EXPENDING CALORIES AFTER EXERCISE?
Excess postexercise oxygen consumption (EPOC), often referred to as the “afterburn,” is a highly debated topic in the field of exercise science. After an exercise bout, energy expenditure (EE) remains elevated while the body slowly returns to homeostasis. This is identified through an elevated oxygen consumption (VO2) above preexercise resting VO2 that typically remains elevated for 1 to 2 hours with usual moderate-intensity exercise. The excess oxygen is used to create the energy necessary to restore the body after exercise. Current fitness trends emphasize the concept of increased EPOC to create exercise programs that maximize EE during recovery, often done with the intent to aid in weight loss and weight maintenance.
When investigating EPOC claims, one must consider that EPOC only consists of EE above resting levels. For example, the average resting VO2 is roughly 3.5 ml/kg/min. If we convert this to an absolute value, assuming an individual is 70 kg, the average resting VO2 is roughly 250 ml/min. Consequently, EPOC consists of an elevated VO2 above 250 ml/min. With this in mind, although VO2 after exercise is 350 ml/min, the resultant EPOC is 100 ml/min.
Although the application of EPOC to maximize EE has become popular in the past two decades, the concept dates back to the early 1900s and the work of Hill et al. (1). They termed this concept “oxygen debt” and attempted to link postexercise VO2 to lactate metabolism and phosphagen restoration, resulting from the oxygen deficit created at the onset of an exercise bout (1). Oxygen debt was further defined by the work of Margaria et al. (2), who split the concept into a fast (alactacid) and slow (lactacid) component. Over the subsequent 50 years, many studies were conducted on the topic, leading to the seminal review by Gaesser and Brooks in 1984 (3), suggesting that the phenomenon was too complex and multifaceted to be termed oxygen debt, as it involves far more than simple repayment of the oxygen deficit. Gaesser and Brooks postulated that the variety of biochemical mechanisms contributing to postexercise metabolism are more complex than the oxygen debt terminology allowed for. From this, the term EPOC was proposed and has been used ever since.
Similar to the proposed fast and slow components of EPOC of Margaria et al. (2), current literature separates EPOC into a rapid component immediately after exercise in which EE is large but quickly drops and a slow component that extends to the point where EE returns to rest (Figure 1) (4). Although research regarding EPOC is extensive, the physiological mechanisms underlining EPOC are still under investigation. EPOC is thought to be related to phosphagen restoration, lactate metabolism, elevated body temperature, increased circulation, increased ventilation, elevated catecholamine concentration, adipocyte lipolysis, fat oxidation, and protein synthesis (3–5).
INFLUENCES ON EPOC
EPOC responses have been studied across many different modes of exercise, and regardless of mode, exercise duration and exercise intensity have been identified as the greatest contributors to EE during EPOC (4,5). It has been shown that as exercise duration increases, EPOC increases in a linear fashion (6–8). For example, the EE during EPOC after 60 minutes of exercise would be approximately double that of 30 minutes of exercise at the same intensity (9) (Figure 2). However, it appears that an exercise intensity threshold of ~50% to 60% of maximal VO2; (VO2max) is necessary to produce a meaningful EPOC (5), and below this intensity, exercise duration will not significantly contribute to EPOC. Beyond the 50% to 60% threshold, EPOC increases exponentially (6,10–13). Thus, it is generally accepted that exercise intensity is the primary contributing factor to EPOC, with duration playing an important but much smaller role. Figure 3 represents the relationship between exercise intensity in respect to EPOC.
EPOC AND AEROBIC EXERCISE
EPOC is normally reported in terms of duration (how long EE is elevated above resting values) and magnitude (the amount of EE during EPOC). Several methodological factors contribute to the variability in results across studies, including how long EPOC is measured, continuous versus intermittent VO2 measurement, direct versus estimated VO2, baseline resting VO2 measurement, and time of feeding. For example, one study may measure the full duration of EPOC (until EE returns to resting values), whereas other studies may only measure EPOC for 1 to 2 hours.
Current national physical activity guidelines recommend that individuals perform 150 minutes of moderate-intensity continuous exercise (MICE) (46% to 63% VO2max) per week (14). For 30 to 45 minutes of MICE, EPOC is estimated to last for ~45 minutes and accounts for an additional 15 to 20 kcal (5). In perspective, walking for 5 minutes at 3 mph on a treadmill would elicit the same EE. This EE is quite minimal when considering the total EE needed to create a negative energy deficit.
When investigating EPOC, EE is often not reported in kilocalories expended, but VO2 in liters. To convert liters of oxygen to kilocalories, it is assumed that 1 L of oxygen is equivalent to 5 kcal of energy.
If an individual were interested in performing continuous exercise that emphasizes the EPOC response, exercise duration would need to be at least 60 to 80 minutes, and at an intensity of ~70% to 75% VO2max. EPOC duration ranges from 7 to 24 hours with this type of exercise with an additional EPOC added of ~170 kcal (4,5,7,8,10,11). Although this may seem like a reasonable postexercise EE, one also must consider the overall EE required to have a meaningful effect on weight loss and weight maintenance and the demands of such exercise in relation to the targeted population.
To generalize the EPOC response to submaximal aerobic exercise, Laforgia et al. (5) presented the expected percent of total EE of exercise (exercise EE and EPOC EE) that EPOC accounts for, or ~7% of total EE. For example, if an individual were to perform submaximal aerobic exercise with a total EE of 300 kcal, EPOC would account for ~20 kcal of the 300 kcal. However, because of the large amount of variability associated with the EPOC responses, the accuracy of this generalized assumption must be interpreted with caution.
EPOC AND INTERVAL TRAINING
More recent EPOC research has focused on its response after interval-type training. High-intensity interval training (HIIT) uses exercise bouts ranging from 30 seconds to 4 minutes of vigorous to near maximal intensity (~90% VO2max). However, HIIT protocols can vary relative to the number of intervals, length of rest periods, and interval intensity, making it difficult to generalize HIIT as a singular mode of exercise. Tucker et al. (15) identified an EPOC EE of ~83 kcal after four bouts of cycling at 95% peak heart rate for 4 minutes with 3 minutes of active recovery. This EE accounted for ~25% of the total EE of exercise. However, one limitation of this study was that EPOC was only measured for 3 hours, which may not have captured the full EPOC duration, thereby underestimating the EPOC EE.
A substantially greater amount of research has been conducted on sprint interval training (SIT). SIT usually consists of multiple 20-second to 1-minute intervals of supramaximal exercise (≥105% VO2max) followed by 2- to 4-minute active or inactive rest periods. Earlier studies on lower intensity SIT protocols reported EPOC durations and EE of 8 hours and ~85 kcal, respectively (16,17). Laforgia et al. (5) found that EPOC contributed ~14% to the total EE of SIT-type exercise. Current research, however, using more intense protocols (e.g., consecutive Wingate cycling tests), has resulted in larger EPOC responses (15,18,19); however, EPOC was measured for different durations (30 minutes, 3 hours, and 24 hours), making it difficult to draw conclusions on EPOC duration and EE. Hazell et al. (18) found that SIT, using four consecutive Wingate tests, was associated with an EPOC duration of up to 24 hours and an EE of ~315 kcal. This is a fairly substantial EPOC response, but in regard to overall EE for exercise, submaximal aerobic exercise (30 minutes, 70% VO2max) had an equal total EE (~500 kcal). With this, the EE during EPOC only accounted for ~60 kcal for submaximal training. This presents an important point regarding SIT. Although EPOC EE may be larger, total EE for exercise (i.e., EE during and after exercise) is equal to or less than that of submaximal continuous exercise. Similar results were found by Tucker et al. (15) with a SIT EPOC EE of ~110 kcal and a total EE of ~270 kcal at 3 hours. For submaximal continuous exercise, total EE was greater (~350 kcal) than SIT, but not significantly greater than the total EE of HIIT (~330 kcal) (Figure 4).
Most EPOC literature has been centered around responses to aerobic and anaerobic training, but a growing amount of literature has been conducted investigating EPOC responses to resistance training (RT). It is well accepted that RT is of importance to energy balance via the prevention of losses in fat-free mass during energy-restricted weight loss (14). Because of the relatively low direct EE during RT compared with other forms of exercise, investigators have focused on the EE during EPOC as a means of aiding in weight loss and weight maintenance. However, components of experimental design such as the length of rest periods, speed of movement, exercise selection, quantifying exercise stimulus, and intensity during RT have made comparisons of protocols difficult from study to study (5,20). This has created a large range of reported EPOC values for RT.
Traditional RT workouts normally consist of 2 to 4 sets of 4 to 8 exercises with 8 to 12 repetitions using upper and lower body exercises at ~60% to 70% of one-repetition maximum. Studies using similar RT workouts report fairly consistent results in regard to EPOC duration and magnitude (21–24), typically with a duration of ~60 minutes with a corresponding EPOC EE of ~35 kcal. Once again, when considering the needed EE to create a meaningful energy deficit, this EPOC is minimal and would not significantly contribute to weight loss and weight maintenance. As a result, RT that focuses on hypertrophy and produces high amounts of muscular damage (emphasis on eccentric contractions) has become emphasized because of the associated greater EPOC responses (25–28). Increased muscular damage is thought to produce a larger resultant EPOC because of the increased protein degradation and synthesis during recovery (25). Hackney et al. (27) recorded an extended EPOC duration of up to 72 hours with resting EE elevated by ~9% as a result of eccentric-based training targeting increased muscular damage. This equates to an EPOC EE of ~550 kcal over the 72-hour duration. To date, this study has shown the greatest EPOC response as a result of RT. Although this type of RT has the potential to produce a significant EPOC response, the appropriateness of exercise needs to be considered for nonathletic populations. In untrained individuals, such a program may be highly impractical because of the degree of muscular damage resulting in excessive delayed-onset muscle soreness and feelings of unpleasantness. Heden et al. (28) found that one full-body set of 10 exercises at a 10-repetition maximum with an extended eccentric phase (4 seconds) elicited a similar EPOC to three sets of the same exercise and was similar to results of the study of Hackney et al. They found that resting EE was elevated by ~5% over a 72-hour period. This equates to an EPOC EE of ~300 kcal over the 72-hour duration. This may be a more reasonable protocol for the untrained, sedentary population. However, an extra 300 kcal over 3 days is a comparatively modest total EE.
FAT LOSS AND EPOC
Some emphasis has been placed on the idea of increased fat oxidation during EPOC after interval training and RT. This has been thought as potentially leading to enhanced fat loss. Although studies have shown a switch in substrate use to fat during EPOC after interval training, the relative contribution seems to be minimal and seems unlikely to be a factor in altering body composition (e.g., net fat storage) (15). Research has shown the possibility of interval training for enhancing body composition compared with MICE (29). This may involve but has not been identified as being specific to EPOC.
For EPOC to be a relevant point of emphasis in terms of weight loss, it needs to provide a considerable contribution to the EE of an exercise bout. EPOC has been shown to make a measurable, but relatively small, contribution to the overall EE of an exercise bout for most types of training. Several methodological factors play a role in EPOC studies, which make it difficult to draw firm conclusions regarding the duration and magnitude of EPOC for different types of exercise. However, our findings lead us to the following conclusions regarding the EE of EPOC:
- EE of EPOC for a typical MICE session seems to be minimal, lasting less than 1 hour and equating to 15 kcal to 20 kcal.
- EE of EPOC for HIIT and SIT is greater than MICE but in regard to total EE is not thought to contribute to energy balance any more than continuous aerobic exercise.
- EE of EPOC for traditional RT is minimal, but RT programs that focus on eliciting muscular damage (e.g., eccentric centered training) may contribute significantly to energy balance.
- Although EPOC responses for interval training and muscular damage centered RT can be significant, these types of training may be too demanding and not well tolerated for sedentary, less trained individuals.
- The focus of EE for exercise should remain on the EE during exercise rather than EPOC.
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