Keto Diet for Runners?
The Hype- Keto Diets
You may have heard and even thought about trying it out yourself (if you haven’t already)- ‘going keto’ for hopes to improve your endurance performance. The rationale (in a nutshell) is that your body will adapt to utilizing more fatty acids, thus sparing your glycogen stores. Athletes (and non-athletes) are interested in this diet’s proposed benefit of altering body composition by decreasing fat mass.
As many of you already know, our glycogen stores are quite limited and we need them for higher intensity exercise. Running out of glycogen feels like hitting a wall- which is how that phrase got coined. So a keto diet makes sense then, right? Well, let’s look at what the research shows as well as dig into a bit of physiology of the metabolic rationale.
Note that ‘Keto’ (ketogenic) diets are roughly 15-20% protein, 5-10% carbohydrate, and 70-80% fat. The following metabolic explanations are similar to other low carbohydrate diets; the extent of these metabolic adaptations are generally reflecting the degree of carbohydrate restriction. Ketones are used to a small degree as fuel in normal conditions. For example, after an overnight fast, they supply ~2 to 6% of the body’s total energy needs.
Summary of the Process- If you are interested- Otherwise skip to the next section!
Ketone Production increases as carbohydrate sources become less available. In normal conditions, glucose is broken down to pyruvate, which much of the pyruvate is converted to acetyl CoA (in aerobic conditions). Acetyl CoA gets fed into TCA cycle (aka Krebs cycle) to produce ATP (energy). If carbohydrate sources are not available, acetyl CoA is produced from fatty acids that undergo β-oxidation.
Simultaneously, oxaloacetate (produced from the TCA cycle) gets used for gluconeogenesis. As a result, less oxaloacetate is available to combine with acetyl CoA to form citrate (critical for TCA cycle to continue). The excess acetyl CoA can go on to form ketones- mainly acetoacetate and β-hydroxybutyrate. The ketone bodies are then transported in the bloodstream and taken up by other tissues for energy use, such as the brain and muscle cells. In the muscle the ketones get converted to 2 acetyl CoA before entering the Krebs cycle and ETC to produce ATP!
Ketones and Endurance Athletes
Low carbohydrate diet’s effect on athletic performance has been studied in varying intensities and durations. The metabolic adaptations of a ketogenic diet, in regards to endurance exercise, has been a popular topic, especially in the promotion of these types of diets. There are studies that succeed in demonstrating desirable metabolic shifts from a ketogenic diet (varying in their degree of study-strength), some of these reported adaptations in human studies include:
post exercise decreases in lactate accumulation
higher gene expression in markers of fat oxidation
increased muscle mitochondrial volume
Improvement in body composition
While these metabolic adaptations are impressive, evidence documenting short or long term improvements on endurance performance compared to moderate-high carbohydrate diets is lacking. However, there is plentiful, strong evidence demonstrating that higher carbohydrate diets improve endurance exercise performance.
Studies suggest that low carbohydrate diets negatively impact an athletes all-out exertion and power; therefore, low carbohydrate diets are usually discouraged for athletes involved in sprinting sports. Do keep in mind, even distance runners may frequently be challenged with running segments that require all-out-excursion.
It has been suggested that long distance endurance athletes may not have decreased performance compared to similar athletes following a moderate carbohydrate intake. One study by Volek et. al. observed that keto-adapted ultra endurance runners had similar muscle glycogen utilization and repletion as compared to those consuming high carbohydrate diets. The keto-adapted athletes also had high rates of fat oxidation, which would be expected.
Considering that everyone is different and therefore may respond differently to various diets, some individuals may feel they are better off on a ketogenic diet. This highlights the fact that one diet type may not be appropriate for everyone. If you feel strongly about trying or continuing the ketogenic diet, consider overall health, outside of just exercise performance. Since ketogenic diets are very high in fat (70-80%), it may be difficult to stay under the recommended saturated fat intake of <10% of total energy intake. Therefore, a healthier ketogenic diet would include mostly plant-based healthy fats and plenty of non-starchy vegetables. Ideas for healthy plant-based fat sources include olive oil, avocados, nuts and nut butter, and seeds.
In a Nutshell
Insufficient carbohydrate (or total energy intake) increases fat metabolism and ketone production
Fatty acids are less efficient at producing ATP (this includes ketones as fuel) and require significantly more oxygen than using carbohydrates as fuel
Keto diets may produce desirable metabolic adaptations, such as burning more fat as fuel
Relying on a larger percent of ketones and fatty acids during exercise limits intensity
Keto adapted athletes have been shown to have impaired glycogen utilization- so much for working hard to ‘reserve’ it!
The current research does not support exercise performance improvements of low carb (including keto) diets
Each individual athlete may respond differently to diet interventions, and some may feel the ketogenic diet works well for them
References
Volek JS, Freidenreich DJ, Saenza C, Kuncesa LJ, Creighton BC, Bartley JM, Davitt PM, Munoza CX, JM Andersona, Maresha CM, Lee EC, Schuenkec MD, Aerni G, Kraemer WJ, Phinneyd SD. Metabolic characteristics of keto-adapted ultra-endurance runners. 2016. Metabolism Clinical and Experimental 65 (2016) 100-110.
Dunford M & Doyle JA. Nutrition for Sport and Exercise 4th ed. Cengage. 2019.
Burke LM, Whitfield J, Heikura IA, Ross MLR, Tee N, Forbes SF, Hall R, McKay AKA, Wallett AM, Sharma AP. Adaptation to a low carbohydrate high fat diet is rapid but impairs endurance exercise metabolism and performance despite enhanced glycogen availability. The Journal of Physiology, 22 July 2020. https://doi.org/10.1113/JP280221
Ma S, Suzuki K. Keto-Adaptation and Endurance Exercise Capacity, Fatigue Recovery, and Exercise-Induced Muscle and Organ Damage Prevention: A Narrative Review. Sports. 2019; 7(2):40. https://doi.org/10.3390/sports7020040