EFFECTS OF A CARBOHYDRATE-PROTEIN BEVERAGE ON CYCLING ENDURANCE AND MUSCLE DAMAGE (for full study click here - PDF version)
SAUNDERS, M. J., M. D. KANE and M. K. TODD. Med. Sci. Sports Exerc., Vol. 36, No. 7, 2004.

Introduction: The purpose of this study was to determine if endurance cycling performance and post-exercise muscle damage were altered when consuming a carbohydrate and protein beverage (CHO+P; 7.3% and 1.8% concentrations) versus a carbohydrate-only (CHO; 7.3%) beverage.

Methods: Fifteen male cyclists (mean VO2peak=52.6±10.3 ml·kg-1·min-1) rode a cycle ergometer at 75% VO2peak to volitional exhaustion, followed 12-15 hours later by a second ride to exhaustion at 85% VO2peak. Subjects consumed 1.8ml/kgBW of randomly assigned CHO (Gatorade®) or CHO+P (Accelerade®) beverage every 15 minutes of exercise, and 10 ml/kgBW immediately following exercise. Beverages were matched for carbohydrate content, resulting in 20% lower total caloric content per administration of CHO beverage. Subjects were blinded to treatment beverage and repeated the same protocol seven to 14 days later with the other beverage.

Results: In the first ride (75% VO2peak), subjects rode 29% longer (p<.05) when consuming the CHO+P beverage (106.3±45.2 min) than the CHO beverage (82.3±32.6 min). In the second ride (85% VO2peak), subjects performed 40% longer when consuming the CHO+P beverage (43.6±12.5 min) than when consuming the CHO beverage (31.2±8.7 min). Peak post-exercise plasma CPK levels, indicative of muscle damage, were 83% lower following the CHO+P trial (216.3±122.0 U/L) than the CHO trial (1318.1±1935.6 U/L). There were no significant differences in exercising levels of VO2, ventilation, heart rate, RPE, blood glucose or blood lactate between treatments in either trial.

Conclusion: A carbohydrate beverage with additional protein calories produced significant improvements in time to fatigue and reductions in muscle damage in endurance athletes. Further research is necessary to determine if these effects were the result of higher total caloric content of the CHO+P beverage, or due to specific protein-mediated mechanisms.

Protein Metabolism

Proteins contain carbon, hydrogen, oxygen, nitrogen, and sometimes other atoms. They form the cellular structural elements, are biochemical catalysts, and are important regulators of gene expression. Nitrogen is essential to the formation of twenty different amino acids, the building blocks of all body cells. Amino acids are characterized by the presence of a terminal carboxyl group and an amino group in the alpha position, and they are connected by peptide bonds.

Digestion breaks protein down to amino acids. If amino acids are in excess of the body's biological requirements, they are metabolized to glycogen or fat and subsequently used for energy metabolism. If amino acids are to be used for energy their carbon skeletons are converted to acetyl CoA, which enters the Krebs cycle for oxidation, producing ATP. The final products of protein catabolism include carbon dioxide, water, ATP, urea, and ammonia.

Vitamin B6 is involved in the metabolism (especially catabolism) of amino acids, as a cofactor in transamination reactions that transfer the nitrogen from one keto acid (an acid containing a keto group [-CO-] in addition to the acid group) to another. This is the last step in the synthesis of nonessential amino acids and the first step in amino acid catabolism. Transamination converts amino acids to L-glutamate, which undergoes oxidative deamination to form ammonia, used for the synthesis of urea. Urea is transferred through the blood to the kidneys and excreted in the urine.

The glucose-alanine cycle is the main pathway by which amino groups from muscle amino acids are transported to the liver for conversion to glucose. The liver is the main site of catabolism for all essential amino acids, except the branched-chain amino acids, which are catabolized mainly by muscle and the kidneys. Plasma amino-acid levels are affected by dietary carbohydrate through the action of insulin, which lowers plasma amino-acid levels (particularly the branched-chain amino acids) by promoting their entry into the muscle.

Body proteins are broken down when dietary supply of energy is inadequate during illness or prolonged starvation. The proteins in the liver are utilized in preference to those of other tissues such as the brain. The gluconeogenesis pathway is present only in liver cells and in certain kidney cells.