Keto Diet and Endurance Sports

Introduction

There has been considerable interest in ketogenic (keto) or low carbohydrate high fat (LCHF) diets in recent years. But are these appropriate for endurance athletes looking to improve their performance? Keto or LCHF diets have been associated with weight loss, improved body composition, metabolic health benefits and increased mental clarity, both anecdotally and through research. Their impact on endurance sports performance, however, has been subject to much debate, both on social media and in the scientific literature. There is still much to learn about if, or when, they might be a useful strategy for athletes. But new research studies published between 2016 and 2020 have helped to build our understanding about whether these diets may benefit athletic performance and in what type of events. In this blog I share these findings so that you can make up your own mind about whether to try a ketogenic or LCHF diet.

First, a bit of background about these diets. A ketogenic diet will restrict carbohydrate intake to 50g per day or less, generally less than 5% of calories consumed, while fat intake will make up around 75-80%. The aim is to reach and remain in a state of 'ketosis', where your body can produce ketone bodies through fat metabolism that can be used for energy in place of glucose from carbohydrate metabolism. A low carbohydrate high fat (LCHF) diet is not low enough to induce ketosis, as carbohydrate intake is generally maintained at 10-20% of calories while fat might make up 60-70%, but it is likely to result in some level of increased ability to burn fat as fuel due to the carbohydrate restriction. By comparison, a standard diet would be composed of 45-60% of calories from carbohydrate and 30-35% from fat, with the switch from fat to carbohydrate metabolism occurring at a lower intensity during exercise than it would in an athlete following a keto or LCHF diet. The protein intake in all three types of diet is similar at 15-20% of calories.

One benefit for athletes following ketogenic or LCHF diets is that they may be able to rely on stored fat as a source of fuel for longer, due to increased fat burning capacity. This spares the more limited muscle glycogen stores, with a reduced need to take on carbohydrate during training or racing. This may be helpful for some endurance athletes due to the risk of experiencing gastrointestinal distress from a high carbohydrate intake and is often given as a reason for following a high fat diet.

But will an increased capacity to burn fat improve performance?

It has been shown in several studies that switching to a ketogenic or LCHF diet improves fat oxidation rates. For example, a 2016 study ¹ compared elite ultra-marathon and iron-distance triathletes during 3 hours of running on a treadmill at 64% of VO₂ max (easy to moderate pace). One group followed a high carb diet with 60% of calories from carbohydrate while the other group had adapted to the LCHF diet approach (average 10% carbs, 70% fat) over an average of 20 months. During the study, the LCHF athletes demonstrated average fat oxidation rates that were 59% higher than the high carb athletes. Peak fat oxidation – the crossover to using carbohydrate as fuel – occurred at 70% of VO₂ max in the LCHF group compared with 55% of VO₂ max in the standard diet group. But this study did not measure performance, so it doesn't answer any questions about that.

Another study on elite athletes published in 2017 ², in this case race walkers, took a very different approach but one that gives us plenty of useful information about the impact of a ketogenic diet on performance, at least in this very highly trained population. Athletes were educated about three different types of diet: standard high carbohydrate (8.6g/kg/day, consumed pre, during and post training), periodised carbohydrate (the same quantity overall but with intake varied to match different types of training sessions, including some done with low carb stores) or ketogenic (under 50g of carbs per day with 78% of calories from fat). They chose which one to follow. At the start of the trial, they undertook both a 10km race and a 25km training session, done at 50km race pace. These were repeated at the end of the three week trial, after all groups had undertaken the same intensive training programme and followed their chosen diet, with all food provided by dieticians during a training camp.

Although all athletes demonstrated an increase in their maximum aerobic capacity after three weeks of intensive training, only the high carb and periodised carb groups saw an improvement in their 10km race time, while the ketogenic group's time stayed the same. Moreover, while the high and periodised carb groups saw a reduction in the aerobic capacity required to maintain 50km race pace, in the ketogenic group it stayed the same despite huge increases in fat oxidation capacity (which suggests that the short period of adaptation to a keto-diet was not an issue). This finding reflected reduced exercise economy: switching from carbohydrate to fat as their energy source meant that they required more oxygen to produce each unit of energy (ATP). As a result, the benefits of increasing aerobic capacity through training were negated. This reduction in economy would be a significant issue for endurance athletes looking for improved performance from a ketogenic diet. It is also worth noting that athletes on the ketogenic diet experienced a greater perception of effort throughout the three week training period and recorded lower training volumes than the other groups.

After attracting a great deal of comment and controversy on social media, the researchers involved in the above race walker study decided to test whether the same results would be seen if the study was repeated. The new research was published in June 2020 ³ and the findings were indeed replicated. While the 10k performance of those race walkers following a high carbohydrate diet improved by 4.8% after 3 weeks of hard training, that of the race walkers following a ketogenic diet declined by 2.3%. This was due to reduced economy, ie they were less efficient at burning oxygen to produce speed. The researchers also extended the study to look at whether there was a 'super-compensation' effect shown by those athletes following the ketogenic diet if they returned to a high carbohydrate diet for 2.5 weeks taper prior to a 20 km race. This does not appear to be the case: they returned to their pre-diet trial performance level, rather than improving on it. Meanwhile the race walkers who had trained for 3 weeks on a high carb diet, and then continued with it for a further 2.5 weeks taper, performed at a similar level to their improved post-trial 10k. It should be noted that this study was done in elite athletes and the results might not be the same for recreational athletes. Also, the distances raced were fairly short (10-20k), requiring a certain level of intensity, so the results might not apply to ultra-endurance events.

Can you still perform at high intensity while following a keto or LCHF diet?

So, we know that high fat diets increase the ability to burn fat as fuel at higher intensities, but also that you need more oxygen to convert fat to energy than carbohydrate, which makes you less efficient and therefore slower. We also know from earlier studies ⁴ that the mix of carbohydrate (high intensity) and fat (lower intensity) required for energy varies according to the type/duration of event (think 10k vs marathon or sprint triathlon vs iron distance), the training status of the athlete, and both recent and long term dietary intake. For optimal performance, metabolic flexibility – the ability to switch between fuel sources – is required so that high intensity pace can be switched on when needed by rapidly generating energy, eg for a surge in a cycling race. This can only happen if the athlete has maintained the ability to burn carbohydrate efficiently, as fat cannot be converted to energy as fast as carbohydrate.

Previous studies have shown that the ability to sprint while following a short term LCHF diet, even with a one day carb re-load, are compromised ⁵ and that the process of converting stored muscle glycogen is impaired. ⁶ This may not be of concern for an athlete undertaking an ultra-endurance event or a recreational athlete who is looking to finish an event such as a middle or long distance triathlon or a marathon, rather than to be competitive, but it is certainly something to consider is you are looking to "race" rather than "complete" your endurance event.

However, this may not be the whole story. The period of adaptation to a high fat diet and the training status of the athlete may be relevant. Three studies published more recently in 2018-19 have shown that the ability to sprint or perform high intensity work while following a very low carbohydrate diet may be maintained in some athletes. In the first study ⁷ , 20 male endurance-trained subjects followed the same training programme and either a high carb diet (65% carb, 20% fat) or a keto diet (6% carb, 77% fat) for 12 weeks. Before and after, they performed a 6 second peak power test. The keto group saw an increase in  sprint power (+0.8 w/kg) while the high carb group did not (-0.1 w/kg).

The second study ⁸ involved 18 moderately trained males who followed a very low carb diet (<50g carbs/day, 63% fat, 29% protein) or their habitual diet (48% carbs, 17% protein, 35% fat) for four weeks. Before and after the trial, they undertook a high intensity interval session. (5×3 mins). Both groups performed the intervals at a similar % of VO₂ max in both trials and reported a similar rate of perceived exertion. The low carb group's ability to perform high intensity work was no different, despite evidence of increased fat burning ability. It is worth noting that the diets in this study were not well controlled and that the fat intake was too low and the protein intake too high for this to be considered a ketogenic diet, although carbs were less than 50g/day. This may have had an impact on the results. The same authors then conducted a different study ⁹ where 24 recreational athletes followed a very low carb high fat (VLCHF – 8% carbs, 69% fat, 23% protein) diet or their habitual diet for 12 weeks. No decrease in high intensity continuous or intermittent exercise lasting up to 25 minutes was seen in the VLCHF group, while both groups improved their time to exhaustion in a graded exercise test. The VLCHF group lost 3.6 kg over 12 weeks compared with a loss of 0.9 kg in the habitual diet group. This may help explain why they did not see a performance decline as might be expected with this type of exercise.

Can you maintain endurance performance on a ketogenic or LCHF diet?

The 20 endurance-trained athletes who performed the 6 second peak power test discussed above ⁷ also undertook a 100 km TT, in which there was no significant change for either group in their time, despite the keto group consuming only water in the 'after' test vs 30-60g of carbs per hour in the 'before' test. This suggests that a keto-diet may be beneficial for endurance events where athletes struggle with high carbohydrate intakes. The keto group had also lost an average of 5.9 kg in weight during the 12 week diet trial, which might be beneficial for performance in real life cycling events. The 100km TT was performed on a stationery bike at a self-selected resistance which prevented the improved economy from weight loss affecting the study results. It is also worth noting that the keto group showed significantly greater fat oxidation than the high carb group.

There is some evidence that weight loss resulting from a ketogenic diet may offset the decline in performance that might be expected from a reduced ability to oxidise carbohydrate at higher intensities. A small study ¹⁰ on 8 recreational runners showed an average weight loss of 2.5 kg but no significant difference in their 5km time trial performance after 3 weeks on a ketogenic diet, when they could consume as many calories from fat as they wished while restricting carbohydrates to less than 50g per day. The authors suggest that eating in this way is a good way to improve body composition while being able to maintain performance, at least in recreational middle aged runners.

Two other recent small studies, both from the same research group, are more in line with previous findings on ketogenic diets, as far as performance goes. One ¹¹ , a pilot case study with just 5 recreational athletes aged 49-55 (4 female, 1 male) who all followed a diet with less than 50g carbs/day and unlimited fat found that while their ability to utilise fat was increased, their time to exhaustion in a graded cycling test dropped significantly as their ability to perform at high intensity was compromised. However, they did see other benefits: an average weight loss of 4kg, improved skin conditions and quicker recovery from training. All chose to continue with a modified lower carb, higher fat diet after the trial due to enhanced well-being.

In the other study ¹² , 8 runners with sub 3:30 marathon times spent 31 days on their habitual diet (48% carbs, 38% fat) and 31 days on a ketogenic diet (4% carbs, 78% fat). At the start and end of each trial period, subjects completed a graded exercise test to assess VO₂ max, followed two days later by a time to exhaustion trial done at 70% of VO₂ max. These showed that the ketogenic diet impaired efficiency when running above 70% of VO₂ max in the graded exercise test but overall time to exhaustion remained the same when running at 70% of VO₂ max. This reflects other trial results, but what is interesting is the individual variation in the TTE test: 3 subjects lasted longer than before while 5 gave up sooner. This emphasizes the point that the effect of a ketogenic diet is individual, which has also been shown in previous studies. ^1,2 It's also worth noting that those runners who had maintained more ability to metabolise carbohydrate (higher respiratory exchange ratio) lasted longer in the test. It's a small study but perhaps makes an important point about ketogenic diets and performance.

Does carbohydrate feeding prior to and during races have any benefit when following a ketogenic or LCHF diet?

In 2018, a paper was published that looked at different strategies for fuelling iron distance triathlons ¹³. it was suggested that following a low carbohydrate or ketogenic diet might be appropriate for those looking to complete this ultra-endurance event in more than 12 hours, as the fat oxidation rates that might be achieved from such a diet would likely be sufficient to match the energy demands of their event. However, elite and top level age groupers with target finish times between 8 hrs and 11 hrs might benefit from following an LCHF or ketogenic diet in training, but would need to maximise their glycogen stores prior to the race by significantly increasing the amount of carbohydrate consumed in their diet. They would then need to take on carbohydrate during the race itself to be able to maintain economy at higher intensities, enabling them to be competitive. Bearing this in mind, it has been questioned why an elite athlete would need to follow an ketogenic diet at all, if the intention is to race with full glucose availability, negating the potential benefits of ketogenesis (ie using ketone bodies as a source of fuel). ¹⁴ There would need to be other benefits from a ketogenic diet for the athlete, eg body composition benefits, prevention of gastrointestinal problems, reduced levels of post-exercise inflammation, etc. These potential health benefits and their consequent impacts on performance are yet to be researched comprehensively.

In the meantime, there are two case studies on elite athletes using a LCHF approach in training, but racing with restored glycogen stores, which provide insights into the effect on performance. The first ¹⁵ on an athlete who had followed an LCHF diet for two years and performed well at various distances on water alone  found that feeding carbohydrate during the event benefited performance in a high intensity 20km cycling time trial but not in a prolonged 100km time trial. The second ¹⁶  found that the athlete experienced his worst ever long distance triathlon race results during 8 months on an LCHF diet but was back to his normal level of performance after just 5 weeks of returning to a high carbohydrate diet. All this really suggests is that the effects of LCHF diets are individual. Also see the 2020 race walker replication study ³ discussed at the start for more on the effect of restoring glycogen levels prior to racing.

So, what conclusions can be drawn?

These would be my observations from the research.

• Ketogenic and LCHF diets may have a more positive effect on performance in recreational than highly trained athletes. This may be the result of weight loss combined with benefits from increased training, rather than related to fat burning ability.
• Ketogenic and LCHF diets are likely to compromise performance above 70-80% of VO _{2 max.} This could include "shorter" endurance events up to marathon distance or those where bursts of speed are required to be competitive, such as road racing.
• Ketogenic and LCHF diets may be beneficial for those competing in ultra-endurance events conducted at a steady pace below 70% of VO₂ max, particularly when there is risk of GI distress from consuming large amounts of carbohydrate or it may be difficult to carry large amounts of nutrition.

If you do decide to try this dietary approach, two books to check out can be found on Amazon here and here.

I would also add that ketogenic diets and LCHF diets are not the only ways to improve the ability to burn fat: see my blogs on Low Glycogen Training and the Sleep Low nutrition strategy  for more on this. These approaches encourage metabolic flexibility as discussed earlier.

Finally, a related area of interest is the use of ketogenic supplements, which may enable athletes to experience the benefits of a ketogenic diet while maintaining the ability to consume and use carbohydrate for fuel at higher intensities. For more on this, see my blog here.

Jo Scott-Dalgleish BSc (Hons) is a BANT Registered Nutritionist who writes about nutrition for endurance sport. She also offers nutritional therapy consultations to UK-based triathletes, runners, cyclists and other endurance sports participants looking for improved health and performance, either in West London or online. For further details and to make an enquiry, please visit https://www.nutritionforendurancesports.co.uk/

References

¹ Voleket al. Metabolic characteristics of keto-adapted ultra-endurance runners.Metabolism. 2016. March; 65(3): 100-110. https://www.ncbi.nlm.nih.gov/pubmed/26892521

² Burke et al. Low carbohydrate high fat diet impairs exercise economy and negates the performance benefits from intensified training in elite race walkers. J Physiol. 2017. May 1;595(9):2785-2807. https://www.ncbi.nlm.nih.gov/pubmed/28012184

³ Burke et al. Crisis of confidence averted: impairment of exercise economy and performance in elite race walkers by ketogenic low carbohydrate high fat (LCHF) diet is reproducible. PLoS One. 2020. Jun 4; 15(6): e0234027.  https://pubmed.ncbi.nlm.nih.gov/32497061/ FULL TEXT AVAILABLE

⁴ Burke LM. Re-examining high fat diets for sports performance: did we call the 'nail in the coffin' too soon? Sports Med. 2015 Nov;45 Suppl 1: S33-49. https://www.ncbi.nlm.nih.gov/pubmed/26553488

⁵  Havermann et al. Fat adaptation followed by carbohydrate-loading compromises high intensity sprint performance. J Appl Physiol. 2006. Jan;100(1):194-202. https://www.ncbi.nlm.nih.gov/pubmed/16141377

⁶ Stellingweff et al.Decreased PDH activation and glycogenolysis during exercise following fat adaptation with carbohydrate restoration. Am J Physiol Endocinol Metab. 2006 Feb;290(2): E380-8. https://www.ncbi.nlm.nih.gov/pubmed/16188909

⁷ McSwiney et al. Keto-adaptation enhances exercise performance and body composition responses to training in endurance athletes. Metabolism. 2018. Apr; 81:25-34. https://www.ncbi.nlm.nih.gov/pubmed/29108901

⁸  Cipryan et al. Effects of a 4-week very low carbohydrate diet on high intensity interval training responses. J of Sports Sc & Med. 2018. 17, 259-268. https://www.ncbi.nlm.nih.gov/pubmed/29769827

⁹ Dostal et al. Effects of a 12 week very-low carbohydrate high-fat diet on maximal aerobic capacity, high intensity intermittent exercise and cardiac autonomic regulation: non-randomized parallel-group study. Front Physiol. 2019. July 17; 10:912 https://pubmed.ncbi.nlm.nih.gov/31379612/

¹⁰ Heatherly et al. Effects of ad libitum low-carbohydrate high-fat dieting in middle-age male runners. Med Sci Sports Exerc. 2018. Mar; 50(3): 570-579. https://pubmed.ncbi.nlm.nih.gov/29112626/

¹¹  Zinn et al. Ketogenic diet benefits body composition and wellbeing but not performance in a pilot case study of New Zealand endurance athletes. J Int Soc Sport Nutr. 2017 Jul 12; 14:22 https://www.ncbi.nlm.nih.gov/pubmed/28706467

¹² Shaw et al. Effect of a ketogenic diet on sub-maximal exercise capacity and efficiency in runners. Med Sci Sports Exerc. 2019. Oct; 51(10): 2135-2146 https://www.ncbi.nlm.nih.gov/pubmed/31033901

¹³ Maunder E, Kilding AE, Plews DJ. Substrate metabolism during Ironman triathlon: different horses on the same courses. Sports Med. 2018 Oct; 48(10): 2219-2226. https://pubmed.ncbi.nlm.nih.gov/29777386

¹⁴ McSwiney et al. Impact of ketogenic diets on athletes: current insights. Open Access J Sports Med. 2019. Nov 15:10; 171-183. https://pubmed.ncbi.nlm.nih.gov/31814784

¹⁵  Webster et al. A carbohydrate ingestion intervention in an elite athlete who follows a low-carbohydrate high-fat diet. Int J Sports Phsiol Perform. 2018. Aug 1; 13(7): 957-960 https://pubmed.ncbi.nlm.nih.gov/29252062

¹⁶  Mujika I. Case study: long term low carbohydrate high fat diet impairs performance and subjective well-being in a world-class vegetarian long distance triathlete. Int J Sport Nutr Exerc Metab. 2018. Aug 30: 1-19 https://pubmed.ncbi.nlm.nih.gov/30160554

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Keto Diet and Endurance Sports

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