Creatine supplementation improves muscle energy production during high-intensity exercise, but it is less clear whether such improvements are evident during endurance exercise. Australian researchers examined the effects of 5 days of creatine loading on measures of skeletal muscle energy balance. Before and after supplementation, highly trained subjects cycled for 45 min at a standard pace and then completed a performance ride performing a standard amount of work as fast as possible. Muscle metabolites from biopsy samples obtained from the thigh were measured at rest, after 45 min of cycling, and after the performance ride. As expected, creatine supplementation resulted in a significant increase in muscle total creatine of about 17%. The novel finding was that creatine supplementation was associated with a significantly lower accumulation of muscle inosine monophosphate (IMP). Muscle IMP normally accumulates when there is a mismatch between energy supply and demand. That is, when more energy is needed than can be supplied during exercise, then IMP increases. So the lower IMP indicates that creatine supplementation improves energy balance in muscle. The study provides evidence that creatine supplementation may also be a valuable supplement for endurance athletes.
McConell GK, Shinewell J, Stephens TJ, Stathis CG, Canny BJ, Snow RJ. Creatine supplementation reduces muscle inosine monophosphate during endurance exercise in humans. Med Sci Sports Exerc. 2005 Dec;37(12):2054-61.