For over a century, cyclists have known that stacking meals with carbohydrates is a requirement for riding fast [1, 2].

While lower cycling intensities draw on fat as an energy source, the harder you ride, the more your body taps into on-board carbohydrate (glycogen) to fuel your fastest efforts [1].

As exercise intensity goes up (expressed in % of VO2max), so does your body’s reliance on carbohydrate as a fuel source [3].
Adapted from Brooks, 1997

Loading Up

Early attempts to maximize muscle glycogen led to the discovery of “supercompensation”; the body’s ability to rebound from low levels of glycogen, by later driving higher levels of glycogen storage in muscle [4].

This research led to carb-loading protocols, as seen below.

In short, early carb-loading (as seen on the left of the image) began with exhaustive exercise, followed by a few days of a low-carb diet, then finished up with a high-carb diet in the days leading up to a competition.

While these strategies boosted glycogen storage, they also came with downsides like GI problems, lethargy, and poor recovery [1].

*image from Jeukendrup, A.E. and M. Gleeson, Sport nutrition. 2019.

More moderate carb loading (as seen on the right of the image above) generates a similar boost in muscle glycogen without many of the drawbacks of the first yo-yo approaches [5, 6].

Moderation is achieved by incrementally reducing training volume while simultaneously increasing carb intake in the days leading up to a competition.

While carb-loading may be an effective method to boost glycogen storage, is it the best way to fuel before your event? Does carb-loading really matter?

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  1. Jeukendrup, A.E. and M. Gleeson, Sport nutrition. 2019.
  2. Krogh, A. and J. Lindhard, The Relative Value of Fat and Carbohydrate as Sources of Muscular Energy: With Appendices on the Correlation between Standard Metabolism and the Respiratory Quotient during Rest and Work. Biochem J, 1920. 14(3-4): p. 290-363.
  3. Brooks, G.A., IMPORTANCE OF THE ‘CROSSOVER’ CONCEPT IN EXERCISE METABOLISM. Clinical and Experimental Pharmacology and Physiology, 1997. 24: p. 889-895.
  4. Bergström, J. and E. Hultman, Synthesis of muscle glycogen in man after glucose and fructose infusion. Acta Med Scand, 1967. 182(1): p. 93-107.
  5. Sherman, W.M., et al., Effect of exercise-diet manipulation on muscle glycogen and its subsequent utilization during performance. Int J Sports Med, 1981. 2(2): p. 114-8.
  6. Sherman, W.M. and D.L. Costill, The marathon: dietary manipulation to optimize performance. Am J Sports Med, 1984. 12(1): p. 44-51.
  7. Thomas, D.T., K.A. Erdman, and L.M. Burke, Position of the Academy of Nutrition and Dietetics, Dietitians of Canada, and the American College of Sports Medicine: Nutrition and Athletic Performance. J Acad Nutr Diet, 2016. 116: p. 501-528.
  8. Hargreaves, M., J.A. Hawley, and A. Jeukendrup, Pre-exercise carbohydrate and fat ingestion: effects on metabolism and performance. J Sports Sci, 2004. 22(1): p. 31-8.
  9. Jeukendrup, A.E. and S.C. Killer, The myths surrounding pre-exercise carbohydrate feeding. Ann Nutr Metab, 2010. 57 Suppl 2: p. 18-25.
  10. Cornford, E. and R. Metcalfe, Omission of Carbohydrate-rich breakfast impairs evening 2000-m rowing time trial performance. European Journal of Sport Science, 2018: p. 1-8