When exercising, especially moderate and high-intensity exercises, carbohydrates become the main energy source. In fact, when exercise is prolonged over time and glycogen is depleted (the storage form of carbohydrates), performance will be impaired as all energy will have to come from lipolysis (which allows lower exercise intensity to be sustained), and even feel bonked (due to hypoglycaemia). This is the reason why, in all sports, it is advisable to get to the competition with the highest glycogen storage levels possible.
Having shown the importance of carbohydrates availability (glycogen) during the competition, a research study has assessed a group of people who did a high-intensity training session with the aim of depleting glycogen storage.1 After that session, some of them had a high-carb meal (8 g/kg) in order to restore glycogen levels, while others had a low-carb meal (2 g/kg). The following day, the participants performed a 60-minute intermittent sprinting protocol (which could be similar to a football match), and the results showed that those who had eaten less carbohydrate not only had lower glycogen levels (almost half) before the test than those who had eaten more carbohydrate, but they also covered a shorter distance overall (-4.9%) and at high intensity (-8.1%) (Figure 1). Therefore, it is clear to say that it is essential to get to the competition with glycogen stores as full as possible. But how can we achieve this?
The renowned Australian researcher Louise Burke conducted a review that analysed the different nutritional strategies that have been studied in order to maximise glycogen synthesis (See Figure 2).2 Until a few years ago, “re-synthesis” glycogen protocols were based on the so-called “window of opportunity” or “metabolic window”, which supported that glycogen synthesis was greater if carbohydrate intake occurred in the later hours (especially the two hours after) following an exercise session than if it occurred later in the day..3 This “metabolic window” could occur because, following an intense training session in which glycogen levels are depleted, a number of mechanisms (including increased insulin sensitivity and cell membrane permeability to glucose, as well as stimulation of the enzyme glycogen synthase) would be activated to promote glycogen re-synthesis.4 Therefore, based on these physiological responses, it has been proposed that if there is little time between an intense training session and the next session or competition (e.g. when doing two training sessions on the same day or when competing several times in one day), it is recommended to ingest carbohydrates as soon as possible.
Also based on the activation of mechanisms that favour glycogen re-synthesis after glycogen depletion, it has been traditionally supported that, in order to reach competition with glycogen stores as full as possible, it would be advisable to deplete them a few days before the competition (either following low-carbohydrate diets [<2 g/kg] or by very intense training) and then carry out a “loading” or “super-compensation” phase of carbohydrates (approximately 8-12 g/kg per day) during the two or three days prior to the competition.5 However, it is important to note that this strategy can be risky as it can lead to fatigue as a result of training or low carbohydrate availability. In addition, it has subsequently been observed that similar glycogen super-compensation can occur without the need to deplete glycogen in the days prior to competition – thus avoiding the risk of fatigue – by simply increasing carbohydrate intake in the days prior to competition (Sherman et al. 1981). In fact, one study found that glycogen stores can be optimally replenished in as little as one day by ingesting a high amount of carbohydrate (10 g/kg) and reducing the level of physical activity (e.g. by training very lightly or resting the day before the competition).6
To sum up, it is important to get to the competition with the highest possible glycogen storage. Therefore, it is advisable to consume an adequate amount of carbohydrates (> 1 g/kg per hour) after training sessions, especially in the 2-4 hours after it takes place, when glycogen synthesis is increased. Moreover, other factors such as the glycaemic index or carbohydrate absorption rate can make glycogen synthesis difficult; the recommended carbohydrates are those of rapid absorption such as glucose (rather than others such as fructose) if rapid recovery is required, for example, when there are two training sessions or competitions in the same day. On the other hand, it is fundamental to raise carbohydrate intake (8-10 g/kg) days prior to competition and to reduce training load.
Pedro L. Valenzuela
- Skein M, Duffield R, Kelly BT, Marino FE. The effects of carbohydrate intake and muscle glycogen content on self-paced intermittent-sprint exercise despite no knowledge of carbohydrate manipulation. Eur J Appl Physiol. 2012;112(8):2859-2870. doi:10.1007/s00421-011-2253-0
- Burke LM, van Loon LJC, Hawley JA. Postexercise muscle glycogen resynthesis in humans. J Appl Physiol. 2017;122(5):1055-1067. doi:10.1152/japplphysiol.00860.2016
- Ivy JL, Katz AL, Cutler CL, Sherman WM, Coyle EF. Muscle glycogen synthesis after exercise: effect of time of carbohydrate ingestion. J Appl Physiol. 1988;64(4):1480-1485. doi:10.1152/jappl.1922.214.171.1240
- Prats C, Helge JW, Nordby P, et al. Dual regulation of muscle glycogen synthase during exercise by activation and compartmentalization. J Biol Chem. 2009;284(23):15692-15700. doi:10.1074/jbc.M900845200
- Bergström J, Hultman E, Roch-Norlund a E. 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. 1972;29(2):231-236. doi:10.3109/00365517209081080
- Bussau VA, Fairchild TJ, Rao A, Steele P, Fournier PA. Carbohydrate loading in human muscle: An improved 1 day protocol. Eur J Appl Physiol. 2002;87(3):290-295. doi:10.1007/s00421-002-0621-5
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