Every week a new way of eating hits the mainstream media. They are often described as a way to improve health and promote fat loss quickly and easily. Many of these diets are merely fads that have shown little value when finally placed under scrutiny by high-quality researchers.
Which is why the ketogenic diet has literally taken the world by storm.
Not only has the anecdotal evidence surrounding the keto diet been very positive, but it also has the research to back it up.
But what remains somewhat unclear is how it impacts exercise performance – which is what we are going to uncover today.
What is the ketogenic diet?
So what is a keto diet?
To put it simply, the ketogenic diet is a way of eating that is high in fat, moderate in protein, and extremely low in carbohydrate (Westman, 2003).
Just to be clear, when I say low, I really do mean low.
To give a bit of insight into how low this truly is, you should know that an average sized apple contains around 20 grams of carbohydrates.
Like I said – low.
With this in mind, you might be wondering why someone would actually want to keep their carbohydrate intake 10 grams per day. It comes down to a physiological state known as ketosis.
Yep, that’s right – the main goal of the ketogenic diet is to get your body into ketosis.
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How can you achieve ketosis?
Your body has the ability to use both fats and carbohydrates to create energy.
But, in the scenario where your body runs out of carbohydrates, then it will increase the rate it breaks down fat to produce energy.
Makes sense really.
Now, somewhat interestingly, when the body breaks down fats for energy, it produces little compounds known as Ketones as a by-product.
Taking all of this into consideration, Ketosis essentially refers to a state where your body is producing high amounts of ketones – which is simply accomplished by restricting your carbohydrate intake to extremely low levels.
What are the benefits of ketosis?
Ketones are quite interesting, as while they are indeed a side effect of fat metabolism, they can also be broken down in the brain for energy. In this manner, they also impact upon the body in a number of unique ways.
As a result, the benefits of the keto diet include (Krikorian, 2012; Paoli, 2013; Vargas, 2018):
- Enhanced cognitive function
- Increased rates of fat loss
- Reductions in blood sugar, blood cholesterol, and blood pressure levels
- Declines in inflammation
All of which contribute to some serious health outcomes!
Now, this is all well and good, but how does the keto diet influence exercise performance? Even more importantly, how does the keto diet affect high-intensity exercise performance?
What is high-intensity interval training (HIIT)?
High-intensity interval training (or HIIT, for short) describes a form of aerobic exercise that revolves around short bouts of intense exercise, alternating with lower intensity periods of recovery exercise (Wewege, 2017).
A simple HIIT protocol could be running at near maximal intensity for 30 seconds, then walking for 60 seconds to recover. You could then repeat this process for 25 minutes to get in a very solid (and simple) HIIT session.
HIIT has become extremely popular because it allows you to get in an excellent workout in a very short amount of time – ultimately meaning that you can obtain all the benefits of aerobic exercise, albeit in much less time.
These benefits include (Kessler, 2012, Heggelund, 2014):
- Significant reductions in fat mass.
- Declines in resting blood pressure and resting blood sugar.
- Improvement in blood cholesterol levels.
- Increases in mood.
- Reductions in feelings of stress and anxiety.
- Prevention of depression and anxiety.
- Vastly superior improvements in aerobic and anaerobic fitness.
Considering that many of the benefits associated with the ketogenic diet actually coincide quite nicely with those experienced with regular HIIT, you might be thinking that they would offer the perfect combination – but is this really the case?
How does the Ketogenic diet affect HIIT?
When we start looking at the keto diet and HIIT, things start getting a little bit interesting.
As already discussed in brief, during exercise, your body has the capacity to break down both fats and carbohydrates for energy – however, where this energy comes from is heavily dictated by the intensity of that exercise (Aslankeser, 2017).
Under normal dietary circumstances, carbohydrate metabolism typically increases as exercise intensity increases, while fat metabolism subsequently decreases.
With this in mind, at lower exercise intensities, fats are typically your predominant source of energy.
As a result, during HIIT training, we normally see large increases in the amount of energy being derived from carbohydrates – and fat metabolism becomes nearly non-existent.
But what happens if carbohydrates aren’t readily available for energy? Such as when someone is undertaking a ketogenic diet?
Well, to be honest, it is not all that shocking.
Recent research has shown that when active individuals undertake a ketogenic diet and then partake in HIIT training, the amount of energy they derive from fats sees a huge spike compared to normal circumstances (Cipryan, 2018).
This means that to maintain a given energy output, you will simply burn more to make up for the lack of carbohydrates available.
It is important to note that in this study, this change in fat metabolism had no negative impact on exercise performance adaptations – meaning that individuals on the ketogenic diet become very efficient at using fats for energy, even at higher exercise intensities.
Pretty amazing if you ask me!
What are the benefits of combining the ketogenic diet with HIIT?
Taking the above into consideration, there is a myriad of benefits associated with performing HIIT while on the ketogenic diet (Cipryan, 2018).
First and foremost, despite not necessarily being optimized for performing work above lactate threshold, you will still see substantial improvements in both aerobic and anaerobic fitness, which is obviously pretty darn important.
Secondly, you will see some very positive improvements in your body composition.
You see, given that you are going to be metabolizing more fat for energy on the ketogenic diet than you would be under normal circumstances, it stands to reason that your rate of fat loss will also be enhanced.
Finally, the unique combination of both the ketogenic diet and HIIT will cause a substantial improvement in mood. This is a very nice side effect, in my personal opinion.
Looking at the above information, it should become apparent that there really aren’t any cons associated with the keto diet when it is used with HIIT. This is because being on the ketogenic diet doesn’t necessarily appear to impact the outcomes of HIIT – however, it does seem to limit its performance slightly.
This is because, during exercise above lactate threshold, energy is normally produced predominantly from carbohydrates, rather than fats.
Which will roll into our next section quite nicely….
Is the ketogenic diet better for aerobic or anaerobic exercise performance?
Taking the above into consideration, following the ketogenic diet might actually inhibit your ability to perform high-intensity activity slightly (Chang, 2017; Wroble, 2018).
This will in turn make it less than ideal for those individuals who actively participate anaerobic field sports or short track events
However, this is not the same for those individuals who participate in longer duration aerobic events (Volek, 2015; Chang, 2017).
You see, fats provide more energy per gram than carbohydrates.
If you become completely adapted to using fats for energy (thanks to the ketogenic diet), you will become more efficient at breaking down and using fats for energy.
Through this rather important interaction, the ketogenic diet appears to have a seriously positive effect on long aerobic exercise performance.
So, if you are into marathon running, long-distance cycling, triathlons, or anything similar, then the ketogenic diet could be for you.
But, I should also note that it can take up to 12 weeks to become completely adapted to using fats for energy – so you might need to commit to the ketogenic diet for a good portion of time before seeing any real improvements in performance.
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Does the ketogenic diet effect energy levels?
When first starting to exclude carbohydrates from the diet, some people will experience some negative symptoms, including lethargy, mental fogginess, headaches, muscle cramps, and general feelings of fatigue (Kossoff, 2009).
While these symptoms will disappear after a week or two (once you have become adapted to using fats for energy), they are enough to put some people off the ketogenic diet entirely.
With this in mind, during the first two weeks on the ketogenic diet your energy levels can decline noticeably – however, this isn’t the case once you have become adapted.
Recent research has demonstrated that once people have become adapted, they tend to feel good on the ketogenic diet.
Like really good.
In fact, their energy levels tend to increase significantly, they see vastly improved sensations of mood and wellbeing. They even see marked improvements in cognitive function. This indicates that on the whole, the ketogenic may actually have a very positive effect on your energy levels (Zinn, 2017).
Take Home Message
The ketogenic diet appears to have some serious health benefits – particularly in comparison to what many would deem a typical western diet. With these come improvements in mood, cognitive function, and incredibly, aerobic exercise performance.
However, it is important to note that as the ketogenic diet does not contain any carbohydrates, it can limit your ability to perform high-intensity exercise.
Interestingly, when the keto diet is combined with something like HIIT, you will still see large improvements in both anaerobic and aerobic fitness, as well as some serious improvements in body composition.
So if you train for health, function, and aerobic performance, or for simply looking good, then the ketogenic diet could be for you!
Westman, Eric C., et al. “A review of low-carbohydrate ketogenic diets.” Current atherosclerosis reports 5.6 (2003): 476-483.
Krikorian, Robert, et al. “Dietary ketosis enhances memory in mild cognitive impairment.” Neurobiology of aging 33.2 (2012): 425-e19.
Vargas, Salvador, et al. “Efficacy of the ketogenic diet on body composition during resistance training in trained men: a randomized controlled trial.”. Journal of the International Society of Sports Nutrition 15.1 (2018): 31.
Paoli, Antonio, et al. “Beyond weight loss: a review of the therapeutic uses of very-low-carbohydrate (ketogenic) diets.”. European journal of clinical nutrition 67.8 (2013): 789.
Kossoff, Eric H., and Jong M. Rho. “Ketogenic diets: evidence for short-and long-term efficacy.” Neurotherapeutics 6.2 (2009): 406-414.
Wewege, M., et al. “The effects of high‐intensity interval training vs. moderate‐intensity continuous training on body composition in overweight and obese adults. A systematic review and meta‐analysis.” Obesity Reviews 18.6 (2017): 635-646.
Kessler, Holly S., Susan B. Sisson, and Kevin R. Short. “The potential for high-intensity interval training to reduce cardiometabolic disease risk.” Sports medicine 42.6 (2012): 489-509.
Heggelund, Jørn, et al. “High aerobic intensity training and psychological states in patients with depression or schizophrenia.” Frontiers in psychiatry 5 (2014): 148.
Aslankeser, Zübeyde, and Şükrü Serdar Balcı. “Re-examination of the contribution of substrates to energy expenditure during high-intensity intermittent exercise in endurance athletes.” PeerJ 5 (2017): e3769.
Cipryan, Lukas, et al. “Effects of a 4-Week Very Low-Carbohydrate Diet on High-Intensity Interval Training Responses.” Journal of sports science & medicine 17.2 (2018): 259.
Chang, Chen-Kang, Katarina Borer, and Po-Ju Lin. “Low-carbohydrate-high-fat diet: Can it help exercise performance?.” Journal of human kinetics 56.1 (2017): 81-92.
Volek, Jeff S., Timothy Noakes, and Stephen D. Phinney. “Rethinking fat as a fuel for endurance exercise.” European journal of sport science 15.1 (2015): 13-20.
Wroble K et al., “Low-carbohydrate, ketogenic diet impairs anaerobic exercise performance in exercise-trained women and men. A randomized-sequence crossover trial.” The Journal of sports medicine and physical fitness (2018).
Zinn, Caryn, et al. “Ketogenic diet benefits body composition and well-being but not performance. A pilot case study of New Zealand endurance athletes.” Journal of the International Society of Sports Nutrition 14.1 (2017): 22.
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