UltraFit Magazine - Issue 113, Page 24

by Paul Taylor

This will be a four-part series that explores the use of carbohydrate as a performance aid in intermittent team sports, such as Soccer, Aussie Rules, Rugby and Hockey. This first part will focus on the need for carbohydrate stores (muscle and liver glycogen) in the above sports and will explore the major factors involved in replenishment of these stores. Parts 2, 3 and 4 will then focus on the best strategies to use before, during and after training or competition, respectively.

The time and type of activity involved in intermittent team sports means that muscle and liver glycogen contribute significantly to energy metabolism, particularly in the second half of a match (Ekblom, 1986). It has been reported that active people can deplete muscle glycogen with 30-60 minutes of high-intensity intermittent exercise (Coyle, 2004), whilst studies have shown that glycogen content in the leg muscles decreases rapidly during a game, with many subjects having essentially no glycogen left at the end of a game, and some having very low levels at half-time.

In one of the pioneering studies, Saltin (1973) investigated the effects of pre-game muscle glycogen levels on workrate profiles of nine soccer players. Four players began the match with low levels of glycogen (45 mmol/kg ww), whilst five players had normal muscle glycogen levels (96 mmol/kg ww). Significant glycogen depletion occurred in all players, but the low starting levels of the four players resulted in them having less than 10mmol/kg at half-time, with glycogen levels fully depleted at full-time. Players with low pre-match glycogen covered 25% less distance (4100m vs 5900m) than the players with normal levels. Interestingly, they covered more of the distance by walking (50% vs 27%) and less by sprinting (15% vs 27%) than those with normal pre-match levels.

A study by Jacobs et al (1982), on elite-level soccer players, reported average muscle glycogen values after a competitive soccer match to be 43% of the pre-match levels. Furthermore, by 24 hours post-game these levels were not restored (63%). Therefore, the contribution of carbohydrate to performance in soccer and other intermittent team sports is highly significant. There is obviously potential to improve performance by the adoption of nutritional strategies that can both optimise muscle and liver glycogen prior to the commencement of competition or training, and attenuate their decline (along with blood glucose exercise creates all the conditions necessary for glycogen resynthesis in the muscle. An intake of the appropriate amount of substrate at the correct times (to be discussed later) following exercise can therefore lead to optimisation of glycogen stores.

There is some more recent research to show that exercising/training in a carbohydrate-depleted state can induce greater enzyme and other cellular adaptations that can bestow an added advantage when it comes to competition, which obviously needs to happen when glycogen stores are full. Much of this research has been misinterpreted, however, most recreational and competitive athletes actually do this by accident, as most do not consume enough carbohydrates on a routine basis.

How do we apply all of this to team sports? The answer is that when you combine this knowledge with the fact that opportunities for glycogen replenishment during a game are very limited, it becomes obvious that the best strategy to maximise glycogen during competition/exercise involves consideration of its replenishment following training or competition. We will discuss how to do that in the next issue.

References

Burke L Deakin V Clinical Sports Nutrition McGraw-Hill Book Company NSW 2nd Ed 2002
Coyle EF (2004) Fluid and fuel intake during exercise J Sports Sci. 22(1) 39-55
Ekblom B (1986) Applied physiology of soccer Sports Med 3 50-60
Ivy JL Lee MC Bronzinick JT Reed MC (1988) Muscle glycogen storage following different amounts of CHO ingestion J Appl Physiol 65 2018-23
Jacobs I Westlin N Karlsson J (1982) Muscle glycogen and elite soccer players Eur J Appl Physiol 48 297-302
McCoy M, Proietto J Hargreaves M (1996) Skeletal muscle GLUT-4 and postexercise muscle glycogen storage in humans J Appl Physiol 80 411-15
Saltin B (1973) Metabolic fundamentals in exercise
Medicine and Science in Sports and Exercise 5 137-46