Morgan King MSc, RD
High fat vs high carb has always been a topic of interest for sports performance. It has been of interest because 1) fat has unlimited storage capacity 2) fats produce more energy compared to carbohydrates and 3) endurance exercise utilizes fat as an energy source in combination with carbohydrates. But before we delve into the literature let us quickly review how our body converts food into energy used for exercise.
Exercise requires extra energy on top of our basic living energy needs (Breathing, Organ function, sleeping and eating). The body has three different energy systems that are used during exercise to produce energy. The three systems are aerobic, anaerobic, and phosphocreatine systems. Endurance sports and other prolonged activities such as running, cycling, and rowing use all the energy systems, just at different time points of exercise, but the aerobic system is the dominant energy system. All of these systems use substrates, which are carbs, fats and proteins, which are then metabolized with or without the use of oxygen to produce energy.
The aerobic system is reliant on oxygen consumption to aid in the production of energy (1). Whereas the other systems such as anaerobic and Phosphocreatine do not need oxygen to produce energy. For all systems, the preferred substrate for energy is carbohydrates as it is the fastest substrate in the Krebs cycle. The downfall of carbs as a main fuel for exercise is that our body has limited storage capacity (2) which needs to be topped up during prolonged exercise (3).
Fats are also used during exercise at the same time as carbohydrates, the proportion of which fat or carbs are being used changes and depends on exercise intensity (1).
So, are high-fat diets helpful for endurance exercise performance…………
Fats can be used for low and slow (<60% VO2 Max)
Fats and carbs are being used at the same time during exercise, but the proportion ratio changes are dependent on exercise intensity (1). In exercises that are low to moderate intensity, fat utilization is the predominant source of energy. As soon as exercise increases to moderate to high intensity, the dominant energy source shifts and becomes reliant on carbohydrates (1, 3, 4).
Those who are exercising under a consistent VO2 max of 60% (VO2 max= Max heart rate) and on a low-carb high-fat diet will not see detrimental performance effects (3). Research that has been published unanimously states that any activity that rises above that 60% VO2 max and relies on fat as the primary source of energy, the system becomes less efficient and cannot keep up with providing energy due to a lack of oxygen, which in turn results in diminished performance (1, 3, 5).
So, Burke et al argue that a steady slow state of exercise will be able to keep up with the energy needs, whereas if there are sprinting bouts involved, the system becomes unable to keep up. Overall, it comes down to the fact that fat metabolism is less efficient than carbs. To metabolize fat, our body requires a higher oxygen demand compared to carbs and for that reason, the system is not able to provide energy efficiently for higher intensities (1,5). In other words, athletes may lose out on the ability to shift into higher-intensity gears (sprints), during their performance because of the oxygen demand (1,3, 5,6).
Can only adapt to either tolerating High fat or High Carbs Diet during exercise
While it is possible to adapt to a high-fat diet over a short duration (3-4 weeks) (1), during this time athletes often report having increased feelings of fatigue, rate of perception exertion (RPE, i.e perceived effort for exercise feels much harder) and decreased feeling of overall energy (3). While some athletes may return to their normal baseline while on a low-carb and high-fat diet, some athletes never return to their baseline (1).
So, what happens if you combine high carbs and a high-fat diet during exercise performance? You would think having the best of both worlds would optimize performance even further. Sadly though, you can only maximize one at a time.
Burke L. conducted a study with athletes who have adapted to high-fat diets and were given carbohydrates during exercise. It was found that there was an inhibition of carbs metabolism during exercise (5). It was hypothesized that because these athletes’ guts were untrained to consumption of carbs during exercise and had glucose inhibition which resulted in no muscle glycogen sparing (which is the point of consuming carbs during exercise) like it normally does when adapted to ingesting carbs during exercise (1, 5).
In the end, this means you can only shift to optimize either fat or train the gut to utilize carbs during exercise, and not both at the same time.
Gut distress…
Often endurance athletes have gut distress during exercise. Some foods are often culprits of gut distress, namely, fibre and fat. Much like fibre, fat slows down digestion in the body which can increase the feelings of gut distress and it is typically advised to reduce the amount of fat before exercise (4).
The athletic level of an elite vs recreational athlete can also influence the ability to adapt to high-fat diets. Those who are highly trained compared to non-trained have less GI upset when adapting to a high-fat diet (3). In addition, the intensity of the exercise can influence the effects of fat during exercise. The lesser the intensity of the exercise the less fat will have a gut distress effect (4). Whereas, those who are exercising at a higher intensity will likely suffer gut distress effects from fat (4).
Choosing what type of diet will depend on what your athletic goals are. However, most of the studies to date are in favour of having improved performance with a high-carb diet for endurance activities that require periods of higher intensity.
References
1.Burke L. Ketogenic low CHO, High Fat diet: The future of endurance sport? J physiol 599, 2021; 819-843.
2.Caitlin P. Bailey & Erin Hennessy (2020) A review of the ketogenic diet for endurance athletes: performance enhancer or placebo effect?, Journal of the International Society of Sports Nutrition, 17:1, 33, DOI: 10.1186/s12970-020-00362-9
3. Murphy N., Carrigan C, and Margolis. High fat ketogenic diets a physical performance: A systematic review. Adv Nutr, 2021:12; 223-233.
4. Wilson P.The Athlete Gut. 2020. Velopress.
5.Burke L., Whitfield J., Heikura I., et al. Adaptation to a low carbohydrate high fat diet is rapid but impairs endurance exercise metabolism and performance despite enhance glycogen availability. J physiol 2021, 771-790. DOI: 10.1113/JP280221
6. Burke LM, Ross ML, Garvican-Lewis LA, Welvaert M, Heikura IA, Forbes SG, Mirtschin JG, Cato LE, Strobel N, Sharma AP, Hawley JA. Low carbohydrate, high fat diet impairs exercise economy and negates the performance benefit from intensified training in elite race walkers. J Physiol. 2017 May 1;595(9):2785-2807. doi: 10.1113/JP273230. Epub 2017 Feb 14. PMID: 28012184; PMCID: PMC5407976.
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