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Ketogenic diets have grown in popularity and there are certainly some well-established health benefits to them but can they work for an endurance athlete?


What is a ketogenic diet?  A  ketogenic diet is one that is very low in carbohydrate, usually under 50g a day (typically less than 5% of calories consumed versus a standard diet which is usually in the region of 45-60%), and high in fat (around 75-80% of energy intake).    The aim of a ketogenic diet is to reach and maintain a state of ketosis, where the body is using ketone bodies as source of energy instead of glucose.   However, be aware, a ketogenic diet is not the same as LCHF (Low-carb High-fat) – a common mistake people make is to replace carbohydrates with protein but this is not the same as many amino acids in protein can be converted into glucose and enter the glucose pathway which are are trying to avoid.

When carbohydrates are in plentiful supply, the body uses glucose as its primary energy source but if it is scarce the body will look to use alternative energy pathways using fat (or trigclycerides) and produce higher levels of ketone bodies  (acetoacetate, β-hydroxybutyrate and acetone) as the primary fuel.   Using increased fat as a fuel has been shown to have some well researched health benefits such as helping to reverse Type II Diabetes, epilepsy, to help manage pain from many chronic health conditions via a reduction in inflammation, improved blood lipid profiles and for weight loss.

However, is it a suitable strategy for endurance athletes?  Will an increased ability to burn fat help performance?  

The answer as usual is it depends – principally on an athlete’s goals (eg performance or weight loss), genetics, athletic level/trained status (eg what VO2 max are you going to be racing at) and consideration to your underlying health.

  • Your goals.  As an endurance athlete what is your primary goal when looking to a ketogenic diet – to improve performance or to lose weight?   A ketogenic diet is likely to help you lose weight which indirectly may result in improved performance but following a ketogenic diet is not necessarily going to improve your performance in the short term especially if weight loss is not something you need or want to achieve.   More on performance below.
  • Your perception of ‘endurance‘.  The definition of endurance is ‘the ability to endure an unpleasant or difficult process or situation without giving way’- for some this may be an olympic distance triathlon, for many it may be an Ironman or multi-day run/cycle event.  The benefits are likely to be greater the longer you go – for anything under a half ironman it is less likely a ketogenic diet will improve performance unless it is indirect via weight loss.
  • Underlying health concerns: if, for example, your cells do not produce enough insulin as in Type-I-diabetes the risk of entering ketoacidosis (dangerously high levels of blood ketones) is more likely and a ketogenic diet would not be recommended.  In healthy individuals, enough insulin should still be produced to prevent excessive amounts of ketones from forming.  Other mitochondrial issues that inhibit efficient utilisation of fat – if you suffer from unexplained fatigue it is worth chatting to a suitable qualified nutritional therapist before you embark on this diet.
  • Your level as an athlete.   Consider what VO2 max level you will be racing at.  Elite athletes are likely to race Ironman and half Ironman at VO2 levels significantly above 60-70% VO2 max, the level above which basic physiology shows that glucose is required to fuel performance.  So while there are benefits to be able to burn fat in the later stages of a long race, it is metabolic flexibility that is important, rather than your ability to burn fats at the expense of glucose.  It may be useful here to review some basic physiology:

We have 3 main fuel sources (ATP) – 2 aerobic and 1 anaerobic systems.  The one that utilises fats is aerobic but typically only able to fuel exercise intensity around 65-70%.   Thus, why the level of an athlete’s trained status and VO2 max or intensity that they are likely to race at is important.  However, it is not quite as simple as that, as for endurance athletes, if you are able to burn fats as a fuel efficiently, the level at which you can switch from carbohydrates to fat has been shown to increase if you have the metabolic flexibility to do so.

Performance Benefits:  What do the studies show?  Studies show mixed results which can be hard to translate across due to different lengths of study (very important when considering individual’s difference in time to adapt to burning ketones as a primary fuel source), different trained athlete status, different sports, different genders.  Here are some examples:

  • It has been shown in highly-trained ultra-endurance athletes that long-term keto-adaption can result in much higher rates of fat oxidation but that glycogen repletion rates after a 3 hour run are unchanged.  This implies fat adaptation does not preserve glycogen reserves however the crossover rate where athletes used carbohydrate as fuel on the ketogenic/LCHF diet occurred at 70% of VO2 max versus 50% in the standard diet group.
  • It has been shown elsewhere in elite endurance athletes (walkers) that 3 weeks of adaptation to a LCHF diet resulted in increased rates of fat oxidation but significantly reduced economy (ie more oxygen is required to produce the same energy output), negating potential performance benefits.  It is also interesting that the athletes on the ketogenic diet experienced a greater perception of effort (ie they found it harder)
  • Two other studies looking at Ironmen athletes and gender difference as one of their determinates, found that increased fat oxidation through a higher fat diet may have greater rewards for men than women.
  • It has been shown that the ability to sprint when following a short-term LCHF diet is compromised as the process of converting stored muscle glycogen to glucose is impaired.    Fat oxidation is a much slower source of fuel and is something to consider if you are looking to ‘race’ and not just ‘complete’ your event and also for athletes taking part in races under half iron man in distance.

For individuals who you may read about where performance has improved on a ketogenic diet I would want to know a little extra information such as how much weight they lost as this can certainly have an indirect effect on performance but are they are able to race at a higher intensity on it?  If not, how are they measuring performance?   I would also want to find out what other changes they made to their diet by reducing their carbohydrate intake – has inflammation fallen by the removal of all processed foods and alcohol? Benefits that could be achieved without the extremes that a ketogenic diet require in terms of carbohydrate limitation.


What can you make of this and what use is to you as an athlete?  Many endurance athletes I speak to have tried a ketogenic diet and found it great to start with and then unsustainable.   A solution to this which I commonly recommend, is it to trial it for a month during the ‘off season’.  If you get on well, as your training ramps up, consider a periodised or cyclical approach taking into account your training schedule where certain days are high fat and others are higher carbohydrate.   This approach is also likely to help improve your metabolic flexibility.  You will need to track your macro intake and measure your ketone bodies to do this properly.

Potential Benefits for endurance athletes:

  • Off season weight loss or weight maintenance.
  • Metabolic flexibility – this can be very beneficial in events such as an Ironman or ultra-endurance races and may help avoid ‘bonking’
  • Potential reduction in gut issues such as diarrhoea, cramps and nausea, through lower reliance on gels during the marathon or later stage of a longer race.
  • Improvement in blood glucose management and lipid markers (eg cholesterol).  If you have a family history of type 2 diabetes or cardiovascular disease, this may be a significant side (main?!) benefit for you.
  • Lowered inflammation in the body may result in improved recovery.

Some Potential Pitfalls:

  • Lack of being able to perform at higher intensities.
  • Genetics – you are not designed to metabolise fat well and will be fighting an uphill battle on this diet.
  • Compromised immune and thyroid function.  Insufficient carbohydrates for many athletes, particuarly woman, can be tough.
  • Potential loss of muscle mass if protein levels are reduced.
  • Limitation of food groups can result in lack of essential nutrients if enough care and attention is not given to a high fat diet.

To summarise: what should you do?

  • Get a comprehensive functional blood chemistry panel run to get a good overall idea of your health and your starting position.  This can also be used to choose selective markers to monitor progress and health.
  • Look at your diet as it is now.  Start with addressing the basics of your diet such as removing processed foods and increasing your vegetable and healthy fats (eg avocado, oily fish, nuts, seeds, hemp oil, flax, olive and coconut oils) content of your diet.  Ease yourself into a transition to a full ketogenic diet.
  • If you want to experiment a ketogenic diet, do it in the off season having built into it gradually as above. Then consider cycling in and out of ketosis to accommodate your training schedule.
  • Remember we are all unique with unique metabolisms, consider your stage of life, lifestyle and goals.  What works well for one person does not work well for everyone.
  • Don’t forget to buy a monitor to measure your urine ketone bodies.
  • If you are unsure where to start or want a more guided and personalised approach contact Katherine.