The recently published paper by Volek et al in Metabolism has been hailed as a breakthrough paper which it is said has proven beyond a doubt that fat adapted runners are able to utilize fat as fuel, with rates of fat oxidation at levels previously thought impossible. Compared to a group of high carb athletes, the HF cohort was primarily using fat as it’s preferred fuel source, compared the HC cohort which was surprise, surprise, burning carbs. I won’t rehash all of the details - you can read it yourself here - but it’s a well designed study that proves it’s point well. And the take home point seems to be that at relatively low intensities (62% VO2 max), fat adapted and well trained ultra endurance athletes use a different fuel source when compared to their traditional high carb consuming ultra buddies. It is certainly very valuable to finally have some valid science to back up the anecdotes of increasing numbers of endurance athletes, but for me however, as an athlete and a high performance doc, I think this paper served to highlight a number of unanswered questions, and it left me hungry for more, if you’ll excuse the pun! I’ve been thinking about it a lot lately and I can think of several follow up studies which I feel would be immensely valuable!

The paper concludes thus:

“Conclusion. Compared to highly trained ultra-endurance athletes consuming an HC diet, long-term keto-adaptation results in extraordinarily high rates of fat oxidation, whereas muscle glycogen utilization and repletion patterns during and after a 3 hour run are similar.”

I think that a lot of people will be thinking so-what? No big surprise here. Matt Fitzgerald in his book Diet Cults talked about the adaptability of the human body, and how different cultures had evolved very successfully on very different diets. But I think that in the world of the elite endurance athlete, whether we can survive on this vs that is less relevant than what can make us a better performer. We are all striving to maximize our potential. My car can run on regular or premium gas, and with some fiddling could even run on diesel, or vegetable oil, but if it’s a race car, what matters is what makes it go faster.

I think that if one talks to athletes who follow a generally low carb form of day to day diet, most will quite openly admit that they continue to race on carbs. Why do they do this? I think Peter Defty from Vespa puts it well. He stated to me: “Fat adaptation has huge benefits, but, directly, it won't yield the most power output; however, it is an important foundation for the athlete to achieve superior performance.”

What Peter is saying is that once fat adapted, it’s possible to see huge performance gains when CHO is introduced on race day. This point isn’t addressed in FASTER though, and I think that there are those among us who would like to see additional studies addressing the whole performance issue (and to their credit - Volek et al do mention this).

So here are a few questions I have for the authors.

Lactate levels were increased in the LC cohort in the final hour of their 3 hour treadmill run. I have to wonder why is this? One could surmise that this was due to either increased workload, increased muscle fibre recruitment, increased gluconeogenesis resulting in a sugar spike and therefore increased lactate production, or even decreased lactate clearance in the keto group. Intensity was controlled at 62% VO2max for both cohorts, so that can be ruled out, and let’s face it, this is a pretty low intensity. Aside from this I don’t think we really know the mechanism, and I wonder if it’s even relevant, though interesting. I’d love the authors to comment on this, and potentially looking at higher intensity, or even more prolonged sessions, though I won’t be first in line for a 5 or 6 hour treadmill session!

Both cohorts were fed different pre-race meals. I think we know that endurance athletes are likely in a very metabolically flexible position; you have to be when you’re involved in sporting events that can last 20+ hours or more. We also know that the composition of the pre-test meal can influence R/Q values. A local exercise physiologist told me once that he can tell as soon as a runner jumps on the treadmill to have an R/Q test performed whether they had bacon & eggs for breakfast, or whether they had pancakes and syrup! I’d like to see what happens when the fat adapted athletes are fed a high carb meal. What happens to their R/Q values - is fat oxidation switched off in favour for glycogen use? Those of us who ‘train low’ and ‘race high’ would like to know the answer to this, as it may help to determine our pre-race meal. We know that the HC athlete feels awful and struggles to perform when carbs are restricted, but what of the LC athlete fed ‘rocket fuel’!

No apparent sparing of muscle glycogen post-exercise was observed. This is one of the oft-touted benefits of being fat adapted, and personally, I have found my reliance of carbohydrate fuel sources to be reduced when I follow a strict high fat approach, and I know of others who report the same. This glycogen sparing effect is clearly of huge interest to ultra runners or, for that matter, anyone who plans too push their workouts or races beyond 2 hours, so what gives here? If you look closely at the study design, there was no apparent difference between the HF and the HC groups, but the post workout shakes were different. Having said this, immediate post exercise muscle biopsies were performed before recovery shakes, and then repeated at 2 hours. Muscle glycogen was reduced by around 60% in both groups, and at 2 hours both groups were at around 35%. No big difference, except that the they were each fed a different shake. So perhaps it was the higher carb post workout shake that replenished the HC athletes’ muscle glycogen at 2 hours, and perhaps it was a spike in post workout blood sugar from hepatic gluconeogenesis that replenished the muscles of the HF group. It’s interesting to speculate, and the authors do, but I guess at this point we really just don’t know. And of course, whether it is even relevant in the real world of competition remains to be seen, but the potential relevance would be in the use of an immediate post exercise high carbohydrate shake to more rapidly and more fully replenish glycogen stores when a second or multiple workouts are planned for the day. Again, interesting to speculate but no proof from this trial.

What of the female ultra endurance athletes! This was a male only study. Anecdotally, many low carb female endurance athletes report increased carbohydrate requirements when training and racing. This was a male only study, and I’m sure the females are interested in what a similar study of females would look like. I know that many low carb females like to cycle through higher carbohydrate cycles, but I don’t know what data there is out there to support this practice.

The final issue which is not addressed by the authors in this particular paper, and which may ultimately come to light in later analyses, is the issue of soreness and inflammation. I do not know if this issue was looked at. Anecdotally, LC athletes report less post exercise soreness and faster recovery. To date I don’t believe inflammatory markers have been studied in this situation. I see this as being very relevant to multi day event participants, and these events have grown, and continue to grow in popularity. The ability to recover completely from day to day in these events, is likely the key to winning performances, and for that matter, on enjoyment!

So after some deeper reflection, I’ve concluded that this study really just confirms what we suspected: that at relatively modest exercise intensities, fat adapted male athletes have very different metabolic profiles, and utilize fuel sources quite differently to their high carbohydrate counterparts. It’s a great start to have this kind of proof published in a peer-reviewed journal, but I have to say it leaves me wanting more!

Let’s look at female athletes, let’s look at different intensities, let’s look at low carb athletic performance when fuelled by high carb pre and intra exercise meals. Let’s look at the effects of different diets on soreness, inflammation and recovery. There is also some recent evidence that there are genetic responders and non-responders to high fat diets. This is all very interesting stuff, and I think that these are some of the real issues to those of us who are keen to maximize our performance through dietary manipulation and strategic use of different fuel sources whilst competing.

For now I see no reason to change what I'm doing. Day to day I'm a low carb eater but I train hard and race on Vitargo. And I supplement with VESPA, an amino acid, on those long training days and when racing. Vitargo is the only carbohydrate source that I really tolerate and there are some great studies out there showing why it is typically so well tolerated and effective.

Hopefully this post will generate some thoughtful discussion, and if not, well I'll see you on the trails!

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