Carbohydrate Loading

Carbohydrate loading (carbo loading) involves increasing the daily intake of carbohydrate to maximize muscle glycogen levels. Most endurance athletes undergo a period of carbohydrate loading prior to endurance competition. This often takes the form of increased carbohydrate intake over a period of a few days prior to competition, as well as consuming a high carbohydrate meal 3-4 hours before the race and a carbohydrate drink just before and during competition.

Carbohydrate Loading Endurance Competition

The basic principle behind carbohydrate loading is that increasing carbohydrate consumptions in the build up to an important race ensures muscle glycogen levels are maximized. A high carbohydrate meal 3-4 hours prior to competition helps to ensure that any muscle of live glycogen that has been depleted overnight is replaced. This is based on research showing that the consumption of a high carbohydrate meal (3-4g carbohydrate per kg of body mass), approximately 3-4 hours before exercise may help to ensure glycogen levels are maximal and significantly enhance exercise performance and time to exhaustion (Wee et al., 2005; Schabort et al., 1999; Chryssanthopoulos and Williams 1997; Sherman et al., 1989; Neufer et al., 1987;). It is generally recommended that athletes

Whilst carbo loading it’s important to ensure that you have an adequate intake of fluids as the storage of glycogen requires additional fluid storage – for every gram of glycogen stored there will be an additional ~3 grams of water stored.

High GI Carbohydrates vs Low GI Carbohydrates for Carbo Loading

The speed at which different sugars enters the blood stream is measured by the Glycemic Index (GI). Sugars that are fast releasing (enter the blood stream quickly) have a high GI, whereas foods that enter the blood stream more slowly have a low GI.

Research is not completely clear as to whether the prior consumption of high GI or low GI foods is more beneficial for endurance exercise performance (Jamurtas et al., 2011). Prior consumption of HGI food (3-hours prior to exercise) has been found to increase muscle glycogen levels compared with LGI food (Wee et al., 2005), and improve carbohydrate utilization but does not appear to have any superior benefit, over LGI, in terms of endurance exercise performance (Febbraio et al., 2000; Jamurtas et al., 2011). Although HGI food consumption has been shown to enhance carbohydrate utilization it has a negative effect of fat metabolism (Little et al., 2009). This is the opposite of LGI foods which appear to increase free fatty acid availability, help to maintain fat metabolism, spare muscle glycogen and may lead to lower muscle lactate levels (Wee et al., 2005).

In addition to increasing free fatty acid availability, fat metabolism and sparing muscle glycogen stores some studies have demonstrated greater performance improvements with LGI compared with HGI food (Moore et al., 2009; DeMarco et al., 1999; Thomas et al., 1991). Researchers looking at pre-exercise LGI and HGI consumption (30-60minutes prior to exercise) found that LGI carbs led to greater improvements in time to exhaustion (Thomas et al., 1991), time to exhaustion following a 2-hr submaximal cycle (DeMarco et al., 1999) and 40km time trial performance (Moore et al., 2009).

Although it is not completely clear as to whether LGI or HGI foods are better for prior exercise consumption research is more supportive of the use of LGI foods (Moore et al., 2009; Wee et al., 2005; Siu and Wong, 2004; DeMarco et al., 1999; Thomas et al., 1991).

It is important to note that muscle glycogen can reach maximal levels after a period of carbohydrate loading (Saltin and Hermansen 1967) and therefore may not increase further with the consumption of a high carbohydrate meal prior to competition – particularly if an athlete tapers there volume of training and increases carbohydrate intake over a period of days prior to exercise.

Consuming a large high GI meal prior to exercise may negatively effect performance by causing a a significant increase in insulin levels, which can lead to increased breakdown of glycogen stores. The reason for this is due to the way High GI foods cause a rapid rise in blood sugar levels, which can lead to a rapid rise in insulin levels. The rise in insulin levels has two negative effects that may adversely affect endurance exercise performance: Firstly) if insulin levels rise too high it will overcompensate for the actual amount of sugar in the blood and leads to a state of low blood sugar (hypoglycaemia); Secondly) high insulin levels are known to have a inhibitory effect on the mobilization and utilization of fat as an energy source. The combination of inhibited fat metabolism, and reduced blood sugar levels, can lead to increased depletion of glycogen stores as there must be increased reliance on thses to compensate for the low blood sugar levels and reduced rates of fat metabolism. Because fat metabolism is decreased and glycogen metabolism increased, as soon as you then start your race, carbohydrate metabolism will then have to further increase and therefore the rate of glycogen depletion will be even greater, resulting in an earlier point of glycogen depletion. The risk can reduced by consuming low GI foods as the final high carb meal prior to competition.

Basic recommendations for carbo-loading for endurance competition

When preparing for a long distance race of 2hours+ duration, like a marathon, you should increase your carbohydrate intake, by around 100-200g per day, over the last 3-4 days before the race. If you combine this with a decrease training volume (~25-50% decrease in training volume) over the last week prior to the race it should ensure that your carbohydrate intake is sufficiently greater than the amount metabolised during exercise and will help to fully restore muscle glycogen stores going into the race. For races of around 30-90 minutes duration, glycogen stores should be adequate as long as you maintain normal dietary intakes of carbohydrate (4-5g per pound of body weight per day for an endurance athlete), and just reduce training volume by 25-50% over the last week before the race.

Carbohydrate Loading Summary:

  • Carbohydrate loading (Carbo loading) normally involves increasing carbohydrate consumption for a period of 3-4 days prior to competition in order to maximize levels of muscle and liver glycogen stores.
  • Training volume is also reduced in order to ensure adequate recovery and replenishment of muscle glycogen levels.
  • A further high carbohydrate meal is normally consumed 3-4 hours before competition and has been shown to improve endurance exercise performance and time to exhaustion.
  • Whilst it is not fully clear whether the best approach is to consume High or Low GI foods prior to exercise, research has generally found Low GI foods to be more beneficial.
  • Low GI foods appear to aid fat metabolism and spare muscle glycogen levels.
  • Consuming a meal that is too High GI may negatively affect endurance performance by increasing insulin levels, which may lower blood glucose and inhibit fat metabolism.
  • The basic recommendations for carbo loading are to increase daily carbohydrate consumption by approximately 100-200g for 3-4 days. This should be combined with a reduced training volume (25-50% reduction) which ensures that your carbohydrate intake is in a positive balance leading to maximized muscle and liver glycogen levels. For shorter duration races ~30-90 minutes, or less, carbo loading may not be necessary and a reduced training volume may be sufficient to maximize glycogen levels.

Carbohydrate Loading References

Chryssanthopoulos C and Williams C. (1997) Pre-exercise carbohydrate meal and endurance running capacity when carbohydrates are ingested during exercise. Int J Sports Med 18: 543–548, 1997.

DeMarco HM, Sucher KP, Cisar CJ, (1999) Butterfield GE: Pre-exercise carbohydrate meals: application of glycemic index. Med Sci Sports Exerc 1999, 31:164-170.

Febbraio MA, Keenan J, Angus DJ, Campbell SE, Garnham AP. (2000) Preexercise carbohydrate ingestion, glucose kinetics, and muscle glycogen use: effect of the glycemic index. J Appl Physiol. 2000 Nov;89(5):1845-51.

Jamurtas AZ, Tofas T, Fatouros I, Nikolaidis MG, Paschalis V, Yfanti C, Raptis S, Koutedakis Y. (2011) The effects of low and high glycemic index foods on exercise performance and beta-endorphin responses. J Int Soc Sports Nutr. 2011 Oct 20;8:15. doi: 10.1186/1550-2783-8-15.

Little JP, Chilibeck PD, Ciona D, Vandenberg A, Zello GA. (2009) The effects of low- and high-glycemic index foods on high-intensity intermittent exercise. Int J Sports Physiol Perform. 2009 Sep;4(3):367-80.

Moore LJ, Midgley AW, Thomas G, Thurlow S, McNaughton LR. (2009) The effects of low- and high-glycemic index meals on time trial performance. Int J Sports Physiol Perform. 2009 Sep;4(3):331-44.

Neufer PD, Costill DL, Flynn MG, Kirwan JP, Mitchell JB, and Houmard J. (1987) Improvements in exercise performance: effects of carbohydrate feedings and diet. J Appl Physiol 62: 983–988, 1987.

Saltin B., Hermansen L. (1967) Glycogen stores and prolonged severe exercise. In: Blix G., editor. Nurition and Physical Activity. Vol. 5. Almquist and Wiksell; Stockhol, Sweden: 1967. pp. 32–46.

Schabort EJ, Bosch AN, Weltan SM, and Noakes T. (1999) The effect of a preexercise meal on time to fatigue during prolonged cycling exercise. Med Sci Sports Exerc 31: 464–471, 1999.

Wee S.L., Williams C., Tsintzas K., Boobis L. (2005) Ingestion of a high-glycemic index meal increases muscle glycogen storage at rest but augments its utilization during subsequent exercise. J. Appl. Physiol. 2005;99:707–714.

Sherman WM, Brodowicz G, Wright DA, Allen WK, Simonsen JC, and Dernbach A. (1989) Effects of 4 h preexercise carbohydrate feedings on cycling performance. Med Sci Sports Exerc 21: 598–604, 1989.

Siu PM, Wong SH. (2004) Use of the glycemic index: effects on feeding patterns and exercise performance. J Physiol Anthropol Appl Human Sci. 2004 Jan;23(1):1-6.

Thomas DE, Brotherhood JR, Brand JC. (1991) Carbohydrate feeding before exercise: effect of glycemic index. Int J Sports Med. 1991 Apr;12(2):180-6.

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