Does HIIT take a hit on a ketogenic diet?

Much of the research points to the utility of a keto diet for increasing fat oxidation for longer, slower training. After a period of adaptation, athletes are then able to tap into an alternative fuel source which affords them an extensive supply of fuel at a steady rate, unlike carbohydrate (glycogen) stores which are limited and easily depleted in a moderate-long training session. Thus, it stands to reason athletes are able to go for longer than if they are straight carb-burning athletes in an endurance event – something that Maunder and colleagues discuss in this most recent paper outlining the practical application of a low carbohydrate diet for athletes of varying abilities. However many of the recent randomized trials (such as this one here) have found that performance, particularly at the top-end of the spectrum, is compromised when athletes switch to a lower carbohydrate approach. Further, the relative effort at a given heart rate is increased. You go slower, but it feels harder. Ouch. This understandably makes an athlete’s coache a little nervous to recommend their athletes go on a lower carbohydrate diet.

Interesting though, this is not the experience of many people I work with who transition to a lower carb diet. If anything, performance improves for the athlete (something I’ve written a lot about over the last few years, including this blog here). Given enough time, any reduction in power that occurs early in the transition phase appears to be reversed and the athlete comes out leaner, stronger and fitter in their endurance training. Reducing reliance on carbohydrate as a training fuel reduces the oxidative damage that occurs during training, thus inflammation is reduced. They aren’t placing their body under as much oxidative stress and therefore the athlete can train more consistently during the season with less risk of stress-related injury and illness. This may also be due to a higher presence of beta hydroxybutyrate in the bloodstream, which act as signalling molecules and increase the transcription of enzymes that encode antioxidant genes superoxide dismutase, catalase 2 and glutathione peroxidase. This helps scavenge free radicals created through training and protects the athlete from tissue damage. This may be one of the reasons why they are seeing better results with their key races.

Two of my mates felt similarly, and experienced similar benefits of adhering to a low carbohydrate diet, experiencing no detriment to high intensity training, despite what the research deemed. So they decided to test the hypothesis.

They took 18 male endurance athletes who were habitually eating a standard western diet, and randomised them to consume either their normal diet (control group), or a very low carbohydrate ketogenic diet, consisting of no more than 50g carbs per day for four weeks, and performed graded exercise tests before and after the experiment, and a HIIT session (5x3min, work/rest 2:1, passive recovery, total time 34min) before, and after 2 and 4 weeks.

The researchers found that (as expected) fat oxidation levels increased in the experimental group throughout the tests, and total time to exhaustion, performance in the HIIT session and rate of perceived exertion was no different between the groups. Ergo, the ketogenic diet did not impact the athlete’s ability to undergo high intensity training (nor make it seem harder for them). Interestingly, the level of protein in the diet was around 29%, higher than the 17% used in other studies – this could account for the level of ketones present in the blood stream that were lower at the end of the study (0.4 mmol/L), just out of the ‘nutritional ketosis’ range. The difference this may have made to the athletes’ performance, however, we don’t really know.

Many of the studies conducted that have found performance is reduced are likely too short to allow the athletes to adapt to a ketogenic diet, which is thought to take several weeks to months. Hopefully this new research makes you think twice about taking the results of a study like such as the one here, as a reason to dismiss the low carbohydrate diet for athletes.

To recap, then, of what we know is possible for athletes following a lower carbohydrate approach:

As a side note, lots of peeps look at the elite athlete who chows down on carbohydrate in racing and during everyday life and thinks to themselves that, if they can perform to that standard eating a higher carb approach, then why can’t I? A couple of points to note:

  • The elite athlete may train from 20-30 hours per week – by default they spend a lot of time in a depleted state, meaning they are likely training low glycogen as it is impossible to replenish carbohydrate at the rate they are burning it. This is going to afford them the same capacity to train in the lower carb state that provides enhanced training adaptations. The average age grouper may have time to train 12-18h a week maximum, and don’t have the volume available to get into the low glycogen state.
  • They are elite for a reason. They are able to go harder and faster than most people –psychologically they are able to hurt more and potentially go longer before they bonk – we age-groupers have more of a preservation mindset. They may also be able to train harder when in an inflamed state for this reason. I’m not saying this is ideal (far from it). I’m just putting it out there as a reason why there are professionals who are able to see results where others don’t.
  • Even at the top of the field the elite athlete can suffer, and far more than an age-grouper. Years of a nutrient-poor, carbohydrate rich diet and overreaching to the point of overtraining will leave an athlete burnt out and unable to continue on at the level they previously enjoyed. It might appear that elite athletes are bullet proof but I’m sure as you’re reading this you’re thinking of someone who falls into this category. Things aren’t always as they appear, and the golden glow of success can be pretty fleeting.
  • Of course, there are others who are just awesome and continue to turn up and take it out year after year, regardless of diet, training methods, lifestyle etc. Like the people who drink every day, smoke like a chimney and don’t eat vegetables, yet live to 102 years.
  • Re: racing high carbohydrate – that Maunder paper again – worth a read.

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