The Anabolic Effects of HIIT: 3x30s High Intensity Intervals Increase mTOR & Ramp Up Marker of Protein Synthesis by +43% in Men and +222% in Women - Even in a Fasted State!

Image 1: While the study at hand clearly shows that HIIT, even done on an empty stomach, is anabolic, not catabolic, it appears as if women respond better to sprint exercises than men. And this assumption is not based on gene-assays but dates back to the results of a 1999 study by Esbjörnsson which showed a more pronounced CSA increase in the leg muscles of female subjects.
Not all too long ago, the general accepted consensus was that anyone whose main interest is in building muscle must abstain from any strenuous cardiovascular exercise... running on a treadmill? God-forbid! You could lose muscle. Over the last two years or so, this paradigm has began to totter, though. And now, at the beginning of 2012 I would estimate that the number of (recognized) trainers and trainees who recommend doing high intensity interval training (HIIT), if not for general conditioning, then at least as a means to shed fat, initially surpasses the number of the conventionalists who maintain that "classic cardio" training in the "fat-burning zone" was the way to go. Now, if this is not your first visit here at the SuppVersity, you should be aware that the latest scientific research supports the arguments of the advocates of HIIT. And not so much to my, as to the surprise of some researchers, this holds true not only for already well-conditioned gymrats and athletes, who want to finally pass beyond the 10% body-fat barrier, but also for the obese and metabolically deranged diabetic, who is trying to get his blood sugar under control (cf. "Hitting Diabetes With A Hammer").

The advantages of HIIT reach well beyond fat loss, but...

Moreover, a 2011 study by Naito et al., the results of which I have discussed in November 2011, shortly after it was published in Acta Physiologica (cf. "HIT Your Satellite Cells to Increase Your Gains"), already hinted at the fact that the advantages of HIIT reach well beyond its fat-burning effects. Yet although the increase in both satellite cell count and incorporation into the muscle Naito et al. observed speak for themselves, there's still rumors going round that this training style could be catabolic. In that the argument usually revolves around the notion that muscle damage is a major driving force of satellite cell recruitement and that if the latter is a necessary consequence of HIIT it would counter your efforts to build muscle. Now, aside from the fact that this argument is intrinsically flawed (I mean, what to you do in the gym, when you weight train? You break down muscle tissue!), a recently published study from the famous Karolinska Institute in Stockholm, Sweden, attests to the fact that the exact opposite is the case.

... it appears as if women could derive even greater benefit from all-out sprinting than men

Image 2: The exercise stimulus in the study was a Wingate test, one of standard procedures in exercise science.
In an earlier study from 1999 Esbjörnsson and his / her colleagues had observed that the cross-sectional area of the leg muscles of women exhibited a more pronounced hypertrophy response to sprint training than those of their male peers (Esbjörnsson. 1999). With the advent of our advanced understanding of the underlying principles of skeletal muscle hypertrophy and the central, but as those of you who read the Hypertrophy 101 know, in the current discussion possibly overemphasized position of the mammalian target of rapamycin (mTOR), Esbjörnsson et al. did now set out to examine, whether a sex-specific response of mTOR and its downstream targets could explain their previous results (Esbjörnsson. 2012).

To this ends, the scientists recruited nine men and eight women who despite participating in leasure time sports were only "in good shape" and not considered to be athletes. For the experiment the subjects reported to the lab fasted and, after a brief 1min warm-up, performed the well-known Wingate-test, which consists of three consecutive 30s all-out sprints with 20min breaks between the intervals on a braked cycle ergometer (average peak power was ~645W and ~935W for women and men, respectively, on a per-lean body-mass base, the peak and mean power was yet identical)
Figure 1: Illustration of the experimental protocol used in the study.
Before the warm-up and 140min after the 3rd sprint, Esbjörnsson et al. took muscle biopsies from the quadriceps muscles of the subjects to assess the local expression of mTOR and its downstream targets.
Figure 2: Phosphorylated AKT, mTOR, p70S6K and rpS6 (a.u.) in male and female study participants before the first and 140min after the third sprint of the Wingate test (data adapted from Esbjörnsson. 2012)
If you are not well-versed in the the intricacies of the mTOR-cascade, it appears as if the data in figure 2 would disprove the scientists' research hypothesis that "mTOR signalling is more pronounced in women than in men". After all, the increase in phosphorylated mTOR (p-mTOR) and AKT (p-AKT) in response to the three 30s seconds sprints was obviously more pronounced in the male, than in the female participants (mTOR +26% and AKT +17% greater increases; a difference which did not reach statistical significance, though).

Do women just make better use of the same stimulus?

As far as the phosphorylation of p70S6K, of which the current scientific evidence suggests that is a more appropriate measure of the "real-world" protein synthetic effect of mTOR, a completely different picture emerges. While the +43% increase in the male subjects is just about statistically significant (p = 0.04), the +222% increase in p-p70S6K in the female subjects appears to confirm what Esbjörnsson et al. already  had suspected.
Figure 3: Serum leucine and growth hormone levels at rest and after the sprints (data adapted from Esbjörnsson. 2012)
The slightly greater disappearance of leucine from the skeletal muscle of the male subjects (cf. figure 3) is yet only one of three possible explanations (and one you could counter by ingesting BCAAs, for example) Esbjörnsson et al. come up with based on the results of previous studies:
Image 3: If you look at the leg muscles
of some of the female speed skaters,
it is quite obvious that the leg muscles
of women respond pretty well to short,
intense bouts of all-out sprinting.
(the image shows Claudia Pechstein)
  1. Lower accumulation of lactate and ammonia and a faster recovery of ATP levels in type II fibers of women than men
  2. Lower levels of plasma catecholamins (=stress hormones) in response to sprint exercises in women than in men
  3. Slower disappearance of leucine and thusly more sustained elevation of protein synthesis in women than in men
Whether it is any of these, or a combination of all three factors which is responsible for the differential response to statistically (!) not significantly different activations of mTOR and p-AKT, cannot be decided based on the available data.

An alternative explanation, which would, by the way, have real-world implications for the training practice, is (and I prefer to cite this, to avoid being accused of sexism) that...
women do not exhaust themselves as much as men during each bout of exercise and thereby elicit a smaller activation of AMPK, resulting in less inhibition of mTOR.
In view of the fact that previous studies by Esbjörnsson et al. refute this hypothesis, it appears unlike that an "over-expression" of AMPK, of which I have discussed in one of the previous installments of the Intermittent Thoughts that its locally expressed alpha-2 isoform does not inhibit the exercise induced increase in protein synthesis, anyway, could explain why similar exercise stimuli (peak and mean power per fat-free mass were virtually identical for men and women) and within the statistical margin identical mTOR responses induce a more pronounced protein synthetic response in women than in men. And whether the early(-ier) rise in serum growth hormone, which is the last possible explanation the scientists mention, has anything to do with it appears questionably, as well. After all, the data in figure 3 shows quite clearly that the overall GH response was much more pronounced in the male than the female participants.

We don't know about aliens, but for earthlings HIIT is anabolic - regardless of their sex

In essence, it does not even really matter, what the underlying cause of the sex-specific response to sprint training is. As far as I am concerned, the most significant result of the study is not the gender-difference, but the simple, yet as the scientists point out "novel" finding that "repeated 30-s all-out bouts of sprint exercise, separated by 20 min of rest, increased Akt- mTOR signalling in skeletal muscle." And this effect was observed in both men and women. Now, this is allegedly not exactly your "usual" HIIT protocol, if you do yet take into consideration that it was performed after an overnight fast and went without BCAAs, protein shakes all the other "obligatory" anti-catabolics, the average gymrat uses to avoid the purported catabolic effects of high intensity conditioning work, I would dare to say that it HIITs another (if not a final) nail into the lid of the casket of the "HIIT = catabolic" myth.
Disclaimer:The information provided on this website is for informational purposes only. It is by no means intended as professional medical advice. Do not use any of the agents or freely available dietary supplements mentioned on this website without further consultation with your medical practitioner.