As the competitive period begins for many sports, including triathlon, it's important to remember the importance of coming into race day with your glycogen stores as full as possible. Carbohydrates (CHO) are the main energy substrate when we exercise at moderate or high intensity. As a result, glycogen depletion (the storage form of CHOs) is one of the biggest limiting factors for endurance sports performance. A high availability of CHO during exercise will prevent a decrease in blood glucose or hypoglycemia (often described as "bonking" or "hitting the wall"), and will also allow you to maintain a higher intensity of exercise.
Various nutritional strategies have been studied to increase glycogen synthesis and thus reach competition with the stores in the best conditions (Burke et al. 2017). A classic study that popularized the term "window of opportunity" or "metabolic window" found that glycogen synthesis was much higher if CHO intake was done within two hours immediately after an exercise session than if it was done later (Ivy et al. 1988). This is because, although after intense training glycogen levels are depleted, this situation also causes the activation of a series of mechanisms (increased insulin sensitivity and permeability of the cell membrane to glucose, and stimulation of the enzyme glycogen synthase) that favor glycogen resynthesis (Prats et al. 2009). Therefore, if you have little time between an intense exercise session (with the consequent depletion of glycogen) and the next session, such as when doing two sessions on the same day or when competing several times in one day, it is important to consume CHO as soon as possible.
As low glycogen levels stimulate glycogen synthesis, pre-competitive strategies have traditionally been supported that consisted of depleting this substrate for several days, either through low-CHO diets or by performing very intense training, followed by a phase of high CHO intake in the three days prior to competition (a process called glycogen supercompensation)(Bergström et al. 1972). However, this strategy is somewhat risky and can lead to fatigue as a result of training or low CHO availability. In later years it was observed that this glycogen supercompensation can be produced by providing a high amount of CHO during the days prior to the competition even without the preceding glycogen-depletion phase, thus avoiding possible risks (Sherman et al. 1981). And more recently, an interesting study found that glycogen stores could be optimally recharged in as little as one day (e.g. the day before a competition) if the level of physical activity was reduced and a high amount (10 g/kg) of CHO was ingested (Bussau et al. 2002).
There are also certain nutritional strategies that seem to promote glycogen synthesis, especially those that increase gastric emptying, insulin response or cellular osmolarity: that is, those strategies that favor the passage of CHOs into the muscle cell. For example, CHO sources with a high glycemic index or rapid absorption such as glucose could promote glycogen synthesis to a greater extent than others with a slower absorption such as fructose. In addition, the combination of CHO with protein produces an increase in insulin response, and its combination with creatine an increase in cellular osmolarity, which would help to further maximize glycogen synthesis (Burke et al. 2017).
In short, to maximise performance it is advisable to arrive at the competition with full glycogen stores. Ingesting an adequate amount of CHO (> 1 g per kg of weight per hour) after exercise sessions, preferably in the 2-4 hours after, will help increase glycogen synthesis. In addition, if a quick recovery is required (such as when training or competing twice in a day) it is recommended that CHOs be of high glycemic index or rapid absorption like glucose, and not slow absorption like fructose. It is also important to increase the intake of CHO in the days prior to the competition and reduce the training load, evidence suggests that resting the day before competition and increasing CHO intake (10 g/kg) may be sufficient.
References
- Bergström J, Hultman E, Roch-Norlund a E (1972) Muscle glycogen synthetase in normal subjects. Basal values, effect of glycogen depletion by exercise and of a carbohydrate-rich diet following exercise. Scand J Clin Lab Invest 29:231–6. doi: 10.3109/00365517209081080
- Burke LM, van Loon LJC, Hawley JA (2017) Postexercise muscle glycogen resynthesis in humans. J Appl Physiol 122:1055–1067. doi: 10.1152/japplphysiol.00860.2016
- Bussau VA, Fairchild TJ, Rao A, et al (2002) Carbohydrate loading in human muscle: An improved 1 day protocol. Eur J Appl Physiol 87:290–295. doi: 10.1007/s00421-002-0621-5
- Ivy JL, Katz AL, Cutler CL, et al (1988) Muscle glycogen synthesis after exercise: effect of time of carbohydrate ingestion. J Appl Physiol 64:1480–1485. doi: 10.1152/jappl.1988.64.4.1480
- Prats C, Helge JW, Nordby P, et al (2009) Dual regulation of muscle glycogen synthase during exercise by activation and compartmentalization. J Biol Chem 284:15692–15700. doi: 10.1074/jbc.M900845200
- Sherman WM, Costill DL, Fink WJ, Miller JM (1981) Effect of exercise-diet manipulation on muscle glycogen and its subsequent utilization during performance. Int J Sports Med 2:114–8. doi: 10.1055/s-2008-1034594
AUTHOR
Pedro Valenzuela
Researcher in the Physiology Unit of the University of Alcalá and in the Performance Control Unit at the Sports Medicine Center (AEPSAD, CAR of Madrid).
Web: www.fissac.com
