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A 45-Minute Vigorous Exercise Bout Increases Metabolic Rate for 14 Hours – Research Review

Knab AM et. al. A 45-Minute Vigorous Exercise Bout Increases Metabolic Rate for 14 Hours.  Med Sci Sports Exerc. 2011 Feb 8. [Epub ahead of print]

INTRODUCTION: The magnitude and duration of the elevation in resting energy expenditure following vigorous exercise have not been measured in a metabolic chamber. This study investigated the effects of inserting a 45-min vigorous cycling bout into the daily schedule versus a controlled resting day on 24-h energy expenditure in a metabolic chamber.
METHODS: Ten male subjects (ages 22 to 33 yrs) completed two separate 24-h chamber visits (one rest and one exercise day) and energy balance was maintained for each visit condition. On the exercise day, subjects completed 45-min of cycling at 57% Wattsmax (mean±SD, 72.8±5.8% VO2max) starting at 11:00 am. Activities of daily living were tightly controlled to ensure uniformity on both rest and exercise days. The area under the energy expenditure curve for exercise and rest days was calculated using the trapezoid rule in the EXPAND procedure in the Statistical Analysis Systems (SAS) and then contrasted.
RESULTS: The 45-min exercise bout resulted in a net energy expenditure of 519±60.9 kcal (P<0.001). For 14-h post-exercise, energy expenditure was increased 190±71.4 kcal compared to the rest day (P±0.001).
CONCLUSION: In young male subjects, vigorous exercise for 45-min resulted in a significant elevation in post-exercise energy expenditure that persisted for 14-h. The 190 kcals expended post-exercise above resting levels, represented an additional 37% to the net energy expended during the 45-min cycling bout. The magnitude and duration of increased energy expenditure following a 45-min bout of vigorous exercise may have implications for weight loss and management.

Background
In recent years there has been a focus on the calorie burn that occurs after training, referred to in science terms as EPOC (Excess Post-exercise Oxygen Consumption).  A variety of different types of training (usually revolving around brief duration, high-intensity methods such as interval training or circuits) have been proposed with the major effect of such activity being in the EPOC that is created.

Now, EPOC used to be thought to be related to something called the ‘oxygen debt’, essentially the difference in how much oxygen the body needed to sustain activity and how much was available.  We now know that it’s related to a host of other processes but these aren’t really that relevant practically.   What is relevant from a fat/weight loss point of view is how large the EPOC is and whether or not it contributes meaningfully to the overall caloric expenditure of an individual.

In a previous research review with the imposing title of Effects of Exercise Intensity and Duration on the Excess Post-Exercise Oxygen Consumption I examined a monster review on the topic.  In brief, looking at the data in aggregate, the study concluded that intensity was more important than duration in terms of the EPOC created.

And while the relative EPOC was higher for high-intensity activities (that is the percentage increase), the absolute level of EPOC was still pretty irrelevant  (maybe 30-50 calories).  The researchers concluded that the primary impact of exercise was still through the calories burned during the activity itself: the absolute EPOC was fairly irrelevant to the total whether an individual did long-duration low-intensity activity or short-duration high-intensity activity. Their conclusion based on review of the data was:

The manipulation of energy balance for these individuals should not be concerned with generating large EPOCs but focused on both the energy expended during the actual exercise and the design of programmes that enhance compliance.

The compliance issue is actually quite relevant given that the intensities used in the interval studies are generally exceedingly high and unlikely to be performed consistently by most people, especially beginning and/or overweight exercises (for more on this topic, please read Training the Obese Beginner series).  An added issue that I have talked about variously on the site is that many people looking for fat loss train daily; trying to perform high-intensity activity too frequently is a recipe for disaster. Read the previous review for the details, or the series on Steady State vs. Interval Training that it was a part of if you’re particularly bored.

However, several people have asked me about a recent study that seems to contradict the above conclusions, as it found that the EPOC following what was terms ‘vigorous’ activity was significant and it’s that paper I want to look at today.

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The Study

The study recruited 10 male subjects, aged 23-33 years who were capable of bicycling for 45 minutes continuously. Body composition was measured via DEXA and the subjects all underwent VO2 max testing using a fairly standard protocol.   The subjects performed two ‘exercise’ sessions.  In the first they sat quietly in a metabolic chamber (which was measuring the actual energy expenditure); in the second they performed actual exercise.

Basically, the ‘sitting quietly’ was the control condition, the researchers had to see how many calories the subjects burned at rest and in the following hours so that they could determine the difference in caloric expenditure and EPOC following the actual workout.

The exercise bout consisted of 45 minutes of cycling at 57% maximum wattage (so if a subjects Vo2 max wattage occurred at 300watts, they cycled at 171 watts), this put them at roughly 70% of their VO2 max; the duration was chosen to represent a midpoint for some rather standard recommendations for activity.  There was actually a short (4 minute) warm-up before the main set along with a short cool-down but I’m not going to bother detailing it.

Food intake was controlled (this can be a huge confound to a lot of these studies as eating raises metabolic rate and a lot of early exercise and metabolic rate studies confused the increase from eating with an increase from the exercise session); it is important to note that food intake was increased on the exercise day to maintain energy balance  That is, the subjects were not in a caloric deficit during the period following the training session.  The subjects stayed in the metabolic chamber following the exercise session and this is how the caloric expenditure following the exercise bout was determined.

For the exercise bout itself, the session burned a total of 519 +- 60 calories so about 11 calories per minute for the 45 minute exercise session.  This is consistent with it being a vigorous intensity.  And over the next 14 hours (including 3 hours of sleep), the subjects burned an additional 144+-50 calories with the majority of this occuring in the first 9 hours after training.

This EPOC represented a whopping 37% of the actual exercise bout energy expenditure, far higher than what was reported (usually 15% or less) in the LaForgia review I linked to above. And the total impact of the exercise bout was an energy expenditure of ~750 calories above and beyond the resting condition.  The researchers conclude

“The 24-h net energy expenditure difference between exercise and rest days was 750 kcal, a meaningful quantity over time if two or three such exercise bouts are inserted into the weekly schedule and energy intake is controlled.”

Fair enough.  And obviously a workout burning 750 cal/session (including the workout and EPOC) is meaningful.


My Comments

I think the first issue worth addressing is why this particular study found such a difference compared to previous studies, such as the ones reviewed in the LaForgia review I linked above.  Certainly there could be methodological differences; for example this study used a metabolic chamber to get an accurate measure of caloric expenditure, other studies using a different method (such as Douglas bags, think big balloon looking things that folks breathe in for later analysis) might be getting different measurements.

Another has to do with the fact that the subjects were deliberately kept in energy balance, and fed an additional 660 calories to offset the energy expenditure of the exercise bout.  The researchers state:

The increased energy intake balanced against energy expenditure (energy flux) has been shown in several studies to contribute to the elevated 24-h energy expenditure on exercise days or in trained individuals.

Of course this is a problem with applying this study to a dieting/fat loss situation where, by definition, you can’t be in energy balance (since you have to create a deficit by not replacing the exercise induced energy expenditure with more food).  This study needs to be re-done under conditions where energy intake isn’t increased to see if the same big increase in EPOC is seen under dieting conditions (as that would have actual application to fat loss).

The final reason that contributed is likely this: the workout tested in this study was  HARD.  As I noted above, subjects pedalled at a workrate equal to 57% of their max wattage output.  This corresponded to 70% of VO2 max which corresponds to roughly 85% of maximum heart rate.  Quite in fact, the heart rate measured during the activity was 163 +-16 beats per minute.  This would be equivalent to the Sweet Spot training that I discussed the Methods of Endurance Training series.  I’ll come back to this.

Without going into the details of what determines the EPOC, the researchers state fairly simply that:

The magnitude of post-exercise energy expenditure is greatest when the body experiences significant physiologic stress during prolonged and high intensity exercise.

Basically, through whatever specific mechanisms (and a host have been implicated), the ‘afterburn’ effect is maximized when you massively disrupt homeostasis in the body.  And this tends to occur most significantly for workouts that are a combination of high-intensity AND high-volume (i.e. long-duration).

Earlier studies tended to either look at EPOC following longer duration lower-intensity work or short-duration high-intensity work (such as interval training).   And the simple fact is that neither tend to cause the greatest homeostatic changes to the body: the longer duration stuff is too easy and the high-intensity stuff is too short.  In contrast, this workout had the subjects working pretty damn near their maximal steady state level (for most this falls around 170-180 beats per minute).  Anybody who has done this type of training can attest to the fact that it’s work.

Conceptually this is no different than what I discussed in the Reps Per Set for Optimal Growth where an intensity of 80-85% of maximum tends to put you in the range where you get both a high stimulus intensity coupled with a high-volume.  Higher intensities limit volume (both per set and per workout) and lower intensities don’t tend to have the stress associated with it even if you do more total volume.  Somewhere in the middle you get that maximal response where you combine high-intensity AND a high-volume.

In that vein, having done all three types of workout (long-durations at piss easy intensities, interval work of varying durations, long sessions at sweet spot), by far and away the sweet spot training is the hardest. Long duration/low intensity stuff is mainly just dull.  It’s not hard, just boring (and your butt gets sore on the bike).  The high-intensity stuff certainly hurts but the work bouts are too short for you to really suffer; the workout is over before it gets too awful.

In contrast, working for extended periods near your maximal output level is just uncomfortable as hell and the duration makes it pretty gruelling as the minutes tick slowly by.  This is even more true on a bike where the stress tends to be localized to the legs (running at this pace is a bit easier since the stress is ‘spread out’ a bit more.

That’s what this study did.  Most folks can go about an hour at their maximum steady state; this study was 45 minutes at maybe 90% of that intensity.  It’s a tough workout, most can’t do it that frequently and this would be more true if they are dieting and/or including weight training as well.  So while these study results are certainly interesting, and it’s clear that EPOC can be significant at least under the conditions tested (essentially energy balance and a hard/long workout), I find it questionable how relevant this is to real-world dieting situations where calories are lowered and/or weight training is being done.

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