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Importance of Muscle Glycogen in Endurance Exercise

Muscle glycogen (an energy substrate used for muscle contraction) is the most vital fuel for endurance athletes. Levels at the start of exercise are directly linked with perceived fatigue levels, while exercise intensity is directly correlated with consumption of muscle glycogen. Once glycogen levels are depleted, it’s difficult to sustain high exercise intensity.

Improving Endurance with Optimal Glycogen Stores

Therefore one of the fundamental ways to improve endurance is to increase the glycogen stores in skeletal muscle and the liver before the start of exercise5. It’s equally important for endurance athletes to replenish their glycogen stores post-exercise to provide sufficient energy for the next training session or for competition.

Glycogen Replenishment Takes Time

However, muscle glycogen synthesis is a relatively slow process; taking up to 24 hours for complete replenishment. For this reason scientists have conducted numerous studies in an attempt to determine ways to enhance the rate of muscle glycogen resynthesis1, 2, 4.

Whey Protein Hydrolysate Increases Glycogen and Endurance

While eating a high-carbohydrate diet after exercise is effective for rapid replenishment of glycogen stores, a recent series of studies have shown that intake of whey protein hydrolysate and carbohydrate is more effective for replenishment of muscle glycogen after exercise compared with carbohydrate supplementation alone3, 4. Moreover, whey protein hydrolysate combined with carbohydrate is more effective than either whey protein, branched-chain amino acids (BCAAs) or casein hydrolysate for replenishment of muscle glycogen after endurance exercise4. So it would appear both the source and degree of hydrolysis of dietary protein affects post-exercise glycogen repletion in endurance athletes.

Whey Protein Hydrolysate Stimulation of Glycogen Synthesis

From a mechanistic viewpoint, scientists believe that the combination of whey protein hydrolysate with carbohydrate activates separate mechanisms in muscle glucose uptake to that of carbohydrate alone. Studies in rat models have shown that branched-chain amino acids-containing dipeptides in whey protein hydrolysates were responsible for activating skeletal muscle glucose uptake via a mechanism different from that induced by insulin from carbohydrate intake only6. Taken together, these developments in the knowledge of how whey protein hydrolysate regulates glycogen synthesis represent a major boom for endurance athletes. There is enough evidence now to suggest that endurance athletes who don’t use whey protein hydrolysate as a regular part of their training are giving away a vital edge!

1. Saito S, Yoshitake Y, Suzuki M. Enhanced glycogen repletion in liver and skeletal muscle with citrate orally fed after exhaustive treadmill running and swimming. J Nutr Sci Vitaminol. 1983;29:45–52.
2. Ivy JL, Goforth HW Jr, Damon BM, McCauley TR, Parsons EC, Price TB. Early postexercise muscle glycogen recovery is enhanced with a carbohydrate–protein supplement. J Appl Physiol. 2002;93:1337–44.
3. van Hall G, Saris WH, van de Schoor PA,Wagenmakers AJ. The effect of free glutamine and peptide ingestion on the rate of muscle glycogen resynthesis in man. Int J Sports Med. 2000;21:25–30.
4. Morifuji M, et al. Post-exercise carbohydrate plus whey protein hydrolysates supplementation increases skeletal muscle glycogen level in rats. Amino Acids. 2010;38(4):1109-1115.
5. Aoi W, et al. Dietary whey hydrolysate with exercise alters the plasma protein profile: A comprehensive protein analysis. Nutrition. 2011;27(6):687-692.
6. Morifuji M, et al. Branched-chain amino acid-containing dipeptides, identified from whey protein hydrolysates, stimulate glucose uptake rate in L6 myotubes and isolated skeletal muscles. J Nutr Sci Vitaminol (Tokyo). 2009;55(1):81-6.

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