Part 3

Some practitioners recommend adding protein to the carbohydrate feeding to enhance glycogen repletion. Over a decade ago, Zawadzki et al reported that adding protein to a carbohydrate drink produced higher muscle glycogen synthesis rates after exercise than the carbohydrate drink alone (9). Immediately and two hours after glycogen depleting exercise, the subjects ingested 112 g of carbohydrate supplying 0.8 g/kg/hr (CHO), 40.7 g of protein (PRO), or 112 g carbohydrate and 40.7 g protein (CHO-PRO). During four hours of recovery, the plasma insulin response of the CHO-PRO trial was significantly greater than that of the CHO trial. The rate of muscle glycogen storage during the CHO-PRO trial (35.5 micromol/g/hr) was significantly faster than for the carbohydrate trial (25.6 micromol/g/hr). The researchers concluded that the interaction of carbohydrate and protein on insulin secretion enhanced muscle glycogen storage following exercise.

Their findings were plausible, as elevated insulin concentrations may increase muscle glucose uptake and boost glycogen synthase activity. These factors determine the rate of muscle glycogen synthesis when the supply of substrate (glucose) is adequate. However, the findings of the Zawadzki et al study may be criticized because the treatments were not isocaloric. The CHO drink provided 448 calories whereas the CHO-PRO drink provided 611 calories. As carbohydrate and protein both stimulate insulin secretion, the higher calorie content in the CHO-PRO drink compared to the CHO drink may have been responsible for the increased rate of glycogen storage.

More recently, other researchers have tried to establish whether the improved recovery observed by Zawadzki was the result of additional protein or additional calories.

Van Loon et al evaluated post exercise muscle glycogen synthesis rates following the ingestion of a carbohydrate and amino acid (leucine and phenylalanine) and wheat protein hydrolysate mixture and a drink providing an isocaloric amount of carbohydrate (10). After glycogen depleting exercise, the subjects consumed a control beverage providing 0.8 g carbohydrate/kg/hr (CHO), a carbohydrate and amino acid and protein hydrolysate mixture providing 0.8 g carbohydrate/kg/hr and 0.4 g protein/kg/hr (CHO-PRO), or an isocaloric drink providing 1.2 g carbohydrate/kg/hr (CHO-CHO) every 30 minutes. Plasma insulin responses were 88% higher in the CHO-PRO trial and 46% higher in the CHO-CHO trial than in the CHO trial during five hours of recovery. Muscle glycogen synthesis was also higher in the CHO-PRO trial (35.4 micromol/g/hr) and CHO-CHO trial (44.8 micromol/g/hr) than in the CHO trial (16.6 micromol/g/hr). The researchers concluded that adding a mixture of protein hydrolysate and amino acids to a carbohydrate drink providing 0.8 g/kg/hr and increasing carbohydrate intake to 1.2 g/kg/hr both accelerate the rate of glycogen synthesis.

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