• Marcus Baxby

Keto for CrossFit?

As some of you may know, my training at the moment involves CrossFit, usually 3 or 4 times per week.

Without boring everyone about why I enjoy it so much, there are a few things you need to know about CrossFit.

CrossFit involves strength, cardiovascular endurance, stamina and skills (mainly around Olympic weightlifting and gymnastics).

It is almost always performed at high intensities.


From a physiological point of view, this means that it uses all three main energy systems –

the PCr system (for explosive efforts of less than 10 seconds, eg. An Olympic lift),

the anaerobic glycolytic system (for up to three minute high intensity efforts),

and the aerobic system, for longer, low-moderate intensity efforts.

CrossFit generally consists of combinations of exercises, in an ‘interval’ format, meaning that the first two systems are predominantly utilised for the majority of workouts.

The aerobic system will be utilised as well, but mostly in the background, or during longer, ‘single modality’ workouts such as a long run, bike or row (or combination thereof).

ATP is the energy currency of the body, meaning that to have energy to do anything, we need to produce ATP.

The PCr system utilises the small amount of creatine phosphate that is stored in the muscles.

This generates ATP rapidly, but is in limited supply, meaning that the capacity of the PCr system is limited to around 10 seconds, and that it is used for short, explosive efforts.

In CrossFit, this could be an Olympic lift, a max effort squat, or an explosive jump.

Supplementing with creatine monohydrate has huge benefits for this system, and improves maximal strength, and the ability of the body to recover from maximal efforts.

The anaerobic glycolytic system does not use oxygen, but uses glucose (hence anaerobic glycolytic),

The glucose is broken down into ATP and pyruvate. The pyruvate, in turn, is broken down further into more ATP, producing even more energy.

This system is utilised for ‘prolonged’ high intensity exercise, usually lasting between 10 and 30 seconds, but sometimes up to 3 minutes in well-trained individuals.

In CrossFit, this system is relied on a lot.

As we said, the anaerobic glycolytic system uses glucose.

The main source of glucose is… carbohydrates in the diet.

We will come back to this.

The final system is the aerobic system, which relies on oxygen, combined with the oxidation of either carbs, or fats, or both.

Because CrossFit uses all three energy systems (the PCr, anaerobic glycolitc and aerobic systems), glucose is required in order to perform these types of workouts.

I came across an article recently (from a popular CrossFit news site), which had the title ‘if you’re serious about CrossFit, you should try Keto’.

For those not familiar, ‘Keto’, aka the ketogenic diet, is a protocol whereby carbohydrates are kept at extremely low levels (less than 10% of total calories), protein is relatively low (between 15-25% total calories) and fat is jacked up to between 60-80% of total calories.

For context, fat normally comprises about 20 – 35% of total calories.


A classic keto meal

The idea behind the article was that people on Keto diets would:


a. Lose weight

b. Not lose strength; and

c. Become able to utilise fat instead of carbohydrates for exercise

So let’s dissect each of these one by one

a. Weight loss

People on ketogenic diets do often lose weight, that’s true.

However, as has been proven in the research time and time again, nobody loses weight independent of energy balance.

There is sometimes greater adherence to a keto diet (often because of mistaken beliefs about its ‘magical’ properties), which leads people to stick to it for longer.

Thereby eating fewer calories.

And losing weight.

A calorie deficit remains the principle.

Keto is just one method.

b. Not losing strength


The author (who by the way identified himself as a ‘Certified Ketogenic Health Coach’, not a nutritionist or doctor – vested interest anyone?) did, to his credit, cite some scientific research.


However, the studies on the effects of keto were not long enough for strength to be lost, didn’t study enough people (only 12 participants) and as we know, the most important factors in muscle and strength are a) training and b) protein, both of which the participants continued to broadly the same degree.

The studies didn’t prove what he thought they did, and while people didn’t lose strength on average, they did lose some leg muscle mass, which could negatively affect performance had the study been carried out over a longer period of time.

c. Utilising fat instead of carbohydrate

This is the crux of the argument, and where a good understanding of performance nutrition is required.


The first thing to say is that the body, under normal circumstances, utilises fat at relatively low intensities.

Walking, slow running, carrying out daily chores.

It’s also very important to note that we are talking about fat from food – NOT fat tissue on the body.

Carbs, on the other hand are the body’s preferred fuel source, particularly at higher intensities.

Now, the author correctly identified that lower carb diets increases the body’s ability to use fat for fuel.


But his understanding stopped there.

When carbohydrates are low (like they are in a keto diet), the body has no option but to use fat instead, even at higher intensities.

This is known as becoming ‘fat adapted’.

Restriction of carbohydrates means that the body is forced into using fat for fuel, instead of carbohydrates.

This can happen in as little as 5 days.

The body may also start to utilise protein for making glucose, instead of dietary carbohydrates (again, because it has no choice when there are no dietary carbohydrates!)

This is a process called gluconeogenesis, and this is a rare, and a much less efficient way of producing glucose.

Becoming fat adapted can be useful (and can a crucial part of becoming a better endurance athlete).

The reason for this, however, is not because fat is a superior fuel source, but because it is useful for the body to be able to switch effectively between fat and carbohydrates for fuel at appropriate times.

This ability to switch between carbs and fats at the appropriate time is known as ‘metabolic flexibility’

Increasing metabolic flexibility can be a crucial part of a training programme, particularly where endurance is involved.


An over-reliance on carbohydrates as a fuel source could be damaging to performance, as energy supplies may run out quickly, particularly in longer events, and therefore it is useful to be able to use fat at appropriate times.

The longer the body is able to use fats, the more the carbohydrates are spared, and remain stored for when they are needed most.

This can be beneficial for longer distance events, as if the body is able to oxidise fat at higher intensities, muscle glycogen (stored carbohydrates) is saved for later, or more intense portions of the race, such as hill climbs or sprint finishes.

It’s worth noting that endurance exercise itself increases fat adaptation, but there is some evidence that reducing carbohydrates for some training sessions can further increase the capacity to utilise fat at higher intensities.

Team sports, endurance athletes, and yes, CrossFitters, can all benefit from doing some training sessions with low carbohydrate availability so as to increase their ability to utilise fat as fuel – particularly for endurance-based sessions.

Several methods of training with low carbohydrate availability have been explored in the research, and much of the literature, as well as many of the discussions amongst practitioners within performance nutrition, now centre upon the way in which carbohydrates are periodised in order to maximise the response from a training regimen.

Training with low carbohydrate availability stimulates the machinery used for fat metabolism, and improves aerobic, oxidative metabolism capacity within cells.

Train Low is the most popular carb periodisation strategy, and usually involves training first thing in the morning, without eating, particularly a longer cardio session.

Another strategy is to do a glycogen depleting session, not eating carbohydrates afterwards (thereby not replenishing muscle glycogen), and doing another session later in the day.

For much longer sessions, athletes could start a session with low carbohydrate availability, then eat some carbs (such as an energy gel or energy drink) during the session, so that carbs are available for the end of the session.

A combination ‘Sleep Low’ strategy involves doing a glycogen-depleting session in the afternoon or evening, not replenishing carbohydrates afterwards (just eating protein), then going to bed, and waking up to do another training session.

The strategy can therefore be adapted to the specific athlete.


However, while it is useful to have the ability to use fat where appropriate, it’s ALSO useful to be able to burn carbohydrates at higher intensities.

The research that has been done in this area shows that, while athletes who reduced carbohydrates on a chronic basis burned more fat (became fat adapted), their performance either stayed the same or got worse.

In addition, reducing carbs long-term, such as with a ketogenic diet, means that the ability to burn carbohydrates at high intensities is actually lost.

So you become unable to burn carbohydrates at high intensities, when they are most needed.

You lose your ‘top gear’ so to speak.

To circle back to the start, CrossFit is mostly performed at high intensity.

And carbohydrates are the fuel source that works best at those high intensities.

So yes – you can still do CrossFit on a keto diet.


(as in, you will be physically able to).


But if you want to optimise performance, and perform at your best, you will need carbohydrates.

So if you’re serious about CrossFit?

Don’t do Keto.

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