But what do scientific data truly say about the effectiveness of this dietary approach? <\/strong><\/p>\n <\/p>\n The ketogenic diet is a special form of the Low-Carb-High-Fat (LCHF) diet<\/em>. Its goal is to drastically reduce carbohydrate intake and significantly increase the fat content of the diet, in order to prompt the body to utilize ketone bodies<\/strong> as an alternative energy source. <\/span>(1<\/a><\/strong>).<\/span><\/p>\n The classic ketogenic diet consists of a ratio of 3 to 4:1 of dietary fat to carbohydrates and protein (<\/span>2<\/a>).<\/span> Depending on the study, diets are then defined as ketogenic diets if the proportion of carbohydrates is less than 5% (<\/span>3<\/a><\/strong>) –<\/span> or between 5 and 10% (<\/span>4<\/a><\/strong>)<\/span> – of the total energy. This corresponds to a carbohydrate intake of 30-50g per day – but only for people with low to moderate activity (<\/span>5<\/a><\/strong>). Ambitious endurance athletes consume between 4,000 – 6,000 calories per day. If 5% of the energy intake comes from carbohydrates, this would correspond to 50-75g of carbs. <\/span><\/p>\n <\/p>\n <\/p>\n The consistent implementation of a ketogenic diet shifts energy metabolism towards increased fat oxidation while simultaneously reducing glucose utilization. This process, known as keto-adaptation<\/strong>, is considered a central mechanism for applying this dietary approach in sports (<\/span>6<\/a><\/strong>).<\/span><\/p>\n In a state of ketosis, the body increasingly produces ketone bodies<\/strong> \u2014 primarily acetone, acetoacetate, and beta-hydroxybutyrate (\u03b2HB). \u03b2HB serves as the main measure for the degree of ketosis, with a value above 0.5 mmol\/L<\/strong> in the blood indicating what is known as nutritional ketosis<\/em> (<\/span>7<\/a><\/strong>).<\/span><\/p>\n <\/p>\n Ketone concentration in different nutritional situations (<\/span>7<\/a><\/strong>)<\/span><\/em><\/p><\/div>\n <\/p>\n Theoretically, in this metabolic state, the body is supposed to become so efficient in fat and ketone utilization<\/strong> that the same performance can be achieved without continuously supplying carbohydrates during training or competition (8<\/a><\/strong>).<\/span><\/p>\n <\/p>\n Proponents of the ketogenic diet argue that the limited carbohydrate stores<\/strong> in the body could represent a central weakness during prolonged endurance activities (9<\/a>, <\/strong><\/span>10<\/a>, 11<\/a><\/strong>).<\/span> In the body, carbohydrates are primarily stored as glycogen in muscles (250\u2013600 g) and liver (90 g); additionally, approximately 30 g of glucose are available in the blood \u2013 totaling around 1000\u20132400 kcal. However, even a partial depletion of these stores can lead to a reduction in performance<\/strong>. <\/span><\/p>\n In contrast, the body \u2013 even with a low body fat percentage \u2013 possesses enormous fat reserves: A 70 kg person with 10% body fat carries approximately 49,000 kcal of stored energy<\/strong>. This explains why the idea of targeted fat metabolism improvement is fundamentally attractive to many athletes (12<\/a><\/strong>). <\/span><\/p>\n <\/p>\n Simplified illustration of energy sources in endurance sports (13<\/a>)<\/em><\/p><\/div>\n <\/p>\n Due to the constant consumption of carbohydrates, nutrients are diverted from the stressed muscles to the intestines to support digestion, which can lead to reduced training economy and digestive disorders <\/span>(14<\/a>). In addition, humans can only absorb limited amounts of glucose and fructose per hour via the SGLT1 and GLUT-2, as well as GLUT5 transporters in the small intestine. Higher amounts of carbohydrates can lead to a ‘backlog’ of sugar molecules in the intestine, which can lead to gastrointestinal problems – such as diarrhea (15<\/a><\/strong>). <\/span><\/p>\n <\/p>\n <\/p>\n <\/p>\n After adapting to the ketogenic diet, the body should be able to use fat more effectively for energy production. This would be advantageous because, in contrast to the limited carbohydrate stores, the body has very large energy stores in the form of body fat. Even in a person with a low body fat percentage of approximately 10%, this would still represent an energy reserve of approximately 49,000kcal (in a 70kg person). <\/span><\/p>\n The idea is that a well-trained fat metabolism can produce energy quickly enough in the sub-maximal range and is not dependent on a high carbohydrate intake during exercise, which in turn should make most gastrointestinal problems disappear. <\/span><\/p>\n <\/p>\n Several studies show that a ketogenic diet can indeed induce a metabolic adaptation favoring fat oxidation<\/strong>:<\/p>\n In a pilot study, Kreutzer et al. (2003) observed increased fat and decreased carbohydrate oxidation during a 30-km cycling test (<\/span>20<\/a><\/strong>).\u00a0<\/span> Prins et al. found in a 5 km time trial (running against the clock) that with a high-carbohydrate diet, 3-7% of the energy was obtained from fat and 93-97% from carbohydrates. With a ketogenic intervention, however, the energy supply was 36-52% fat and 48-64% carbohydrates (<\/span>21<\/a><\/strong>). <\/span><\/p>\n <\/p>\n Percentage of energy supply through fatty acids: low-carb high-fat vs. high-carb low-fat (<\/span>21<\/a>)<\/span><\/em><\/p><\/div>\n <\/p>\n Maximum fat oxidation was also significantly higher in the ketogenic diet group than in the carb group. At an exercise intensity of 60% of VO2max, fat oxidation<\/strong> in the keto group was even 117% higher<\/strong> at 1.15 g\/min than in the high-carb group (0.51 g\/min) (<\/span>21<\/strong>)<\/span><\/a>.<\/p>\n <\/p>\n Fat oxidation rate in low-carb (green) vs high-carb (black) (21<\/strong><\/a>)<\/span><\/em><\/p><\/div>\n <\/p>\n Other studies have confirmed this observation of a shift towards increased fat oxidation and reduced carbohydrate oxidation (<\/span>22<\/a><\/strong>). <\/span>Furthermore, a higher concentration of key enzymes (\u03b2-hydroxyacyl-CoA-dehydrogenase) of fat metabolism was also observed (23<\/a><\/strong>), as well as a stronger activation of signalling proteins that lead to increased mitochondrial biogenesis (24<\/a><\/strong>). These are important physiological processes for aerobic energy supply and could lead to improved performance in endurance athletes in the long term.<\/p>\n <\/p>\n The research on ketogenic diets and their effect on the performance of endurance athletes is very inconsistent. The studies differ greatly in their design\u2014for example, about the participating athletes, the training conditions, and the test protocols\u2014and also in their quality. Many studies include only a few participants, and numerous subjects often drop out of the study prematurely or do not adhere consistently to the guidelines. This suggests that a ketogenic diet is difficult for many people to implement in practice in the long term.<\/p>\n <\/p>\n In fact, to our knowledge, there is currently only 1 study (<\/span>25<\/a><\/strong>) in which a ketogenic diet has led to an improvement in performance – compared to a conventional sports diet with many carbohydrates:<\/span><\/p>\n Lamberts et al. (1994) found a significantly better performance after a 2-week ketogenic diet: In the test protocol, the resistance was increased by 25 watts every 150 s until the specified power could no longer be maintained (25<\/a><\/strong>). Here, the subjects were able to maintain the performance requirement significantly longer if they adhered to a ketogenic diet. <\/span><\/p>\n In a high-intensity Wingate test (30s all out on the bicycle ergometer), however, there were no differences in performance in the study.\u00a0<\/span>In another study by the same duo of authors (26<\/a><\/strong>), the effect of a 2-week ketogenic diet\u2014followed by a 3-day refeed with carbohydrates\u2014on cycling performance was tested.\u00a0<\/span>In the test (150 min at 70% of VO2-max, followed by a 20 km time-trial), the group that had previously followed a ketogenic diet performed better (29.5 min fast vs. 30.9 min fast).<\/span><\/p>\n In addition, two studies observed an increase in VO<\/span>2<\/span> max*<\/strong> after consuming a ketogenic diet.<\/span> However, the VO<\/span>2<\/span> max increased just as much as in the other groups in which many carbohydrates were consumed. So there was no relative improvement<\/strong> compared to the high-carb group, but only no deterioration. <\/span><\/p>\n *<\/span>VO2 <\/strong>max<\/strong> is the maximum amount of oxygen (in milliliters) that is consumed per minute and per kilogram of body weight at maximum performance. This value is an indicator of aerobic fitness and should increase with increasing fitness. <\/span><\/em><\/p><\/blockquote>\n Interestingly, both studies were conducted with elite race walkers who chose their own nutritional intervention, and the athletes who chose the ketogenic diet already had slightly higher average VO2 max values at the beginning of the intervention (27<\/a><\/strong>). This suggests that other factors may have simultaneously influenced the choice of diet and the aerobic capacity of athletes, such as genetic variations. <\/span><\/p>\n <\/p>\n Numerous studies confirm that the ketogenic diet significantly improves the fat metabolism of athletes (25<\/a>, 26<\/a><\/span>, 27<\/a><\/strong>). However, a clear disadvantage also emerges: the ability to access carbohydrates as the primary energy source at higher intensities is limited. Most studies have shown an impairment of performance, as well as a deterioration of VO <\/span>2<\/span> max values found (<\/span>28<\/a>).<\/span><\/p>\n \ud83e\udde0 Want to learn more about training fat metabolism?<\/strong> \ud83c\udf99\ufe0fclick here for the podcast episode \u279c<\/a> “Training Fat Metabolism”<\/a><\/p>\n<\/div>\n <\/p>\n The study by Burke et al. (2017) impressively demonstrated this: After three weeks of a ketogenic diet, the fat metabolism markers of the athletes improved significantly, but the efficiency during intense exercise<\/strong> decreased by about 5\u20138% \n
Can the Ketogenic Diet Actually Improve Performance \u2013 or Does it Even Restrict the Body During Intense Exertion?<\/em><\/span><\/h4>\n<\/blockquote>\n
<\/h2>\n
What is the Ketogenic Diet?<\/b><\/span><\/h2>\n
![\"\"](\"https:\/\/athleatcoach.com\/wp-content\/uploads\/keto-food-pyramid.jpg\")
<\/p>\nKeto-Adaptation: when Metabolism Shifts<\/span><\/strong><\/h2>\n
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The ketone bodies can be used as an energy source by both the brain and the muscles. But are they more effective than carbohydrates? <\/span><\/em><\/h4>\n<\/blockquote>\n
![\"\"](\"https:\/\/athleatcoach.com\/wp-content\/uploads\/ktone-levels.-Evans-2017-1-1.jpg\")
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Ketogenic Diet in Endurance Sports: Potentials and Limitations<\/strong><\/span><\/h2>\n
Limited Energy Reserves<\/strong><\/span><\/h4>\n
![\"\"](\"https:\/\/athleatcoach.com\/wp-content\/uploads\/Energiequellen-Ausdauersport.-Algahannam-2021.jpg\")
Digestive Problems<\/span><\/strong><\/h4>\n
And Indeed, Digestive Problems are Very Common among Endurance Athletes:<\/strong><\/h5>\n
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Against this Background, the Question Arises whether the Ketogenic Diet could Represent an Advantageous Alternative?<\/span><\/em><\/h4>\n<\/blockquote>\n
<\/h2>\n
Changes in Substrate Metabolism<\/strong><\/span><\/h2>\n
![\"\"](\"https:\/\/athleatcoach.com\/wp-content\/uploads\/ket-substrat-bereitstellung-in-prozent.jpg\")
![\"\"](\"https:\/\/athleatcoach.com\/wp-content\/uploads\/maximale-fett-oxidation.jpg\")
<\/h2>\n
How Does the Ketogenic Diet Affect Athletic Performance?<\/strong><\/span><\/h2>\n
Positive Effects on Performance<\/b><\/span><\/h3>\n
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However, only 5 cyclists participated in both studies, so it is questionable to what extent these results are meaningful for the general public. <\/span><\/em><\/h4>\n<\/blockquote>\n
<\/h3>\n
Improved Fat Metabolism, but Reduced Intensity Performance<\/strong><\/span><\/h3>\n
\nThen listen to our podcast episode on the topic, where we discuss current study results, practical examples, and coaching experiences.<\/p>\nTime to Exhaustion (TTE)<\/b><\/span><\/h4>\n