High Intensity Exercise & Decreased Post-Exercise Energy Expenditure? Why the Latest Study Results are No Reason To Stop Working Out or Return to the "Fat Burning Zone"

Is HIT obesity incompatible? One thing appears to be sure - the purported post-workout increase in energy expenditure is reversed in the obese.

You may remember the positive results of the 100 squats a day challenge from the SuppVersity news on Friday, right? If you do, you will probably also remember the red box in which I mentioned the ongoing debate that's revolving around the issue, whether or not regular bouts of vigorous physical activity can protect you from the detrimental effects of sitting around 8h+ per day (see "100 Squats A Day Challenge"). While it may not really help us to answer the question whether "sporadic" activity is enough, a recently published study in the journal of the International Association for the Study of Pediatric Obesity certainly provides an interesting and novel angle on the debate.

What happens after you've burned those extra calories?

In a series of three experiments, a group of French researchers tried to elucidate why most of the studies that investigate the effect of physical activity on the change / loss of body fat report sub-optimal results. Within the last two decades, researchers such as Goran et al. (1992), Morio et al. (1998) or Donelly, et al. (2003), have repeatedly suggested that the primary reason for insufficiency of physical activity alone to modulate body weight was not restricted to the compensatory intake of food, but would - at least partly - be mitigated by a compensatory decrease in total daily energy expenditure (DEE).

As usual the existing evidence in ambigous, but a 1999 study by Kriemler et al. provided evidence that the high intensity exercise early in the morning will result in a decreased energy expenditure during the res of the day and the following day (Kriemler. 1990). Since Kriemler and his colleagues relied exclusively on the heart rate of their subjects to assess the energetic expenditure, Thivel et al. re-investigate the purported metabolic slow down in response to high intensity exercise in a series of three experiments.

• The fitter you are the greater the benefits of working out at high intensities (learn more)

  Study 1 involved only obese adolescents and used a combination of heart rate and accelerometer data to evaluate the energy expenditure

• Study 2 was conducted with lean (17% body fat) and obese (44% body fat) adolescents and evaluated the energy expenditure by the means of a SenseWear Armband

• Study 2 involved only obese adolescents and used the most sophisticated measuring technique, a open-circuit whole-body calorimeter in a metabolic chamber

In study 1 and 2 the subjects had to perform a cycling exercise consisting of 3 x 10 min at 70%VO2max (EX), while study 3 compared a 3 x 11 min at 75% VO2max high intensity (very long) interval protocol to a 3 x 20 min at 40% VO2max low intensity extremely long interval protocol (click here to learn how to design a real high intensity interval workout).

Does HIIT slow down the obese metabolism or does it simply increase laziness?

While the general trend, the scientists observed was pretty meaningless (more on that in the "bottom line"), the differences we see between the studies and between lean and obese subjects are quite telling and do in fact relate back to the initially mentioned question of "working out like crazy" and "sitting around lazy" the rest of the day.

Figure 1: "Energy expenditure" in the exercise and control trials of the three studies (Thivel. 2013)

In order to appropriately interpret the data we  have to remind ourselves of what the scientists actually measured in the different studies. Study 1 used accelerometer and heart rate data and did thus access a mixture of metabolic and physical activity. Study 2 used just the SenseWear Armband with a build-in accelerometer, which - despite being more sophisticated than a simple pedometer, still measures only physical activity. Lastly, in study 3, the metabolic chamber actually measures the "true" energy expenditure irrespective of whether that's due to an increase / decrease in metabolic rate or in response to exercise / sitting around.

• Let's start with the third study, since it's the most sophisticated one. What do we see in the middle of figure 1? Right, we see that conducting a 30min HIIT workout reduces the energy expenditure during the afternoon (-9.2%), but the total daily energy expenditure in a well-controlled scenario and measured with sophisticated equipment is still 8.9% higher than in the non-exercise scenario.
• So what about study two then? There is no debating that the afternoon energy expenditure in the obese groups dropped, but wait... what do we actually measure here? Right. That's not necessarily the amount of energy the obese youths actually expended. It's just a measure of their physical activity. It is thus not sure, if the obese youths did, as the scientists imply, not still end up with a higher daily energy expenditure. After all, the heart rate remains elevated after a workout in unfit vs. fit individuals. Plus, even with the "physical activity only" measure the exercise trial yielded a marginal, but statistically non-significant higher total energy expenditure for the day. In the lean subjects the exercise advantage amounted to 233kcal and was thus clearly significant - irrespective of the (imho) inadequate measuring method.
• The results of study one actually speak for themselves: The energy expenditure, which does, as we have noted before, include both, physical activity (accelerometer data) and metabolic rate (heart rate data), simply does not decline after the morning workout.

I guess, I actually don't have to "summarize" the above even more, but since I understand all of you who have been complaining about missing "summaries" in many of the older SuppVersity posts I still will provide you with the obligatory bottom line.

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Now that you know it's not bad for you, read up on all previous HIIT articles at the SuppVersity.

Bottom line: "High intensity" exercise is not useless and it does not (in the presence of adequate nutrient intakes) shut down your metabolism. It may be true that especially people who are not used to working out and will thus be correspondingly fatigued after a by no means "high" intensity "HIIT" workout at 70% of their VO2max (actually this is more of one of those stupid "fat burning workouts") tend to decrease their activity levels during the rest of the day, but the beauty of high intensity workout is that they increase your conditioning and as you are getting fitter, this behavioral but not metabolic compensatory effect is going to decline.

And best of all, as long as you don't succumb to the "I worked out, so I can have my cream pie today" idiocy, you are going to lose weight even while this adaptation process is still taking place. Why? Well, the two studies that don't rely on physical activity (study 1 & 3) only to determine the energy expenditure are telling me that you spend ~9% more energy if you work out for half an our at a not too intense pace in the morning. Whether this will help you to escape the detrimental health effects of sitting around the rest of the day is yet questionable, because health in not determined by the difference between caloric intake and expenditure... but whom am I telling that? You know that anyway, right?

References:

• Donnelly JE, Kirk EP, Jacobsen DJ, Hill JO, Sullivan DK, Johnson SL. Effects of 16 mo of verified, supervised aerobic exercise on macronutrient intake in overweight men and women: the Midwest Exercise Trial. Am J Clin Nutr 2003; 78: 950–956. 
• Goran MI, Poehlman ET. Endurance training does not enhance total energy expenditure in healthy elderly persons. Am J Physiol 1992; 263: E950–E957.
• Kriemler S, Hebestreit H, Mikami S, Bar-Or T, Ayub BV, Bar-Or O. Impact of a single exercise bout on energy expenditure and spontaneous physical activity of obese boys. Pediatr Res 1999; 46: 40–44
• Morio B, Montaurier C, Pickering G, et al. Effects of 14 weeks of progressive endurance training on energy expenditure in elderly people. Br J Nutr 1998; 80: 511– 519.
• Thivel D, Aucouturier J, Metz L, Morio B, Duché P. Is there spontaneous energy expenditure compensation in response to intensive exercise in obese youth? Pediatr Obes. 2013 Feb 28.