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  #1  
Unread 03-26-2008, 12:35 PM
CZa CZa is offline
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Default HIIT - Evidence?

I keep seeing people say how HIIT burns calories for a while after exercise, but I've never seen any proof of this claim. It seems like it's just Chinese whispers or something.

Has this ever been studied (ie. HIIT vs. steady state training)?
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  #2  
Unread 03-26-2008, 12:49 PM
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lylemcd lylemcd is offline
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there's lots of data on EPOC, I reviewed a massive review paper in my newsletter (reprinted below).

whie there is a large epoc after HIIT in terms of %age, the absolute value is still quite small. this is because the total calorie burn of most interval sessions is usually small as well. 20% of 100 calories still doesn't amonut to much and may be less than 5% of 600 calories.

in either case, EPOC makes up the minor portion of the claorie burn.

***
LaForgia J et. al. Effects of exercise intensity and duration on the excess post-exercise oxygen consumption. J Sports Sci. 2006 Dec;24(12):1247-64.
Recovery from a bout of exercise is associated with an elevation in metabolism referred to as the excess post-exercise oxygen consumption (EPOC). A number of investigators in the first half of the last century reported prolonged EPOC durations and that the EPOC was a major component of the thermic effect of activity. It was therefore thought that the EPOC was a major contributor to total daily energy expenditure and hence the maintenance of body mass. Investigations conducted over the last two or three decades have improved the experimental protocols used in the pioneering studies and therefore have more accurately characterized the EPOC. Evidence has accumulated to suggest an exponential relationship between exercise intensity and the magnitude of the EPOC for specific exercise durations. Furthermore, work at exercise intensities >or=50-60% VO2max stimulate a linear increase in EPOC as exercise duration increases. The existence of these relationships with resistance exercise at this stage remains unclear because of the limited number of studies and problems with quantification of work intensity for this type of exercise. Although the more recent studies do not support the extended EPOC durations reported by some of the pioneering investigators, it is now apparent that a prolonged EPOC (3-24 h) may result from an appropriate exercise stimulus (submaximal: >or=50 min at >or=70% VO2max; supramaximal: >or=6 min at >or=105% VO2max). However, even those studies incorporating exercise stimuli resulting in prolonged EPOC durations have identified that the EPOC comprises only 6-15% of the net total oxygen cost of the exercise. But this figure may need to be increased when studies utilizing intermittent work bouts are designed to allow the determination of rest interval EPOCs, which should logically contribute to the EPOC determined following the cessation of the last work bout. Notwithstanding the aforementioned, the earlier research optimism regarding an important role for the EPOC in weight loss is generally unfounded. This is further reinforced by acknowledging that the exercise stimuli required to promote a prolonged EPOC are unlikely to be tolerated by non-athletic individuals. The role of exercise in the maintenance of body mass is therefore predominantly mediated via the cumulative effect of the energy expenditure during the actual exercise.


My comments: In the last year or three, exercise programs for fat loss such as Alwyn Cosgrove's Afterburn, Craig Ballantyne's Turbulence Training and others have been geared around the concept of using certain types of training (either interval style cardio or highish rep/short rest weight training) to cause fat loss through an 'afterburn' effect where calories are burned after workouts to a greater degree than following standard training styles (esp. low intensity cardio). There's little to no doubt that these programs work (I'll come back to this at the end of the review) but this review paper raises the issue of how significant an impact the post-exercise calorie burn (called EPOC which stands for excess post-exercise oxygen consumption) actually is under most circumstances.


The first topic discussed is what EPOC actually represents. An outdated concept is that the post-exercise calorie burn represented an 'oxygen debt' representing the difference between what the body needed and what was available, this turns out to be simplistic and wrong. Lactate metabolism, phosphate resynthesis and fatty acid cycling, along with increases in catecholamine levels are likely the cause of the post-exercise calorie burn. Ultimately, the mechanisms are less important than the fact that EPOC is the result of a metabolic perturbation that has to be repayed afterwards.

I'm not going to detail the next section of the paper as it dealt with a bunch of boring methodological issues. Sufficed to say that accurate measurement of EPOC requires that certain methodologies be adhered to. One huge confound, which is likely the cause of the 'exercise raises metabolism for 24 hours' thing is food consumption. It's easy to mistake the thermic effect of eating with an effect of exercise. Good studies take this into account. Othe issues such as taking into account baseline metabolic rate and subject characteristics are also important.

The next section of the paper deals with continuous exercise and the impact of both duration and intensity on EPOC. Without going into every paper detailed in the review, the picture that has developed from the research is that EPOC goes up linearly with increasing exercise duration but exponentially with increasing intensity. That is, higher intensity exercise generates the higher EPOC. This is true if the duration is the same or if the same number of calories are burned. That is, if two people both burned 300 calories during exercise but one exercised at a high intensity and one at low intensity, the high intensity guy would get about double the EPOC. The problem is that, even under these conditions, the EPOC is still pretty minimal. In one study, subjects who exercised for 80 minutes at 70% VO2 max (about 80% of maximum heart rate) had an EPOC lasting 7 hours. But it only amounted to about 80 calories extra burned. Not to mention that only the most well trained individuals could sustain such a workload in the first place.

Additionally, it appears that there is an intensity threshold to generate any EPOC at all, compared to exercise at 30-50% VO2 max (50% VO2 is about 65% of max HR or the typical 'fat burning' zone), exercise at 75% generates a larger EPOC. However, the total calorie burn is still relatively small overall, averaging perhaps 7% of the total energy burned. So if you burn 600 calories with high intensity continuous exercise, you might burn an additional 45 afterwards. While this certainly adds up over long periods of time, it's still relatively insignificant compared to the total energy expenditure of the exercise bout.

The next section of the paper dealt with suprmaximal work, intervals basically. Interestingly, the data available here finds that relatively short amounts of intervals can generate EPOCs comparable to much longer bouts of continuous exercise. Several studies measured EPOCs from relatively short interval workouts on par with studies using much longer (>50 minutes) of moderate intensity work. Still, the total magnitude of the EPOC was relatively small, equal to roughly 13% of the total energy used during the exercise bout. So while the relative amount of calories burned after interval training is larger, the total amount is still small. In one study, subjects ran 20X1 minute intervals above VO2 max with a 2' rest between. While the EPOC was about double that found in subjects who performed 30' at 70% Vo2 max, the total EPOC was only about 32 calories (135 kJ).
The next section of the paper dealt a little more with the issue of exercise duration as studies have identified an increase in EPOC with increasing durations. However, the effect is only significant for exercise performed at intensities greater than 50-60% VO2 max (60-72% max heart rate). However, unless folks are willing to do 60-90 minutes+ of training, this still doesn't amount to very much in absolute terms. This is especially true of lower intensity exercise where prolonged durations of 90' or more are necessary to generate a prolonged EPOC; even there the absolute magnitude of calories burned is still small.
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Unread 03-26-2008, 12:49 PM
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Finally the paper examines the impact of resistance training on EPOC. A number of studies have been performed and found fairly prolonged durations of EPOC (15-38 hours) and an increase in metabolic rate of 9-11% over that time period. However, many of the studies used horribly unrealistic numbers of sets (60 sets of 8-12 in one study, 30 sets in another). Interestingly, a study of women found a much shorter duration of EPOC (60-90 minutes); the reasons for this are unknown. Perhaps the most interesting study was the one using a relatively low volume of training (4 exercises for 4 sets each) in experienced lifters; in that study metabolic rate was significantly elevated for nearly 48 hours after lifting. The paper points out that the average person is unlikely to be able to sustain either the volumes (30-60 sets) or intensities used in these studies.


The paper concludes that, despite the variability in studies, the intensity of exercise appears to be of the utmost importance in terms of generating an EPOC. However, most studies indicate that the total magnitude of the EPOC is unlikely to be very large. With interval type training, EPOC may approach 14% of the total energy expended but, generally speaking, interval training doesn't burn as many calories during the bout so while the relative amount may be larger, the total EPOC is still small. For submaximal work, an EPOC of 7% is roughly the average.


This doesn't really amount to much not to mention that, outside of trained individuals, most folks couldn't sustain the durations (90'+) or intensities (80% maximum heart rate for steady state work or supramaximal intervals) required to generate much of an EPOC. I would note that even beginners can work up to that level with a properly set up progressive program. One beef I tend to have with many exercise and fat loss studies is that the intensity or duration of the exercise is never increased as the folks become fitter. But that's a separate topic for another day.
The paper suggests that focusing on maximizing the calorie burn of the exercise bout itself and issues of compliance should be the primary goal. Because even if you burn a few extra calories after the exercise bout, if you increase how many calories you burn with exercise by a couple of hundred, that couple of hundred will have a much larger impact than the 15 extra you burn because of it.


But here's the thing, there seems to be a disconnect with the conclusion of this study and the results people are reporting with interval based types of fat loss programs.


Even looking at the original Tremblay interval study, where EPOC was unfortunately not measured, fat loss was significantly greater for the interval group despite a massively lower time investment and calorie expenditure. Something is going on.


It may be, and I suspect that it is, that EPOC is only part of the picture. Studies have found that interval training may increase enzymes involved in fat utilization more effectively (or at least more quickly than steady state exercise). A followup study by Tremblay found that to be the case and a very recent study found that only 2 weeks of interval training had a fairly significant impact on whole body and skeletal muscle capacity for fatty acid oxidation at rest.


This is assuredly mediated through both effects on gene expression as well as the glycogen depletion that occurs with high intensity activities; glycogen depletion itself enhances full body fat oxidation. Frankly, irrespective of EPOC and what happens during the exercise bout, if you increase the body's utilization of fat for the other 23 hours of the day you aren't exercising, that's a good thing from a fat loss perspective. Coupled with a calorie reduced/controlled diet, enhancing fatty acid oxidation during the day goes a long way towards explaining enhanced fat loss.


Another possibility, implied by the Tremblay study is that interval type training programs are generating some muscle growth. I say implied because the original study found less of a change in total bodyweight than the change in fat; that suggests that muscle was gained. Given the caloric cost of synthesizing muscle, that would give a 'sink' for incoming calories.
Of course, as the volume of training (number of intervals, number of exercises/supersets in interval based weight training programs goes up), so does the caloric expenditure of the bout itself. That's in addition to whatever small extra impact that you may get from the EPOC.
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  #4  
Unread 03-26-2008, 03:07 PM
CZa CZa is offline
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In the Tremblay study, what was the training level of the subjects (untrained, trained, athletes, etc.)?
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  #5  
Unread 03-26-2008, 03:34 PM
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beginners, of course

***
Metabolism. 1994 Jul;43(7):814-8. Links
Impact of exercise intensity on body fatness and skeletal muscle metabolism.

Tremblay A, Simoneau JA, Bouchard C.
Physical Activity Sciences Laboratory, Laval University, Ste-Foy, Quebec, Canada.
The impact of two different modes of training on body fatness and skeletal muscle metabolism was investigated in young adults who were subjected to either a 20-week endurance-training (ET) program (eight men and nine women) or a 15-week high-intensity intermittent-training (HIIT) program (five men and five women). The mean estimated total energy cost of the ET program was 120.4 MJ, whereas the corresponding value for the HIIT program was 57.9 MJ. Despite its lower energy cost, the HIIT program induced a more pronounced reduction in subcutaneous adiposity compared with the ET program. When corrected for the energy cost of training, the decrease in the sum of six subcutaneous skinfolds induced by the HIIT program was ninefold greater than by the ET program. Muscle biopsies obtained in the vastus lateralis before and after training showed that both training programs increased similarly the level of the citric acid cycle enzymatic marker. On the other hand, the activity of muscle glycolytic enzymes was increased by the HIIT program, whereas a decrease was observed following the ET program. The enhancing effect of training on muscle 3-hydroxyacyl coenzyme A dehydrogenase (HADH) enzyme activity, a marker of the activity of beta-oxidation, was significantly greater after the HIIT program. In conclusion, these results reinforce the notion that for a given level of energy expenditure, vigorous exercise favors negative energy and lipid balance to a greater extent than exercise of low to moderate intensity. Moreover, the metabolic adaptations taking place in the skeletal muscle in response to the HIIT program appear to favor the process of lipid oxidation.
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