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High Fat and Ketogenic Diets: Friend or Foe of the High Performing CrossFitter?

I am beginning this post with a disclaimer as this topic is vast and there is only so much that can be addressed in a single post (or at least, one of a length that doesn’t use up your whole evening and / or send you running for the hills!!). This post does not address the health benefits and concerns of a ketogenic diet, or whether it is better or worse for weight loss. This post is focussed purely on looking at ketogenic diets and performance … and considering what this means for a CrossFitter looking to optimise their performance.

What is ketosis and a ketogenic diet?

During starvation blood glucose levels fall as we do not have the stored carbohydrate (glycogen) to maintain them, or the ability to make glucose fast enough from other non-carbohydrate sources such as protein. When this happens, the cells of the body cannot rely on carbohydrates (glucose) to provide the energy they need to survive and function. Some cells can rely on fats directly, however not all of them. The liver produces ketone bodies from fats and other substrates to provide the energy to cells of the body that cannot directly rely on fats – for example, the brain. We are in a state of ‘ketosis’.

We can induce a similar state of ketosis by consuming very little carbohydrate in our diet – usually to less than 50g a day (Westman et al 2007). This is the basis for a ketogenic diet. In addition to low consumption of carbohydrate, protein intake is also relatively low – typically 1-1.5g/kg bodyweight per day, or 10-15% of total calories (Paoli 2014). This is important as if protein is too high, the body can convert sufficient amounts of this into glucose and prevent a state of ketosis being induced. The remainder of calories are therefore consumed as fat.

What drove researchers to consider it for sports performance?

Carbohydrates are typically the major fuel for anything other than low intensity exercise (although even here, it contributes!) (Burke 2015). Fats can fuel low intensity exercise, and contribute a lessor and lessor amount as exercise intensity increases (Burke 2015). When we run out of carbohydrates during exercise we ‘hit the wall’ and exercise intensity typically declines; this is likely because energy cannot be produced as quickly from fats (hence they fuel low intensity exercise) and so we no longer have the energy required to maintain such a high intensity, and performance declines (Burke 2015).

On a ketogenic diet there is little carbohydrate available from the diet and relatively little synthesised within the body. This means there are limited carbohydrates available to fuel exercise, and skeletal muscles must rely on fats to produce the energy they need for contraction.

The theory was that if athletes are on a ketogenic diet, or simply very high fat and low carbohydrate (with the state of ketosis not necessarily being induced) they might adapt to become more efficient at using fats for exercise. This would mean that fats could contribute to more of the energy required in exercise and so might ‘spare’ carbohydrates from use until a higher exercise intensity … meaning either that higher intensity could be produced from the onset of exercise (as you get more energy from fats plus the usual carbohydrates) and / or carbohydrates last longer (because fats are doing more of the work) and so athletes can last longer before ‘hitting the wall’. A key point to note – in all these scenarios carbohydrates would of course need to be given before the race so that the athlete had carbohydrates in their body to utilise!!

So what does the research show?

Yes, within a week of a high fat low carb diet there are changes in skeletal muscle that result in more efficient fat metabolism (Phinney et al 1983; Stepto et al 2002; Yeo et al 2011). However, to cut a long story short, this rarely translates to performance enhancement when exercise is above low to medium intensity … which it will always be for extended periods in a competition scenario, except – perhaps – for rare multiday ultramarathon events performed in tricky terrains.

When carbohydrates are not added back to the diet before exercise performance (at above low to medium exercise intensities) is tested, a ketogenic diet does not appear able to match the performance of a higher carbohydrate diet (Phinney et al 1983; Burke et al 2017; McSwiney et al 2018). This is likely because although the ketogenic athlete may be better able to use fats than the athlete who has been on the higher carbohydrate diet, fats still cannot produce energy as fast as carbohydrates; the highest intensity exercise relies on anaerobic pathways for which carbohydrates provide the major fuel, and fats cannot.

What about when carbohydrates are fed back to the high fat low carb athlete in the hours or day preceding the test of exercise performance? In other words, does better fat metabolism + carbs on top = super athlete, as was theorised. Turns out, it seems not (Helge et al 1996; Burke and Hawley 2002; Havemann et al 2006)! Why? It appears that when carbohydrates are removed from the diet the body reduces the activity of the molecules needed to metabolise it to produce energy (Stellingwerff et al 2006; Burke 2015; Hawley and Leckey 2015). So, even if carbohydrates are returned to the diet the body cannot utilise them effectively to enhance performance. As put very concisely by leaders in the field of carbohydrates and sports nutrition: rather than ‘sparing’ the use of glycogen, a ketogenic diet appears to ‘impair’ it (Burke 2015; Hawley and Leckey 2015).

And what is more …

We know that heavy training and prolonged exercise is associated with immunosuppression and increased incidence of infection. There is evidence that low carbohydrate diets exacerbate this immunosuppression, including the inflammatory response (see Gleeson, Nieman and Pederson 2004). If this translates to increased infections and inflammation, this has the potential to negatively impact training, recovery and long term performance.

So, high carb all the way then?!

Ummmm not necessarily every day! To cut another long story short, the current thinking is that the broadly appropriate approach is to use carbohydrates to ‘fuel for the work required’ (Impey et al 2018). In other words, typically vary your carb intake depending on your training, competition and recovery needs for that day (broad IOC guidance below, taken from the IOC Nutrition for Athletes guidance (2012). For most of us, it is likely our carbohydrate needs are more or less the same day to day, perhaps with a spike on a particularly heavy training day, or pre- and post- competition.

However, on top of this, it may be appropriate for some athletes to train certain sessions on low carbohydrate with depleted muscle glycogen stores (see Impey et al 2018). There is growing evidence that this may enable adaptations to improve fat metabolism (as was the aim with a ketogenic diet), without impairing carbohydrate metabolism (as appears to occur with a ketogenic diet!) (see Impey et al 2018). There are a number of different approaches to ‘train low’, each with research for and against, and certain caveats and challenges with implementation so it is not necessarily simple to adopt this approach … it is out of the scope of this article to consider here, but perhaps another time!

Conclusion … what does it actually mean for CrossFitters?

You will notice that I have not cited any work specifically done in CrossFit. This is because there is very little robust research done on CrossFit – because it is so varied it presents challenges, and because it is a relatively new sport it takes time to build research momentum … but it is coming!

However, what I think we can say with confidence is that a ketogenic or high fat low carb diet does not support high intensity exercise performance for durations that would encompass a typical CrossFit training session or competition. As summarised by Stephen Phinney, an eminent researcher in the field “anaerobic (i.e. weight lifting or sprint) performance is limited by the low muscle glycogen levels induced by a ketogenic diet, and this would strongly discourage its use under most conditions of competitive athletics” (Phinney 2004). And this is before we consider the immunosuppressive impact of exercising when carb depleted!


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