Fat Adaptation & Endurance Performance

In exercise physiology, it is well recognized that high carbohydrate intake is advantageous for endurance training. However, could this also be true for a low carbohydrate, high lipid (fat) diet? In this article, we will investigate the effect a high lipid diet has on endurance performance (known as fat adaptation), as well as some basic understanding of how this is the case.

What is Fat Adaptation?

Fat adaptation is a process the body can undergo in which it relies more heavily on lipid use for energy than carbohydrates [1][2].

Does Fat Adaptation improve Endurance Performance?

Contextually, sometimes.

It seems that in endurance events as long or shorter than a marathon, a low carbohydrate, high lipid diet is roughly equal (shorter events may see a decrease in performance) to a high carbohydrate nutrition protocol on performance [1][4][5][6]. However, if a trainee is engaging in an extreme endurance event as long as 100 kilometers (~62 miles) or more, a fat adapted state may have minor benefit by shaving off a few minutes [3][4]. Interestingly, the effectiveness of fat adaptation is highly heterogenous – meaning, some people are simply genetically inclined to use carbohydrates better and some are better at using lipids, making the latter benefit more from fat adaptation [4][5].

How to become Fat Adapted?

Essentially, consume a very low carbohydrate diet, high fat, and moderate protein diet. The problem is that although there have been a series of studies on the topic, each used different definitions –  typically between 20% of total food ingested being carbohydrate or a mere 5% of total food ingested being carbohydrate [1].

If you want to a true ketogenic style experience, where most people likely would not debate you as low carbohydrate – sticking to the 5% mark would be ideal. The length of time ranges between 2-3 weeks and 6 months or more [1][4]. Again, to be completely sure, months on a low carbohydrate diet would likely ensure it, as a measure of ketone bodies (which tend to be increased weeks in) is insufficient as a value of true fat adaptation – this will be explained in the next section.

Understanding the Physiology

Typically, most exercise physiologists and sports nutritionists would recommend a high carbohydrate diet for just about any event [2]. This is typically seen as a good idea, because one is maximizing the amount of glycogen found in the muscle and liver, leading to more time before you “bonk” (aka, the point at which you feel lethargic and have difficulty continuing the event – also called “hitting the wall”) [2]. Also, the idea is that the body can regulate its use of lipid as needed, but will have higher stores of glycogen to tap into, if needed, to maintain a high throughput of performance for a longer duration of time [2]. However, although the evidence is weak, there is some indicating that in extreme endurance events, forcing the body into a state of greater lipid utilization may help, but why?

Unlike a high, or even moderate, carbohydrate diet, in which glycogenolysis (the breakdown of stored glucose – glycogen – for energy) is a primary driver of energy, a fat adapted individual sees decreases in glycogenolysis activity and lower pyruvate dehydrogenase activity (aka, glucose/carbohydrate centric enzymes) [2]. Not only that, cell membrane transport proteins fatty acid translocase (CD36) may be substantially increased in number (but not fatty acid binding protein, which remains stable), leading to more gateways into the cells [2]. Related, higher intramuscular triglycerides are trapped in the muscle (comparable amounts to glycogen, even), leading to a much greater spatial access to lipids that would otherwise be unavailable as lipids would need to be, largely, taken up from serum, released from adipocytes [2]. Then, an increase in mitochondrial use of fatty acids is seen, not from an increase in number, but an increase in the allowance into the organelle via lower inhibition of CPT-1, the mitochondrial membrane translocation protein [2].


This diagram simply shows the compartments of the cell. As fatty acids enter the cell using the fatty acid transporter, fatty acids build up in the cytosol (inside of the cell) until they are allowed through the CPT1-CAT into the mitochondrion and used for energy (ATP). If you are fat adapted, CPT-1 is more accepting of fatty acids to allow a greater influx into the mitochondrion. 

As briefly mentioned before, ketones begin elevating even a few days into a fat adaptation diet, so they are not the best indication fat adaptation has occurred – just that you are favoring fat centric physiology. This means that some of the outlined adaptations do not necessarily track with ketone production; some may come earlier, some may come later.

Unfortunately, even with all these adaptations leading to much easier access to lipids for much greater yields of energy – the metabolic kinetics into the mitochondrion and the subsequent use of these lipids (fat) for energy will still inevitably be slower than glucose (carbohydrate) use. This is likely why it tends not to be beneficial for events that could benefit from carbohydrates (sub-marathon, marathon competitions), yet could lead to a longevity benefit in ultra-events.


Fat adaptation is a process by which the body becomes more efficient at utilizing lipids – increasing the influx of lipids into the cells and priming those cells to shuttle those lipids into their mitochondria to generate more ATP, this decreases the reliance on glucose (or, alternatively, the decreases in glucose reduce their influx into the cells, leading to the same result – greater reliance on fat metabolism for energy). Unfortunately, this does not seem to help in light to moderate endurance events, but may be beneficial for extremely long endurance events (longer than a marathon, like an ultra-marathon).

Writer: Nicolas Verhoeven
[1] Chang, C., Borer, K., & Lin, P. (2017). Low-Carbohydrate-High-Fat Diet: Can it Help Exercise Performance? Journal of Human Kinetics, 56(1), 81-92. doi:10.1515/hukin-2017-0025

[2] Yeo, W. K., Carey, A. L., Burke, L., Spriet, L. L., & Hawley, J. A. (2011). Fat adaptation in well-trained athletes: effects on cell metabolism. Applied Physiology, Nutrition, and Metabolism, 36(1), 12-22. doi:10.1139/h10-089

[3] McSwiney, F. T., Wardrop, B., Hyde, P. N., Lafountain, R. A., Volek, J. S., & Doyle, L. (2018). Keto-adaptation enhances exercise performance and body composition responses to training in endurance athletes. Metabolism, 83, e1-e2. doi:10.1016/j.metabol.2017.11.016

[4] Burke, L. M. (2015). Re-Examining High-Fat Diets for Sports Performance: Did We Call the ‘Nail in the Coffin’ Too Soon? Sports Medicine, 45(S1), 33-49. doi:10.1007/s40279-015-0393-9
- Burke Review

[5] Erlenbusch, M., Haub, M., Munoz, K., MacConnie, S., & Stillwell, B. (2005). Effect of High-Fat or High-Carbohydrate Diets on Endurance Exercise: A Meta-Analysis. International Journal of Sport Nutrition and Exercise Metabolism, 15(1), 1-14. doi:10.1123/ijsnem.15.1.1

[6] Rothschild, J., & Earnest, C. (2018). Dietary Manipulations Concurrent to Endurance Training. Journal of Functional Morphology and Kinesiology, 3(3), 41. doi:10.3390/jfmk3030041


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