By Loukia Lili-Williams, PhD

Part I of II.

Optimizing the body’s metabolic efficiency has been a quest for many elite endurance athletes. In a previous article, we discussed the long term (macro) adaptations in the skeletal muscle that take place after consistent and repetitive short term (micro) adaptations at the metabolic gene level (Exercise and Nutrition: what is happening at the gene level?). Here, we will touch upon the short and long term metabolic effects of a particular diet, the ketogenic diet or KD. We will lay out historical information of the KD development, and present the most recent research on how a KD may or may not contribute to the performance of the endurance athlete.

an assortment of meats and fats such as salmon, coconut, butter, beef and olive oil The KD is characterized by a combination of high-fat, low carbohydrate, and sufficient protein intake. The diet was originally designed in the 1920’s to alleviate seizures in refractory epileptic patients with promising results [1-3]. Later on, the KD was applied to control other neurological disorders [4, 5], metabolic dysfunctions like diabetes mellitus [6, 7] and obesity [8, 9] (for an article review see [12]). The efficiency of this diet relies on the carbohydrate shortage that mimics the cellular state of starvation due to the lack of glucose availability. In response to the glucose deficit, the human cells take advantage of the lipid excess and utilize fatty acid oxidation to produce ketone bodies as alternative metabolites for aerobic energy production.

In the clinical setting, patients on the KD must consume 65–90% of their daily energy requirement as fat. This is twice that of the typical Western diet. KD meals consist predominantly of foods rich in fat—butter, cream, mayonnaise or oils with meat, fish, eggs, or cheese in quantities sufficient to ensure an adequate protein supply, and only very small portions of vegetables or salad to greatly minimize carbohydrate intake [10, 11].

Outside the clinical setting, and although sparse and at developmental stages, there is some emerging evidence of the potential benefits of the KD in the endurance athlete metabolic efficiency [13-18]. Traditional high-carbohydrate diets for the endurance athlete increase muscle and liver glycogen stores to improve endurance performance. As a result, they increase the rate of carbohydrate utilization during exercise. In order to transition to a more efficient energy system that would decrease the rate of carbohydrate utilization while increasing fat metabolism, scientists and athletes have recently begun experimenting with dietary procedures (similar to the ketogenic diet in a less strict or a “periodization” format, e.g. [18]). Although conducted with a limited number of either untrained individuals or well-trained athletes. with variable workout intensities and/or diet protocols, the current research studies indicate that ketosis (the cellular metabolic state that resembles starvation) from a KD results in metabolic adaptations that can reduce carbohydrate utilization and turn free fatty acids into the major metabolic fuel during exercise [13, 15, 17, 19, 20].

In an early study of eight untrained, healthy volunteers, a short-term (3-day) KD resulted in elevated VO2 max and LT values [14]. Nonetheless, in another study of healthy individuals that underwent a 30-s Wingate anaerobic cycle test, a 3-day, low-carbohydrate diet lowered the mean power output and therefore negatively impacted the anaerobic work capacity but enhanced the sympathoadrenal system activity at rest (increased levels of adrenaline and noradrenaline) [31]. Similarly to the latter, in another more recent, preliminary study of eight off-road competitive cyclists that completed a varied intensity and duration workload under a KD and a mixed diet, the KD showed to decrease the ability to perform high intensity work, due to decreased glycogen muscle stores and the lower activity of glycolytic enzymes [15]. However, the same study showed that high volume training on a KD increased the relative VO2max and VO2 LT (Lactate Threshold) values, it increased fat metabolism during exercise, it reduced body mass and fat content and decreased post exercise muscle damage [15].

Despite the potential excitement over the metabolic benefits that the KD may offer to the endurance athlete (elevated levels of ATP, increased mitochondria number and efficient fatty acid oxidation [18] with a potential of VO2 max increase [14-15]), there are some studies that question the benefits of the KD. Apart from the possibility of dehydration and hypoglycemia [21] or other feasible metabolic disturbances [22], and the likely decreased ability to maintain anaerobic workload [15,31], there are concerns on potential mood side effects. However, most experiments that support changes in mood during a KD have been performed in rats (e.g. [23]) or with patients at a clinical state (e.g.[24, 25]). In healthy individuals, any electrophysiological effects of the KD appear to be short-term and disappear after 3 weeks ([26]). The criticism and contradictory results on these potential behavior/mood fluctuations during a KD suggest that there is no clear correlation between the KD and neurological side effects at least in healthy individuals [27-29].

From our discussion above, there is evidence that a high-level endurance athlete can experience a metabolic switch (from glucose to fatty acid utilization) by following a non-clinical KD and/or a previously proposed KD “periodization” protocol (e.g. [18]). From the scientific perspective however, there has not been an adequate amount of research to demonstrate consistent results across all types of high-endurance athletes (for a good review about the debate on the KD in athletes, see [30]). For many athletes that are willing to try the KD under the supervision of a specialist, the results may be significant on energy utilization and race nutrition planning or may not. Athletes should take into account that every individual is unique, diet tolerability may range, adaptation timing could vary, and that the science behind the benefits is at a promising yet preliminary phase.


  1. Pulford, D.S., Ketogenic Diets for Epileptics. Cal West Med, 1927. 27(1): p. 50-6.
  2. A, D.B., The Ketogenic Diet in Epilepsy. Can Med Assoc J, 1931. 24(1): p. 106-7.
  3. Ellis, R.W., Some Effects of a Ketogenic Diet. Arch Dis Child, 1931. 6(35): p. 285-92.
  4. Pacheco, A., W.S. Easterling, and M.W. Pryer, A Pilot Study of the Ketogenic Diet in Schizophrenia. Am J Psychiatry, 1965. 121: p. 1110-1.
  5. Paoli, A., et al., Ketogenic diet in neuromuscular and neurodegenerative diseases. Biomed Res Int, 2014. 2014: p. 474296.
  6. Feinman, R.D., et al., Dietary carbohydrate restriction as the first approach in diabetes management: critical review and evidence base. Nutrition, 2015. 31(1): p. 1-13.
  7. Al-Khalifa, A., et al., Therapeutic role of low-carbohydrate ketogenic diet in diabetes. Nutrition, 2009. 25(11-12): p. 1177-85.
  8. Benoit, F.L., R.L. Martin, and R.H. Watten, Changes in body composition during weight reduction in obesity. Balance studies comparing effects of fasting and a ketogenic diet. Ann Intern Med, 1965. 63(4): p. 604-12.
  9. Vesely, J.M. and L.G. DeMattia, Obesity: dietary and lifestyle management. FP Essent, 2014. 425: p. 11-5.
  10. Sinha, S.R. and E.H. Kossoff, The ketogenic diet. Neurologist, 2005. 11(3): p. 161-70.
  11. Neal, E., Dietary treatment of epilepsy : practical implementation of ketogenic therapy. 2012, Chichester, West Sussex: Wiley-Blackwell. p.
  12. Ruskin, D.N. and S.A. Masino, The nervous system and metabolic dysregulation: emerging evidence converges on ketogenic diet therapy. Front Neurosci, 2012. 6: p. 33.
  13. Helge, J.W., et al., Fat utilization during exercise: adaptation to a fat-rich diet increases utilization of plasma fatty acids and very low density lipoprotein-triacylglycerol in humans. J Physiol, 2001. 537(Pt 3): p. 1009-20.
  14. Langfort, J., et al., Effect of low-carbohydrate-ketogenic diet on metabolic and hormonal responses to graded exercise in men. J Physiol Pharmacol, 1996. 47(2): p. 361-71.
  15. Zajac, A., et al., The effects of a ketogenic diet on exercise metabolism and physical performance in off-road cyclists. Nutrients, 2014. 6(7): p. 2493-508.
  16. Hawley, J.A., Fat adaptation science: low-carbohydrate, high- fat diets to alter fuel utilization and promote training adaptation. Nestle Nutr Inst Workshop Ser, 2011. 69: p. 59-71; discussion 71-7.
  17. Pitsiladis, Y.P., I. Smith, and R.J. Maughan, Increased fat availability enhances the capacity of trained individuals to perform prolonged exercise. Med Sci Sports Exerc, 1999. 31(11): p. 1570-9.
  18. Yeo, W.K., et al., Fat adaptation in well-trained athletes: effects on cell metabolism. Appl Physiol Nutr Metab, 2011. 36(1): p. 12-22.
  19. Burke, L.M. and J.A. Hawley, Effects of short-term fat adaptation on metabolism and performance of prolonged exercise. Med Sci Sports Exerc, 2002. 34(9): p. 1492-8.
  20. Stepto, N.K., et al., Effect of short-term fat adaptation on high-intensity training. Med Sci Sports Exerc, 2002. 34(3): p. 449-55.
  21. Kossoff, E.H., et al., Kidney stones, carbonic anhydrase inhibitors, and the ketogenic diet. Epilepsia, 2002. 43(10): p. 1168-71.
  22. Hartman, A.L. and E.P. Vining, Clinical aspects of the ketogenic diet. Epilepsia, 2007. 48(1): p. 31-42.
  23. Zhao, Q., et al., Detrimental effects of the ketogenic diet on cognitive function in rats. Pediatr Res, 2004. 55(3): p. 498-506.
  24. Wing, R.R., J.A. Vazquez, and C.M. Ryan, Cognitive effects of ketogenic weight-reducing diets. Int J Obes Relat Metab Disord, 1995. 19(11): p. 811-6.
  25. Lambrechts, D.A., et al., Ketogenic diet effects on cognition, mood, and psychosocial adjustment in children. Acta Neurol Scand, 2013. 127(2): p. 103-8.
  26. Cantello, R., et al., Ketogenic diet: electrophysiological effects on the normal human cortex. Epilepsia, 2007. 48(9): p. 1756-63.
  27. Rho, J.M. and M.A. Rogawski, The ketogenic diet: stoking the powerhouse of the cell. Epilepsy Curr, 2007. 7(2): p. 58-60.
  28. Thio, L.L., et al., A ketogenic diet does not impair rat behavior or long-term potentiation. Epilepsia, 2010. 51(8): p. 1619-23.
  29. Xu, K., et al., Diet-induced ketosis improves cognitive performance in aged rats. Adv Exp Med Biol, 2010. 662: p. 71-5.
  30. Phinney, S.D., Ketogenic diets and physical performance. Nutr Metab (Lond), 2004. 1(1): p. 2.
  31. Langfort, J., et al., The effect of a low-carbohydrate diet on performance, hormonal and metabolic responses to a 30-s bout of supramaximal exercise. Eur J Appl Physiol Occup Physiol, 1997. 76(2): 128-33