeter (shown in the picture above). A recent used these techniques to look at subjects energy expenditure and energy sources following exercise (Effects of post-absorptive and postprandial exercise on 24 h fat oxidation, 2013. Shimada K, et al.).
The aforementioned study found that fat oxidation increases when exercise is performed in the post-absorptive state, or after no more glucose is available. This occurs after a fast, such as in the morning before breakfast. Participants were put in a room calorimeter were energy expenditure, and energy utilization was measured. Participants cycled on an ergometer for an hour at 50% of VO2 max. In one trial the participants exercised before breakfast and in the other they exercised after breakfast. As expected, carbohydrates were primarily used in participants who exercised after breakfast while fats were used by those who exercised before breakfast. Those who exercised before breakfast had a negative carbohydrate balance. The figure below demonstrates this relationship.
This finding suggests that exercising before breakfast leads to more fat oxidation, which may help with weight loss. In addition, the increase in fat oxidation is the result of enzymatic changes that can have long lasting effects. Increases in fat oxidizing and fat transporting proteins have been observed in carbohydrate depleted subjects following exercise (Effect of training in the fasted state on metabolic responses during exercise with carbohydrate intake, 2008. Bock KD, et al.). This increase in energy utilization following exercise stems from the EPOC effects mentioned above: muscle repair and glycogen replenishment.
In terms of training, these results may suggest that training with low glycogen stores may be beneficial. Elite endurance athletes have an ability to quickly tap into their fat stores, saving precious carbohydrates during intense exercise. This prolongs or eliminates "hitting the wall". However, the jury is still out on whether or not this approach is beneficial to competitive fitness (Does training fasted make you fast? 2009. McConnell G).
In conclusion, exercising on a fast increases fat oxidation. This increase in fat oxidation is not compensated for in the 24 hours post exercise session. This suggests that superior weight loss can be achieved by exercising on a fasted body. Now that is some motivation for getting up early to get a work out in!