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The Bioavailability Issue
What percent of a supplement is actually absorbed by the body? Several studies show more than 95 percent of oral creatine is.1,2 What differs between subjects is the magnitude of muscle creatine uptake. Up to one-third of individuals supplementing with creatine do not show robust increases in muscle creatine and phosphocreatine.1,3 And, it is unclear whether people supplementing with creatine over a period of time continue to uptake it.
The predominant factors regulating creatine uptake are the amount and activity of creatine transporter protein and circulating insulin concentrations. Creatine can enter muscle cells via a specific transport protein, which is often inaccurately referred to as the creatine "receptor." Recent studies by Theo Wallimann, Ph.D., and associates at the Institute for Cell Biology in Zurich, Switzerland, suggest creatine transporter protein is downregulated in animals following chronic creatine exposure.4 However, unpublished studies with human subjects by Paul Greenhaff, Ph.D., of the University of Nottingham, and colleagues in England show no downregulation after several weeks of use.5 Greenhaff's lab has also shown that a large amount of an insulinogenic carbohydrate (93 g glucose, for example), or very high physiological or pharmacological concentrations of insulin, enhance muscle creatine uptake.2,6
An unpublished study by Jeff Stout, Ph.D., and associates at Creighton University, Omaha, Neb., describes superior increases in muscle performance with a proprietary effervescent creatine preparation. To explain the apparent differences between effervescent and "standard" creatine, the researchers offer the suggestion of increased absorption and/or uptake. The researchers did not, however, attempt to measure true absorption and, more importantly, did not examine muscle creatine uptake.7 Therefore the issues of superior absorption or uptake remain unaddressed.
—A.L.A.
References
1. Harris R, et al. Elevation of creatine in resting and exercised muscle of normal subjects by creatine supplementation. Clin Sci 1992;82:367-74.
2. Green AL, et al. Carbohydrate ingestion augments skeletal muscle creatine accumulation during creatine supplementation in man. J Appl Physiol 1996;271:E821-6.
3. Casey A, et al. Creatine supplementation favorably affects performance and muscle metabolism during maximal intensity exercise in humans. Am J Physiol 1996;271:E31-7.
4. Guerrero-Ontiveros ML, et al. Creatine supplementation in health and disease. Effects of chronic creatine ingestion in vivo: down-regulation of creatine transporter isoforms in skeletal muscle. Mol Cell Biochem 1998;184:427-37.
5. Greenhaff PL. Personal communication. 1999 Jan.
6. Steenge GR, et al. Stimulatory effect of insulin on creatine accumulation in human skeletal muscle. Am J Physiol 1998;275:E974-9.
7. Earnest CP. Personal communication. 1999 Jan.
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