Differential response of muscle phosphocreatine to creatine supplementation in young and old subjects

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Abstract
This study compared the effects of short-term creatine supplementation on muscle phosphocreatine, blood and urine creatine levels, and urine creatinine levels in elderly and young subjects. Eight young (24 ± 1.4 years) and seven old (70 ± 2.9 years) men ingested creatine (20 g day–1) for 5 days. Baseline muscle phosphocreatine measurements were taken pre- and post-supplementation using nuclear magnetic resonance spectroscopy (NMR). On the first day of supplementation subjects had blood samples taken immediately before and hourly for 5 h following ingestion of 5 g of creatine, and a pharmacokinetic analysis of plasma creatine levels was conducted. Twenty-four hour urine collections were conducted for 2 days prior to the supplementation period and for 5 days during supplementation. Old subjects had significantly higher baseline plasma creatine levels than young subjects (68.5 ± 12.5 vs. 34.9 ± 4.7 μmol L–1; P < 0.02). There were no significant differences between groups in plasma creatine pharmacokinetic parameters (i.e. area under the curve, elimination rate constant, absorption rate constant, time to maximum concentration, and maximum concentration) following the 5 g oral creatine bolus. Urine creatine, assessed pre and on 5 days of supplementation, increased (P < 0.001), with no difference between groups. Urine creatinine did not change as a result of creatine supplementation. Young subjects showed a significantly greater increase in muscle phosphocreatine compared with old subjects, and post-supplementation muscle phosphocreatine levels were greater in young subjects (young 27.6 ± 0.5; old 25.7 ± 0.8 mmol kg–1 ww) (P=0.02). There were no differences in blood or urine creatine between groups in response to supplementation, but old subjects had a relatively small increase (young 35% vs. old 7%) in muscle phosphocreatine after supplementation.