The purpose of this study was to determine the different effects on a phenomenon called Excess Post Exercise Oxygen Consumption or EPOC, as well as other markers of metabolism in response to Steady State, High Intensity Interval or Sprint Interval Exercise (supra-maximal effort).
Side note on EPOC: During low intensity exercise, energy systems primarily rely on oxygen, to create energy which costs energy. During high intensity, the body relies on “the glycolytic system” which can produce energy, without oxygen. The oxidative system (using oxygen) is far too slow for producing energy on a short term, high intensity basis.
EPOC then is sort of the body’s way of making up for the energy spent (exercise creates energy debt, EPOC tries to pay it). It’s sort of like an IOU.
During high intensity, all of this energy is spent. But the oxygen system can’t keep up. So for some time after exercise, the body slowly tries to make up the difference in terms of energy. This is EPOC, and it is though that inducing enough of a deficit will create a sustained “consumption of energy” for a long time, after you finish exercising.
It’s basically a theory of inducing long-term fat loss, after exercise has been completed.
10 recreationally active mean completed the study. Each participant was exposed to 4 different experimental procedures, separated by 72 hours to avoid interference.
During the study and for 3 hours after pulmonary ventilation and gas exchange were collected. (This is to measure the amount of oxygen being consumed and also things like, how much fat is being used as energy compared to glucose, etc.).
Each individual was exposed to the following protocols on separate days.
Volume of oxygen consumed and energy expenditure during exercise were significantly greater for steady state (1.9 ± 0.4 L·min-1 ) and high intensity (2.0 ± 0.4 L·min-1) groups than the sprint interval (1.4 ± 0.3 L·min-1 ) (p<0.001)group.
Net Oxygen consumed and net Energy expenditure was greater during exercise for steady state (56.7 ± 11.6 L; 284 ± 58 kcal) and high intensity (49.2 ± 9.0 L; 246 ± 45 kcal) than sprint intervals (32.2 ± 6.5 L; 161 ± 33 kcal) (p<0.001).
Additionally, there was a trend for High intensity to be greater than Sprint intervals.
Post Exercise RER and fat oxidation
What this all means
You’ve heard it all before: “High intensity interval training is superior to steady state cardio,” or “You’ll burn more fat than steady state, with high intensity intervals.”
This study actually examined the different responses to a myriad of different types of cardio and then observed what actually happens, in terms of EPOC and fat being oxidized.
What it ended up revealing was that using supra-maximal sprinting didn’t provide an immense benefit above that of Steady State and Higher Intensity Cardio.
There was certainly a trend for benefits such as greater fat oxidation than the other groups, but it wasn’t a massive difference. Although, it must be noted that 3 hours post exercise is a short amount of time and there was certainly a trend for greater fat oxidation as time went on.
This increase over time is intriguing and certainly could reach statistical significance over time.
Unfortunately, according to this study, using sprints might not be all that more beneficial than doing a bit longer duration, at a slightly lower intensity.
Where to go from here
True high intensity interval training is unlike anything you’ve ever done in your life. No seriously. You’ve never done true high intensity exercise.
Granted, “high intensity” is relative to ability, high intensity exercise in the lab is done on wingates, which, no one in the real world has access to (mostly) and is the most difficult athletic event you can do in less than an hour. (Don’t believe me? Go do a 10 x 10 second wingate protocol with 45 seconds rest, then come back to me).
So just for a moment, lets take that into account. When considering using high intensity for normal people, what can we actually expect?
For one, we can expect a tendency for a bit more oxidation of fat after exercise; a definite plus for HIIT, but not a golden ticket.
Second, we can expect that in terms of weight loss, there’s only a “potential tendency” for increased fat loss.
Not to mention, in order to actually observe the little benefits from sprinting, you’d need to train very hard. This is something that many people are not comfortable with. For example, 3 of the 13 actual participants couldn’t complete the study due to vomiting and nausea – something common with HIIT if you’re doing it correctly.
The truth is, this idea that super high intensity interval training is the answer to fat loss, isn’t as concrete as everyone thinks.
Many people have seen an abundance of fat loss as a result, but it’s difficult to say it’s directly because of HIIT. Everyone is an individual and responds to stimulus differently.
Could it be that they just actually increased their activity level significantly to the point that it forced their body to burn fat? Would they have achieved the same with less intense exercise?
It’s impossible to tell really. The fact is, this study shows that the effects of HIIT may not be as apparent as many believe it is.
It makes 100% sense that doing HIIT would promote a different response than that of steady state. In fact, EPOC was definitely higher in HIIT than Steady state, but in terms of net energy expended, steady state was still high than HIIT.
It’s extremely possible that this EPOC response is widely over exaggerated and/or it’s rectified on a much shorter time frame than people believe.
HIIT however, may induce differing hormonal responses. Studies have shown that HIIT increases catecholamines, which act on fat receptors to release fatty acids into the blood stream for oxidation.
So basically, while you might not have this magical deficit of energy to the extent that people have claimed, HIIT might produce a different hormonal response that manifests in greater fat loss.
It is very possible this is one of the responses that people have received much benefit from. It’s certainly a potential reason why fat oxidation had a higher tendency for the HIIT group.
But then again, it’s difficult to infer that’s the answer. Truth is, we just don’t really know, and we don’t really know that it’s superior.
What should you do then?
I personally suggest having a well-rounded cardio routine. If you’re hopes are to improve body composition and health, I suggest using a combination of HIIT, steady state, and everything in between.
One of my personal favorites is doing a couple HIIT sets, and then doing steady state.
Theoretical at best, but since HIIT increases catecholamines, there is a chance that you can increase the amount of fat burned during steady state as a result. Purely theoretical but fun to theorize.
Alternatively, use undulation in terms of cardio intensity. One day to steady state, the next HIIT, then the next do sprints, etc. Hitting all different intensities is probably a better way to go.
Why this should matter to you
It should matter because many people make HIIT out to be the greatest thing since creatine.
The truth is, we just don’t know if that’s true. You shouldn’t put all your cards into one basket.
Not to mention, true HIIT is super hard. Not to mention, do it regularly and you begin impairing your performance for other exercise sessions. Based on this evidence, it’s cool to use HIIT, but chances are you should use a combination of other types of cardio as well.
A year ago, HIIT was THE answer. Now, people are starting to question it.
Theory changes as the data becomes stronger. Open your mind and be willing to pivot when necessary.
Follow the data, not your opinion.