Rev Up Your Metabolism: Burn Fat All Day Long
If you ask anyone while they are exercising why they are doing it, the most common answer may be “because I am burning calories!” The combination of the caloric expenditure from exercise plus a healthy caloric consumption is the enticing motivation to the exerciser who is seeking optimal weight loss. However, exercise physiologists have found that there is an added bonus to exercise, known as the “Exercise Afterburn.” Research indicates that certain factors can lead to an increase in post-exercise caloric expenditure, although little data exist on whether the calories burned are from fat or carbohydrates.
Exercise afterburn, also referred to as excess post-exercise oxygen consumption (EPOC), is the number of calories that are expended after an exercise session ceases. During exercise, there is an increase in oxygen consumption (known as VO2) to support the body’s energy needs during the exercise session. EPOC represents the increased VO2 after exercise. This metabolic process was originally known as the “oxygen debt.” Oxygen debt (now known as EPOC) is the amount of extra oxygen required by muscle tissue to oxidize lactic acid and replenish depleted ATP and phosphocreatine following vigorous exercise. It represents replenishment of the oxygen store that was depleted during the time that oxygen uptake from the air was inadequate for aerobic metabolism. The human body usually takes several hours to fully recover to a resting state. Some studies showed that resting metabolic rate (RMR) did not return to a pre-exercise state for two hours after strenuous recreational sports, six hours after a one hour of aerobic class, and 36-48 hours after very strenuous cardiovascular exercise. Research has found that EPOC depends on the intensity and duration of exercise, as well as other influencing factors such as fitness level and gender.
The Effect of Intensity and Duration of Aerobic Exercise on EPOC
Many studies have revealed that the intensity of an aerobic exercise session has the greatest influence on EPOC. As exercise intensity increases, the magnitude and duration of EPOC will increase. Studies that used low intensities such as 50 percent VO2max or less and shorter durations (30 minutes or less) have the least effect of EPOC. In fact there was an absence of a sustained increase in VO2 after exercise in these studies. The greatest EPOC was reported in studies utilizing higher exercise intensity (75%-90% VO2max). Additionally, the duration of EPOC following the highest-intensity exercise was significantly longer when compared to the lower-intensity bouts. EPOC seemed to last up to 10 hours as compared to one to three hours.
A clear relationship has been proven between the magnitude of EPOC and both intensity and duration of exercise. A curvilinear relationship exists in exercise intensities of 30 percent VO2max or less and EPOC. At these lower exercise intensities, EPOC lasted up to 80 minutes, but no longer. It appears that in order to induce an EPOC lasting for several hours after exercise, exercise intensities need to be greater than 50 percent of VO2max. At exercise intensities at or above 50 percent VO2max, there is a linear relationship between the magnitude of EPOC and the duration and intensity. The effect of EPOC has been seen to range anywhere from 11 to 32 L of oxygen (55.5 – 160 calories) after 20 minutes to 76 minutes of exercise at 70 percent of VO2max. The longest effect of EPOC duration in these studies was 10 hours post exercise.
The duration of an aerobic exercise session also affects EPOC. The current research shows that a direct relationship also exists between the duration of exercise and EPOC. Evidence has accumulated to suggest an exponential relationship between the magnitude of the EPOC for specific exercise durations lasting longer than 30 minutes. Furthermore, work at exercise intensities >50-60 percent VO2max stimulate a linear increase in EPOC as exercise duration increases. EPOC values of 6.6 L (33 calories over 128 minutes post exercise), 14.9 L (74.5 calories over 204 minutes post exercise) and 33 L (165 calories over 455 minutes post exercise) have been reported for durations of 30, 45 and 60 minutes. Other studies have shown that EPOC values following a 60-minute exercise duration were 8.6 L (43 calories), 9.8 L (49 calories) and 15.2 L (76 calories). These research findings suggest that given sufficient aerobic exercise intensity of greater than 50 percent VO2max (closer to 70%), exercise duration is an important factor influencing EPOC. Several studies have investigated the effects of combining high intensity with even longer duration closer to 80 minutes in a single exercise session. In these studies the subject’s VO2 was still elevated by an average of five percent for 24 hours post-exercise.
Since there is an exponentially linear relationship between exercise intensity and EPOC, it is not unexpected that at supra-maximal intensity levels, EPOC values are even higher. Brief intermittent bouts of fatiguing supra-maximal exercise, such as one-minute cycling sprints at intensities over 100 percent VO2max combined with three minutes rest periods of cycling at a lower intensity around 60-70 percent VO2max, elevated metabolism for four hours post-exercise.
Another study found post-exercise energy expenditure to be elevated for nine hours when performing 20 intervals on a treadmill consisting of one-minute sprints at 105 percent VO2max with a two-minute jogging pace. This type of training is known as high-intensity interval training (HIIT). High-intensity interval training involves alternating between very high intense bouts of exercise with lower intensities. It appears HIIT is an effective way to increase EPOC values for an extended period of time post-exercise. Although is has not been identified as to whether the increased energy expenditure post-exercise comes from fats or carbohydrates, it is well known that during exercise the proportion of each nutrient is dependent upon the intensity of the exercise. Many studies examining HIIT training have reported an increase in the muscle mitochondrial enzymes, citrate synthase and cytochrome c oxidase, which are responsible for beta oxidation (fat oxidation) within the muscle. The HIIT training significantly increased muscle mitochondrial oxidative enzyme citrate synthase and beta-hydroxyacyl CoA dehydrogenase, inducing a distinct increase in fatty acid oxidation. So perhaps, the extended caloric expenditure would also be from fat oxidation post exercise if performing a HIIT program.
Split Exercise Sessions Versus Single Continuous Exercise
Several studies have shown a higher EPOC value after a split exercise session compared with a single continuous exercise session. One particular study found a significantly greater EPOC following a split bout of exercise (two 25-minute sessions at 70% VO2max) when compared to a continuous bout (a 50-minute run at 75% VO2max). The EPOC values for the split exercise session were almost double that of the single session (3.1 L for 15.5 calories versus 1.4 L for 7 calories). Another study compared EPOC after 30 minutes of continuous exercise versus two 15-minute exercise bouts. The split exercise elicited a sum of EPOC of 7.4L (37 calories) versus 5.3 L (26.5 calories) following continuous exercise. Hence, the total EPOC is significantly greater in a split aerobic exercise session versus one single cardio session. This may help some fitness and figure competitors understand the importance of their double cardio sessions!
EPOC and Resistance Training
Heavy resistance training has shown to produce the largest EPOC response compared to lower-intensity strength training. Resistance training utilizing lower repetitions (6-8) and heavier volumes such as 80-90 percent of 1 repetition maximum workload elicited a greater EPOC than higher repetition, lower volume training. The effect of resistance training compared to aerobic exercise on EPOC has also been examined to see which produces a larger and longer lasting energy expenditure post exercise.
Although some may say that comparing these two styles of exercise training is like comparing apples to oranges, most of the studies performed have indicated that resistance training produces a greater EPOC response over aerobic training. In one study, a higher EPOC was found after intense resistance training (50 sets of 8-12 repetitions at 70% 1RM) compared to aerobic cycling at 50 percent VO2max for 60 minutes. In this particular study, the resting metabolic rate was still elevated almost 15 hours after the resistance training was completed. Another study showed the difference in EPOC between aerobic cycling (40 minutes at 80% heart rate max), circuit resistance training (4 sets, 8 exercises, 15 reps at 50% 1RM) and heavy resistance training (3 sets, 8 exercises, 3–8 reps at 80%–90% 1RM). The heavy resistance training produced the greatest EPOC compared with circuit training and cycling. A third study found that resting metabolic rate remained elevated for 48 hours after a moderate- to high-intensity weight-training workout. These results were determined to be the result of protein degradation and tissue repair. Hence, it is difficult to compare resistance training to steady-state aerobic exercise since it is not practical to measure the energy cost of the resistance training with indirect calorimetry.
EPOC and Fitness Level Training Status
The training status of an individual may also have an effect on EPOC. It is suggested that highly fit trained individuals recover from exercise faster than their untrained counterparts. Not only does their “oxygen debt” return to baseline faster, but so do their hemodynamic responses as well. Heart rate and blood pressure of a trained individual return to baseline values much faster than those of an untrained person. It is very difficult to equate exercise intensity and total work performed for trained and untrained individuals. If exercise intensity and work rate were relative to fitness level, the trained individual would be working at a higher intensity than the untrained individual. Several studies have reported a faster decline in EPOC and a shorter duration of EPOC in trained subjects versus untrained subjects. However, even though people with higher fitness levels appear to return more quickly to a pre-exercise state, the magnitude of their EPOC is much larger due to their higher training intensities and longer exercise duration. So a person with a higher fitness level may have a larger EPOC than an untrained person, but the duration of the post-exercise energy expenditure will be longer in an untrained person.
Influence of Gender on EPOC
Gender is another factor that may influence EPOC. Research shows that energy expenditure in women at rest and during exercise varies with the menstrual phase. Basal metabolic rate has been shown to be lowest one week before ovulation and highest during the 14-day luteal phase following ovulation, affecting EPOC accordingly. When comparing EPOC between men and woman, the same dilemma may arise when comparing trained versus untrained. Gender effects basically come down to intensity and duration of the exercise as well, so many studies show men with a larger EPOC effect over women. In short, the gender effect on EPOC is still unclear due to the difficulties in matching work rates for gender differences.
Possible Mechanisms for the Prolonged EPOC Response
As this article stated in the beginning, there is little existing data on whether the calories expended during EPOC are from fat or carbohydrates. However, we do know that during exercise the crossover concept exists, where there is a shift in the substrate source from carbohydrate to fatty acids during prolonged exercise. There is significant evidence showing an increase in the rate of the energy-requiring triglyceride/fatty acid cycle after prolonged exercise. Since the energy equivalent of oxygen is lower with fat as the substrate as compared to carbohydrates (fatty acids = 4.7 mol ATP/mol oxygen whereas CHO – 5.1 mol ATP/mol oxygen) part of the EPOC can be explained by the substrate shift. This substrate shift could account for 10-15 percent of the post-exercise EPOC response. The reason why glycogen may not be as greatly considered as the EPOC component after aerobic exercise is because glycogen synthesis is low during fasting and no EPOC differences have been found in the fasted versus fed states. The explanation for this theory may have to do with the timing of when food is given post exercise and the ratio of nutrients being fed post exercise. It has been suggested that when food is given during the first 60 minutes of exercise recovery, a synergistic interaction of food and post-exercise replenishment on energy expenditure exists.
EPOC and Weight Management
The effect of the body continuing caloric expenditure after exercise plays a complementary role to an exercise program focusing on weight management. The research suggests that a high-intensity, intermittent-type of training (high-intensity interval training) has a more pronounced effect on EPOC. Also, research has shown that resistance training produces greater EPOC responses than aerobic exercise. It is also suggested that high-intensity resistance exercise disrupts the body’s homeostasis more than aerobic exercise. The result is larger energy requirement needs to be consumed after exercise to restore the body’s metabolic systems to baseline and thus an explanation for the higher EPOC. The underlying mechanisms that cause the higher EPOC observed in resistance exercise include elevated blood lactate, and an increase in circulating catecholamines (epinephrine and norepinephrine) and hormones such as insulin, cortisol, adrenocorticotropic hormone (ACTH). It may appear that EPOC accounts for post-exercise expenditure of somewhere between 50-150 kilocalories. Since a pound of fat is equal to 3,500 kilocalories, the effect of EPOC on weight control must be regarded in terms of a cumulative effect over time.
In conclusion, it appears that much of the conflicting data on EPOC is dependent upon the intensity and duration of the exercise. As a result, smaller and shorter lasting EPOC values are found with lower intensity and shorter duration, whereas the most pronounced effect on EPOC stems from training protocols of high intensity and longer durations. This theory applies to not only aerobic exercise but resistance training as well.
By Tracey Greenwood, Ph.D.
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