Extreme High Intensity Training Might Ruin Your Metabolic Flexibility
Note : The lead author of the paper Dr. Mikael Flockhart and I had a conversation regarding some of the open questions surrounding the study design. I’ve paraphrased his comments in the “Appendix” section at the bottom of the post for those interested.
When looked at the extremes, too little exercise and too much exercise are both known to be problematic. Its easy to show what happens with lack of exercise. But at the upper end of that scale, the waters are murkier.
Its an open question “how much” intensity and volume is tolerable before the body simply shuts down. A new scientific study from the Swedish School of Sport and Health Sciences seems to aim at addressing that question from a cellular metabolism perspective.
Flockart et.al took 6 females and 5 males and had them do progressively increasing interval training blocks for 3 weeks on a cycle ergometer. All test subjects had individualized schedule with strict timing for training, muscle biopsies, oral glucose tolerance tests and diet.
The first 2 weeks were a preliminary study period with easy to moderate amounts of HIIT. The maximum number of HIIT sessions during this period was around 2–3 sessions per week.
In the following 3rd week called “excessive training”, all hell broke loose with the volunteers having to do 5 HIIT sessions totaling 150minutes per week. The 4th week was a recovery week involving a reduced workload but note that participants did not lay off the HIIT schedule completely.
It seems low intensity activities and upper body strength trainings were permitted outside of the lab (how many of the subjects actually did light exercise by themselves is not reported).
The main sessions consisted of 5 x 8 minutes at 90% VO2max after a 10 minute warmup routine in the lab.
Readers familiar with Dr. Stephen Seiler’s investigations will be aware that one of his studies showed 4 x 8 minutes (32 minutes) of HIIT training a week at 90% max HR reaped greater adaptive gains in trained cyclists than did 4 x 4 minutes (16 minutes).
So it was interesting to see that same protocol being used in this study, with one extra bout added for a total of 5 x 8 = 40 minutes of HIIT per session.
In a separate phase of this study, a group of national level elite endurance athletes were outfitted with continuous glucose monitoring (CGM) to see how much time they spent hyperglycemic versus hypoglycemic.
The line of thought with this phase seemed to be to try and see how their glucose metabolism compared to what the extreme training did in the normal subjects.
So lots going on in the study. Getting my head around the discussion and results of this paper required reading the paper twice over. Fortunately, some college level cell biology and biochemistry helped along the way.
Let me try to summarize the main findings of this paper :
- Mitochondrial complex 1 & complex II “intrinsic” respiration was attenuated by 40% after the excessive HIIT week compared to light and moderate HIIT. This, despite a stepwise increase in the mitochondrial content and enzymatic activity along the training phases. Strangely, this respiration never fully recovered even in the final recovery week.
- Oral glucose tolerance tests done on subjects showed an increase in glucose intolerance after excessive training, compared to the first part of intervention. This, despite the fact that muscle GLUT4 levels were most abundant after the excessive training phase.
- The lactate driven glycolytic energy system was substantially diminished in the excessive training week. This, despite the fact that muscle glycogen steady increased throughout the training phases, and circulating free fatty acids were also low. Those who know the physiology of cycling will quickly understand how a poorly functioning glycolytic system will detract from performance.
- The separate cohort of elite endurance athletes subjects monitored by CGM showed around 41 minutes per 24h spent in a hyperglycemic state compared with a separate control group that only spent 22 minutes there. So there were some trends towards glucose intolerance at specific time points in the day (like diabetics show) although majority of the time, they appeared to fall within normal bounds.
The mitochondrial mechanism can be likened to a water wheel, where the flow of protons resembles a flow of water downhill, and the turning of the wheel is like the turning of ADP toward phosphate to cause the bond to form ATP via oxidative phosphorylation. That’s what helps power aerobic exercise.
The paper tries to argue that mitochondrial dysfunction causes the disturbance in glucose metabolism. Its interesting that they chose a bout of extreme exercise to show this.
What was striking is how they report similar kinds of impaired glucose tolerance in the separate group of endurance athletes they studied. Quite strangely, this was also reported in a previous work where a CGM was employed.
On the whole, the paper invites more questions than ones answered.
Firstly, what really causes the mitochondrial respiration to drop and is it really worrisome or just a normal adaptation to “shock” training before a supercompensation?
If supercompensation is a practical phenomena, why didn’t mitochondrial respiration recover fully in the final recovery week? Or, was the study too short (just 4 weeks long) to show the full recovery?
High level endurance athletes are known to polarize their training with majority of their training time doing low intensity work. Despite not knowing who the elite athletes in this study were, is metabolic inflexibility really a widespread phenomena coming from “extreme” training? Is it linked more to intensity, or volume or both?
And are the changes shown in this paper applicable to a broader mode of exercises?
Despite these open questions, it was great fun to read this study and have a peek into the cellular changes from extreme HIIT sessions. The paper is valuable from that standpoint, presenting a barrage of visual plots to show what's going on in terms of group level statistics.
Its interesting to note that 150 minutes per week is the minimum amount of exercise recommended by WHO. That would seem like a walk in the park to many of the dedicated exercise types. The same number is implicated in this study at the far end of the exercise spectrum.
150 minutes of HIIT per week sounds excessive on the surface, yes. But in the age of Zwift and Orangetheory (no links between them of course), it is one step closer to reality.
As Confucius said, do everything in moderation…
Appendix : Comments from Mikael Flockhart
1) In this paper, the aim was to describe the phenomenon linking the attenuation in mitochondrial respiration to glucose intolerance and give a potential explanation.
2) Potential reasons for the partial shutdown of intrinsic mitochondrial respiration was discussed and outcomes linked to oxidative stress.
3) The HIIT sessions in the study were performed by subjects who were new to the intensity prescribed. These were normal people, living their normal lives. This is the complete opposite of studies done in animals living unnatural lives.
4) Regarding low intensity activities outside the lab, some of the volunteers took the bike to school, some were into strength training and some engaged in endurance running. Every session was approved so as not to interfere with the monitored sessions in the study.
5) The upper limit of exercise training where health parameters are affected have not been described in a good way in literature before. Previous research has mostly studied the symptoms and tied it to performance outcomes.
6) Our study took a new swing at this question and just as you state — the results open up for a bunch of new questions. Some of these will be addressed in an upcoming work from our group.
Feel free to reach out to me if you have comment, corrections or thoughts on twitter.
