Endurance sports are becoming increasingly popular, and athletes of all levels are looking for ways to improve their performance through training and nutrition.
For endurance exercises lasting 30 minutes or more, the most likely contributors to fatigue are dehydration and carbohydrate depletion , while gastrointestinal problems, hyperthermia, and hyponatremia can reduce performance and are potentially health-threatening, especially in longer events (> 4 hours). Running a marathon is a prime example of this type of event!
Muscle glycogen and blood glucose are the most important substrates for muscle contraction during marathons. Fatigue during prolonged exercise is often associated with muscle glycogen depletion and reduced blood glucose concentrations, and therefore high pre-exercise muscle and liver glycogen concentrations are believed to be essential for optimal performance, although it is unlikely that either of these factors alone limits prolonged exercise performance.
In addition to glycogen depletion, dehydration can also impair performance in long-distance events. Sweat loss occurs because there is a need to dissipate the heat generated during exercise. Therefore, the nutritional challenge is to prevent severe dehydration (2–3%) and thus contribute to the prevention of fatigue. This recommendation is in line with the most recent guidelines from the American College of Sports Medicine, which states that dehydration of more than 2–3% of body weight should be avoided, but also warns against excessive consumption to prevent hyponatremia.
Pre-test:
- Carb-loading
Before the race, a fundamental strategy to practice is carb-loading (and there's already a post about that on this site). The amount of dietary carbohydrates needed to provide the high carbohydrate availability required to replenish glycogen stores daily or promote glycogen loading depends on the duration and intensity of the athlete's exercise program. These needs can vary around 5 to 12 g/kg of body weight per day, depending on the athlete and their activity. Supercompensated muscle glycogen levels can improve performance compared to low to normal (non-supercompensated) glycogen levels by 2–3% in events lasting longer than 90 minutes.
- Hydration
As I mentioned at the beginning of the post, dehydration can compromise exercise performance, and therefore it is important to start exercise in a hydrated state. When hydrating before exercise, the individual should slowly drink fluids (5–7 mL/kg of body weight) at least 4 hours before the event. If the individual does not produce urine, or the urine is dark/highly concentrated, more fluid should be slowly drunk (another 3–5 mL/kg) about 2 hours before the event.
It is believed that athletes who have difficulty drinking sufficient amounts of fluids during exercise or who lose body water at high rates may benefit from hyperhydration. However, there is also a risk that hyperhydration may substantially dilute and decrease plasma sodium before exercise, thus increasing the risk of dilutional hyponatremia if fluids are aggressively replaced during exercise.
During the test
- Consuming carbohydrates
It has been known for some time that carbohydrate intake during exercise can increase exercise capacity and improve performance. When exercise is prolonged (2 hours or more), carbohydrates become a very important fuel source and their intake is essential.
The amount of carbohydrates needed for optimal performance and minimization of negative energy balance for events lasting longer than 2.5 hours is up to 90 g/h, from multiple transportable carbohydrates (such as glucose + fructose), since exogenous carbohydrate oxidation is limited by the intestinal absorption of carbohydrates from different transporters. It is believed that glucose uses a sodium-dependent transporter, SGLT1, for absorption, which becomes saturated with a carbohydrate intake of around 60 g/h. When glucose is ingested at this rate and another carbohydrate (fructose) that uses a different transporter is ingested simultaneously, oxidation rates well above 1 g/min can be observed.
But remember: you need to train your gut beforehand to absorb a large amount of carbohydrates! So, "train your gut"!
- Training the gut
Since exogenous carbohydrate oxidation is linked to exercise performance, an obvious potential strategy would be to increase the gut's absorption capacity. There is evidence that the gut is indeed adaptable, and this can be used as a practical method to increase exogenous carbohydrate oxidation, which is highly relevant for endurance athletes!
- Maintain fluid balance during exercise.
To avoid significant fluid loss, perhaps the best advice I can give is for athletes to weigh themselves before and after running to assess fluid loss during training and limit weight loss to 2-3% during exercise lasting longer than 90 minutes. Without such planning, it's difficult to obtain concrete advice, as differences between individuals, running distances, course types, and environmental conditions will complicate any suggestions. The addition of sodium and carbohydrates to sports drinks is widely recommended to increase water absorption.
- Caffeine
Caffeine is one of the most common supplements used in endurance sports due to its ability to decrease the perception of effort. Caffeine is absorbed and takes 30 to 90 minutes to reach peak plasma concentration. Therefore, an effective strategy may be to ingest a dose close to 3 mg/kg of body weight 60 minutes before the start of exercise, followed by 1 mg/kg every 2 hours thereafter. However, even when taken only at the end of exercise, caffeine can still be effective. But, be aware of some signs and symptoms of its consumption (increased heart rate, difficulty sleeping, headaches, or anxiety).
In conclusion…
To run a marathon (and of course, to train during the marathon cycle), carbohydrates and fluids play an important role, both before and during exercise.
It's important to start your training/race with high concentrations of muscle glycogen and to be hydrated, which can be achieved through high carbohydrate consumption and adequate intake of fluids and electrolytes.
In all cases, the best scenario is an individualized nutritional strategy that aims to supply carbohydrates to the working muscle at a rate that depends on the intensity of the exercise as well as the duration of the event. Higher carbohydrate intakes may result in better performance, and the intake of multiple transportable carbohydrates will allow for very high carbohydrate oxidation rates and superior performance.
Endurance athletes should try to minimize dehydration and limit body mass loss through sweating to 2–3% of body mass, especially if their event takes place in hot and humid environments.
Other problems in endurance sports include gastrointestinal issues, which are highly individual but can be minimized with certain nutritional precautions.
I wish you, reader and athlete, a good race, and until the next post!
Gabi,
Nutritionist at Z2.
References:
Burke, L. M., Hawley, J. A., Wong, S. H. S., & Jeukendrup, A. (2011). Carbohydrates for training and competition. Journal of Sports Sciences.
Cox, G.R., Desbrow, B., Montgomery, P.G., Anderson, ME, Bruce, C.R., Macrides, T.A. et al. (2002). Effect of different protocols of caffeine intake on metabolism and endurance performance. Journal of Applied Physiology, 93, 990–999.
Foster, C., Costill, D. L., & Fink, W. J. (1979). Effects of preexercise feedings on endurance performance. Medicine and Science and Sports, 11, 1–5.
Hawley, JA, Schabort, EJ, Noakes, TD, & Dennis, SC (1997). Carbohydrate loading and exercise performance: An update. Sports Medicine, 24, 73–81.
Pfeiffer, B., Stellingwerff, T., Hodgson, A.B., Randell, R., Poettgen, K., Res, P. et al. (2011a). Nutritional intake and gastrointestinal problems during competitive endurance events. Medicine and Science in Sports and Exercise (DOI: 10.1249/MSS.0b013e31822dc809).
Pfeiffer, B., Stellingwerff, T., Zaltas, E., & Jeukendrup, A.E. (2010b). Oxidation of solid versus liquid CHO sources during exercise. Medicine and Science in Sports and Exercise, 42, 2030 2037.
