Ketogenic Diet for Athletes
Coach Kevin recently wrote an article comparing high carbohydrate to high fat (or ketogenic diets). Hopefully, by the end, you had a better idea of whether you should stick with the norm (high carb), or consider trying out a high-fat option. This article is for those who are thinking about giving keto a go – Kevin will explain the potential benefits, downsides, and how to do it right from an athlete’s perspective using both his experiences and science.
Perhaps you are open to the idea of a ketogenic or low carb diet but are still unsure whether to take the plunge. These questions should help you decide:
Do you seem to struggle to digest carbs? Either during typical meals or while training/racing (this will be experienced via bloating, stomach / intestinal pain or cramps)
Do your energy levels seem to fluctuate wildly during the day, perhaps needing to take naps after meals or crashing out after steady endurance efforts? If you can’t snack / eat at regular intervals, do you get irritable or tired?
Are you a long or ultra-distance athlete who can’t seem to get the nutritional side of things right after many attempts, despite having considered carbohydrate loading rates, hydration, & practising/validating race nutrition plans in training?
Does your weight fluctuate based on how much or little you are training?
Are you trying to balance losing weight while maintaining training?
The more of the above questions you answered yes to (and they are in order of importance), the more I’d suggest you should consider trying out a ketogenic or low carb diet (I’m using both these terms for now, but I’ll explain the difference later).
If by this point you have decided, yes, you will try a keto diet out, the next question to ask is ‘what time of the training year is it’? If the answer is anything but ‘off or pre-season’ or ‘base training’, then I’d put your plans on hold for a little longer. This type of change (depending on just how reliant on carbohydrates you are) can create havoc with your training and take up to 3 – 6 months or more to (literally) get back up to full speed. In addition, drastically cutting down on carbs can, for some, be a challenging process, and only for those who are well motivated. Sugar has often been compared to an addictive drug, and I can personally attest that the first two weeks of the shift to low carb takes a substantial amount of willpower.
For those of you still with me, let’s take a quick look at what benefits you could potentially expect after a sufficient period of adaptation:
Stable (i.e. not excessive but with no dips either) energy levels all-day
The ability to skip meals without consequence if required
Less hunger if trying to lose weight
Improved fat oxidisation rates during exercise
As an athlete, I’m guessing the last point is the one you are interested in, so why is that an advantage? Well, the more fat you burn for a given intensity, the less glycogen you are burning and, therefore, in theory, the longer you can operate at that intensity. This is due to glycogen stores often being a limiting performance factor (it’s generally tricky to replace glycogen at the rate it is burnt, and the faster you are, the more this is the case). We can break this down a little further by defining two points – FatMax (the power at which you burn the most fat) and Max fat oxidisation (MFO), the maximum rate at which you burn fat:
(The above graph is taken from the study: “Contextualising Maximal Fat Oxidation During Exercise: Determinants and Normative Values”)
The higher your FatMax or MFO, the closer you can operate at race intensities without burning through your glycogen reserves significantly. For an imaginary IRONMAN athlete with an FTP of 300w, if you could bring your FaxMax up from 150w to 225w, then suddenly your IM power (assuming it’s 75% of FTP, a reasonable value for a well-trained triathlete) is now aligned with your maximum fat burn rate. That’s 75 more watts of power output that doesn’t need to be fuelled by carbohydrates, allowing you to take on fewer carbs (if you struggle with that) or spare more glycogen reserves for the run, where it’s often even more challenging to keep fuel supply up with demand. Note that FatMax does not equate to zero glycogen use. Therefore taking on some carbs during racing would still be required.
So if you have decided to give this a try, how best to approach it? I won’t detail the general principles of beginning the transition to a ketogenic diet. There are many, many other articles & websites out there which can help with that. There are a few considerations you need to keep in mind as an athlete, however, as most keto articles are often aimed at less active people looking for health benefits or weight loss. So the main additional areas for athletes to consider are:
Keep in mind that this is not a calorie-restricted diet.
High-fat diets can leave you feeling relatively full. As such (especially if you have not yet developed the enzymes and bacteria to help you easily digest fatty food) there can be a tendency for athletes to undereat. While calorie counting is possible, I would suggest instead going by feel but erring on the side of eating more than you think you need – this is better than being drastically low on calories.
Low Carb or Keto?
Keto is generally defined as daily carbohydrate levels of less than 50g, and sometimes as low as 20g. However, as athletes who burn more glycogen than sedentary people, we have more scope with this value. I would suggest that you stick to the keto guidelines to start with (20 – 50g carbs per day maximum), certainly for the first couple of weeks. But as time goes on and you seem to be transitioning into ketosis period, perhaps add a few more carbs back in on the days when you are doing more training again (especially if this is higher intensity, however, see below!). This approach is usually labelled as Low Carb – there is no real defined amount of carbs for this; it’s just a less strict version of the keto diet. Beware of doing this too soon, however, as your body can quickly switch back to carb-burning mode (you might notice this by getting the ‘keto flu’ symptoms back or by losing the feeling of having consistent energy levels through the day).
Should I keep training?
I think that for the first week or two, when most people struggle the most with the transition (understandably, as you deny your body its preferred fuel source), you should take it very easy with any exercise and keep it low intensity. Also, consider indoor training – it’s easier to stop/call it a day if you feel bad. As time goes on and you feel more comfortable with the diet, you can bring more training back in and perhaps try some higher intensity efforts. The best approach is to keep to very low levels of carbs on easier days and get a small amount back during or immediately after the more demanding sessions.
Sodium levels
One good piece of advice for anyone on a keto diet is to keep well hydrated and increase sodium intake. This is because the drop in blood sugar and therefore also insulin levels caused by a reduction in carbohydrate intake prompts the body to retain less sodium (more comes out in urine). So given that athletes are generally going to have higher sodium requirements even on a traditional diet, this is something you should really keep on top of. If you are not sure how much salt to add to your food, go by what feels right. Your body is very good at telling you what you need if you listen closely! Cravings are a great example of this.
My FTP has dropped; what should I do?
This is a typical occurrence and might signal the end of your keto experiment if the drop is substantial; however, I’d advise patience. This is also the main reason that you should try this diet in the offseason, so there is no real-time pressure to get this level of performance back. The more your reliance on carbs, the greater this drop could be – you have taken away the fuel your body nearly exclusively relied on to produce this level of power. But another way of looking at it is that this is just highlighting your dependence on carbohydrates, and if you keep at it, eventually, the systems controlling your fat utilisation should up-regulate and make up most of the difference. Once this has happened, and you bring some carbohydrates back into your diet, and during training, you should hopefully see previous FTP values come back. If not, excluding long-duration athletes for whom FTP intensities are just not that relevant, then perhaps this diet is just not for you, and you should switch back to something closer than what you were doing before.
Do I never need to eat carbs again?
One phrase often used concerning a keto diet is metabolic flexibility. This is the ability to use either fat or carbs as a fuel source and be able to switch between them easily. Athletes who over-eat carbs (more is better, right?) and have not done sufficient base training can often be overly reliant on carbs & terrible at burning fat – but as a keto athlete, there is a danger that you can swing just as far in the opposite direction. You may be excellent at burning fat, but if you cannot process and utilise carbohydrates, you miss out on a major energy source, especially as intensity increases. To avoid this, you need to strike a balance between fat and carb intake, adjusting for low and high-intensity exercise. There is also the fact that unless you are competing at very low-intensity levels, you’ll still need to take on some carbs during races. I’ll address how to do both of these things in another article, however!
Kevin opened a B&B for cyclists in France in 2014, & then a year later decided to start a cycle coaching (level 3) qualification. This was mainly in order to be able to better support his guests (but also to make his own training more effective too). At the B&B he runs the odd training camp for cyclists but mainly offers coaching advice while guiding guests. An engineer by background, he happy diving into all sorts of training data, but also understanding that coaching is about much more than just the numbers!
After being introduced to triathlon by a good friend & then taking part in races for a couple of years he decided that a level 2 coaching course with British Triathlon was the way forward, completing this in 2019.
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