Protein in Sports Drinks
- Does Adding Protein to a Sports Drink Improve Performance?
Ellen Coleman, RD, MA, MPH Copyright
Sports Medicine Digest Vol 26, No. 1, pages 10-11 (January 2004)
How does carbohydrate intake enhance performance? Can the addition of protein help even more?
Muscle glycogen stores hold the key to performance during moderately high and high-intensity endurance exercise. Because muscle glycogen is so important for prolonged exercise, researchers have spent a lot of time studying techniques to decrease the rate of glycogen utilization during exercise and increase muscle glycogen stores prior to and following exercise.
In one early study, Coyle and colleagues found that although carbohydrate supplementation improved performance during prolonged, strenuous cycling (at 70% of VO2max), it did not reduce the amount of glycogen the muscles actually used. The supplemental carbohydrate enhanced performance by maintaining blood glucose levels and carbohydrate oxidation rather than by sparing muscle glycogen. (See Coyle et al, 1986.)
Carbohydrate supplementation can, however, limit the decline in muscle glycogen stores during prolonged, intermittent (see Nicholas et al, 1999) and variable-intensity exercise (see Yaspelkis et al, 1993). The plasma insulin response to carbohydrate supplementation may explain differences in the ways muscles use glycogen in these exercise protocols. During high intensity exercise, carbohydrate supplementation has a limited effect on plasma insulin concentration. But during intermittent and variable intensity exercise, carbohydrate supplementation increases plasma insulin concentration. These higher plasma insulin levels may spare muscle glycogen and/or increase glycogen resynthesis during rest or lower-intensity exercise--and so enhance endurance.
What About Protein?
In theory, a carbohydrate-protein supplement may be more effective than a carbohydrate supplement in reducing muscle glycogen utilization during intermittent and variable intensity exercise. The addition of protein to a carbohydrate supplement enhances the plasma insulin response to the supplement following prolonged endurance exercise. (See Van Loon et al, 2000.)
In a recent study, Ivy and colleagues at the University of Texas in Austin compared the effects of a carbohydrate and a carbohydrate-protein supplement on performance during variable-intensity endurance exercise. The nine trained male cyclists exercised on three separate occasions at intensities that varied between 45% and 75% of VO2max for three hours, followed by a performance trial at 85% of VO2max until fatigued. Every 20 minutes, the cyclists received either 200 ml of the placebo, a 7.75% carbohydrate solution, or a 7.75% carbohydrate and 1.94% protein solution. The three treatments were provided in a double-blind, randomized trial. (See Ivy et al, 2003.)
The carbohydrate supplement significantly increased time to exhaustion (19.7 minutes) compared to the placebo (12.7 minutes). The carbohydrate-protein supplement significantly increased time to exhaustion (26.9 minutes) compared to the carbohydrate supplement. Compared to the placebo, the carbohydrate supplement improved performance by 36% and the carbohydrate-protein supplement improved performance by 55%. (See Ivy et al, 2003).
Blood glucose and serum insulin were significantly higher during carbohydrate and carbohydrate-protein supplementation compared to placebo, but there were no significant differences between the carbohydrate and carbohydrate-protein treatments. (See Ivy et al, 2003).
Is Performance Boosted by the Protein-or the Extra Calories?
While it was clear that adding protein to the carbohydrate supplement improved performance, the reason for the benefit was not clear. The addition of protein should have promoted a greater sparing of muscle glycogen than carbohydrate alone, thus providing a greater glycogen reserve for the performance trial. However, the insulin responses to the two treatments were the same; there was no evidence that glycogen sparing or synthesis was greater in the carbohydrate-protein treatment compared to the carbohydrate treatment. The measurements of carbohydrate and fat oxidation were also similar for both treatments, suggesting similar muscle glycogen usage. (See Ivy et al, 2003.)
The researchers provide three possible explanations for their results. The addition of protein to carbohydrate may have facilitated muscle glycogen sparing by an unknown process. The carbohydrate-protein supplement may have helped to prevent central fatigue by maintaining plasma amino acids. Lastly, the addition of protein may have provided precursors for the reactions required to maintain Kreb's cycle intermediates in the skeletal muscle. (See Ivy et al, 2003.)
The researches did not address whether the calorie difference between the two supplements may have influenced the study results. The carbohydrate supplement provided 46.5 g of carbohydrate and 186 calories per hour. The carbohydrate-protein supplement provided 46.5 g of carbohydrate, 11.6 g of protein, and 232 calories per hour. Although only a difference of 46 calories per hour, the higher calorie content in the carbohydrate-protein supplement may have been responsible for the improvement in performance compared to the carbohydrate supplement.
Coyle EF et al., Glycogen utilization during prolonged strenuous exercise when fed carbohydrate. Journal of Applied Physiology. 1986;61:165-72.
Ivy JL et al., Effect of a carbohydrate-protein supplement on endurance performance during exercise of varying intensity. International Journal of Sports Nutrition and Exercise Metabolism. 2003; 13:382-95.
Nicholas CW et al., Carbohydrate-electrolyte ingestion during intermittent high-intensity running. Medicine and Science in Sports and Exercise. 1999;31:1280-6.
Van Loon L et al., Maximizing postexercise muscle glycogen synthesis: carbohydrate supplementation and the application of amino acid or protein hydrolysate mixtures. American Journal of Clinical Nutrition. 2000;72:106-111.
Yaspelkis BB et al., Carbohydrate supplementation spares muscle glycogen during variable intensity exercise. Journal of Applied Physiology. 1993;75:1477-85.