Purpose: To study the effects of 8-week creatine monohydrate (CrM) supplementation on blood and urinary clinical health markers in football players.
Methods: 14 football players were randomly assigned in a double-blinded fashion to Cre (n = 7) or Pla (n = 7) group. The Cre group ingested 15 g/day of CrM for 7 days and 3 g/day for the remaining 49 days, whereas the Pla group ingested maltodextrin following the same protocol. Football-specific training was performed during the study. Total body mass was determined and blood and urine samples were analysed for metabolic, hepatic, renal and muscular function markers, before and after supplementation.
Results: A gain of total body mass was observed after CrM intake, but not with placebo. Blood and urinary markers remained within normal reference values. There were no significant changes in renal and hepatic markers after CrM intake. However, total creatine kinase (CK) activity significantly increased, and uric acid level tended to decrease after CrM use. Likewise, serum glucose decreased in the Cre group following supplementation. No significant differences in urine parameters were found in either group after supplementation.
Conclusions: 8 weeks of CrM supplementation had no negative effects on blood and urinary clinical health markers in football players. Properties of CrM may, however, be associated with an increase in CK activity, improving the efficiency for ATP resynthesis, a phenomenon indirectly confirmed by the decreasing tendency in uric acid concentration. Furthermore, CrM seems to slightly influence glucoregulation in trained subjects.
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Creatine (Cr) is a nitrogenous amino acid derivative taken in beef and fish diets, and is naturally synthesised in the body from arginine, glycine, and methionine, primarily in the liver and kidney. It is stored by high-demand tissues (ie, skeletal and cardiac muscle). In skeletal muscle, Cr is phosphorylated to produce phosphocreatine (PCr) for ATP resynthesis in a reaction catalysed by creatine kinase (CK). The Cr−CK−PCr circuit plays an important role in adenine nucleotide homeostasis by preventing an increase in intracellular [free ADP] and [H+], which produces an inactivation of cellular ATPases and a net loss of adenine nucleotides, events that impair muscle contraction. PCr is an "energy carrier", connecting sites of ATP production with sites of ATP utilisation via CK isoenzymes (mitochondria and cytosol).1 Numerous studies have demonstrated that creatine monohydrate (CrM) supplementation increases intramuscular Cr and PCr,2–5 improving power and performance of short-term, high-intensity exercise, particularly exercise of a repetitive nature.4 6 7 Football is considered a high-intensity, intermittent exercise8 with the result that during periods of intensive exercise during matches or training, PCr will significantly prevent any transient increase in ADP likely to impair muscle contraction. Previous studies reported that acute CrM intake can improve the football-specific exercise performance of football players.9 10
Since Cr is a nitrogen-rich compound, it has been suggested that CrM supplementation may cause renal or liver damage. Although many studies have failed to observe any adverse effects of CrM intake at recommended doses,6 7 11–13 there are some isolated case reports linking the supplement to an exacerbation of renal dysfunction in individuals with pre-existing nephropathy,14 15 thus, supplementation may be called into question. Furthermore, in some studies, CrM supplementation moderately increased blood and urinary creatinine (Crn) concentration in healthy subjects.6 7 Although this could be due to an enhanced rate of muscle Cr degradation to Crn, not all studies observed a significant increase of Crn following CrM intake11 16 17, which raises the question is there enough evidence to support the theory that CrM supplementation causes renal damage. Additionally, some researchers reported a moderate increase in muscle and liver enzyme efflux,7 11 while others observed no change following CrM supplementation.6 17
Football is arguably the most popular sport in the world, yet to our knowledge researchers have not thoroughly evaluated CrM intake by football players to evaluate its safety. Therefore, it is the purpose of this study to examine blood and urinary clinical health markers of metabolic, hepatic, renal and muscular function, and the incidence of side effects in football players after 8 weeks of CrM supplementation.
Fourteen healthy, trained male football players participated in the study. Subjects were informed of the procedures, objectives, benefits and possible risks of the test before signing an informed consent statement according to the guidelines of the Ethics Committee of the Clinical Hospital of the University of the Republic. The subjects were semi-professional athletes who were in their competitive season during the study and, as such, playing in matches on average twice a week. Their training was mostly football specific; most exercises were of a technical or tactical nature and less to do with fitness development. Since the subjects were playing in matches twice a week, their schedule was lighter on the days following each game (mostly technical and tactical training, and planning for the next game). They did not undertake power training or weight training, and almost no running training. They performed only one training session a day on match-free days. The researchers did not change the training schedule, which followed that devised by the players' coaches.
Subjects completed a questionnaire concerning medical history. None of the subjects had previously taken a CrM supplement or maltodextrin. Their diets were not standardised, but they all ate a regular, varied diet and were in an iso-caloric state throughout the study. They were asked not to change their dietary habits during the course of the study. Furthermore, when the study was carried out none of the subjects were taking medications regularly.
The mean age and height of the subjects at the beginning of the study were 19.6 years (SEM 3.5) and 175.2 cm (SEM 4.6), respectively. Body mass was measured on a digital Sohnle scale (ES2003 135 KG, Sohnle, Murrhardt, Germany) (accuracy to the nearest 0.1 kg).
Creatine: placebo supplementation
After initial baseline control measurements, subjects were randomly assigned either to the CrM supplementation group (Cre, n = 7) or the placebo group (Pla, n = 7). To administer either CrM or placebo the test included a double-blind procedure.
The Cre group ingested 4.4 g of Cr (5.0 g of CrH2O) three times a day for 7 days, then 2.6 g Cr (3.0 g of CrH2O) each day for 49 days; the Pla group received the same dosage of maltodextrin (Carbo-Complex, 100% pure). Supplements had similar texture, taste and appearance. The subjects were instructed to dissolve the supplement in water with 5 g of sucrose. The solution was ingested with meals. Subjects were asked not to take caffeine drinks with their supplements.4 The biochemical purity of the micronised CrM (Silab, Edatir Laboratories) 99.5% (anhydrous Cr) was verified by HPLC (Hewlett Packard1050, USA) against a certified standard (DSM Fine Chemicals, Gebeen, The Netherlands).
The packets of supplement were administered in unmarked coded boxes containing a 7-day supply. The subjects were asked to return all empty packets before they were given the next 7-day supply. Compliance with supplement intake was supervised by weekly contact with the subjects. After 56 days of intake, consumption was interrupted and on the following day (day 57) subjects reported to the laboratory for postadministration measurements, which were identical to those described for the preadministration phase.
Blood and urine sample collection and analysis
Samples of blood and first urine were collected after a 12 h fast, pre and post supplementation. The last training day, which included technical and tactical training, finished 17 h before sample collection. Presupplementation and postsupplementation samples were taken 48 h following game participation. Blood samples were obtained via venipuncture from a forearm vein. Venous blood was collected in serum separation tubes and centrifuged for 10 min at 2500 r.p.m. using a Labofuge AE centrifuge (Heraeus, Hanau, Germany). Renal function was assessed immediately by measuring Crn and blood urea nitrogen (BUN) in samples, and hepatic function by assessing the amount of total proteins and albumin, and aspartate amine transferase (ASAT), alanine amine transferase (ALAT) and γ-glutamyl transferase (γGT) activities. Metabolism was assessed by examining glucose, uric acid, triglycerides and cholesterol, and muscular function was determined by total CK activity. Biochemical analysis was carried out with commercial reagents (Wiener lab, Argentina) in a CCx automated chemistry analyser (Abbott, USA) following standard procedures. Crn clearance was estimated.18 pH, specific gravity, proteins, occult blood, leukocyte esterase, bilirubin, urobilinogen, glucose, ketones and nitrite were analysed in urine specimens using commercial reagents (Urine strip, Wiener, Argentina). A commercial system for standardised urinalysis (Kova System, Instruchemie BV, The Netherlands) was used to evaluate urinary sediment. Urine specimens were centrifuged at 1500 r.p.m. for 5 min. The sediment was observed using a brightfield microscope (Alphaphot-2 YS2, NIKON, Japan) at low-power (100×) and high-power magnification (400×).
The statistical analyses were performed using Statistica software (version 4.5; StatSoft, USA). Shapiro-Wilk and D’Agostino tests revealed that the data did not follow a normal distribution, so non-parametric statistical techniques were used, including the Wilcoxon matched-pairs test, to find the significant difference between groups.19
Blood and urinary data are presented as median (m) and mean (2 SEM), respectively. Owing to the small sample size, statistical significance was set at p<0.05, and non-significance at p>0.10. All p values between 0.05 and 0.10 were viewed as indicative of data trends approaching significance.
Total body mass was increased post supplementation in the Cre group (m = 64.8 vs m = 66.2 kg, p = 0.02), but remained unchanged in the Pla group (m = 70.0 vs m = 71.0 kg, p = 0.50).
Blood chemistry profiles
Blood measurements analysed were within normal reference values for healthy athletes. No significant difference was seen in estimated Crn clearance, BUN, serum Crn, cholesterol, triglycerides, ratio of BUN:Crn, total protein, albumin, and ratio of albumin:globulin concentrations after supplementation (table 1). The activity of liver enzymes, γGT, ASAT and ALAT, was not influenced by supplementation in either group (table 2).
Considering the small sample size, and the large SEM for CK activity variability, we applied logarithmic transformation for a more refined approach. An increase in log2 serum total CK activity was found for the Cre group (m = 8.0 vs m = 8.6 IU/litre, p = 0.04) post supplementation as compared with the Pla group (m = 8.2 vs m = 8.2 IU/litre, p = 0.50) (fig 1).
Uric acid concentration tended to decrease in the Cre group post supplementation (m = 0.30 vs m = 0.27 mmol/litre, p = 0.06) compared with that for the Pla group (m = 0.29 vs m = 0.26 mmol/litre, p = 0.17) (fig 2).
Postsupplementation glucose concentration decreased in the Cre group (m = 5.27 vs m = 4.61 mmol/litre, p = 0.01), whereas it remained unchanged in the Pla group (m = 5.11 vs m = 5.02 mmol/litre, p = 0.23) (fig 3).
Urinary chemistry profiles
No significant differences in urinary markers were found in either groups post supplementation. One player, however, in the Cre group presented slight hematuria (table 3).
Subjects tolerated the CrM or maltodextrin supplementation very well. No major gastrointestinal distress, musculoskeletal problems or any other clinical problems were reported.
This investigation studied the impact of 8 weeks of CrM supplementation on blood and urinary clinical health markers of metabolic, hepatic, renal and muscular function in football players. The results show that the markers studied remained within normal clinical values for healthy athletes, and are in agreement with previous reports.6 7 12 13 20 None of the subjects experienced side effects from supplement use at the current dosage. However, there was an increase in body mass in the Cre group by the end of the supplementation period, as noted by other authors.6 7 10 17 21
The Cre group displayed an increase in CK activity after CrM intake, although there was individual variability in response. Almada et al.11 also found an increase in CK activity after 8 weeks of supplementation, although the subjects were not attended for physical activity. Kreider et al.7 observed the same response in subjects ingesting CrM during an intense training programme. The increase in CK activity may be related to the fact that with CrM, subjects may endure more intense training, thus enzyme release increases, and/or CrM per se produces an upregulation of the Cr−CK−PCr system.
However, there is controversy regarding the increase in CK activity via Cr−CK−PCr circuit upregulation. This is because this phenomenon is also considered a clinical manifestation of cellular damage. Specifically, it has been observed that since Cr is an osmotically active molecule, it increases intracellular fluid volume,22 and this could cause cellular damage and release of cytosolic material (eg, CK). In the present study neither muscle cramping nor muscle damage appeared to have occurred, and those players showing an increase in CK did not show an increase in ASAT activity (data not shown). Although the increase in CK may be explained by higher intensity and/or load training, the training programme was similar for both groups. The Pla group did not show significant changes, suggesting that the observed increase of CK activity is principally associated with upregulation of the Cr−CK−PCr system. Unfortunately, we cannot be entirely certain about the reasons for the significant increase in CK activity observed, based upon the present data.
Likewise, the Cre group showed a tendency towards a decrease in uric acid concentration. The reasons for this phenomenon are unclear but could be related to CK-catalysed resynthesis of ATP. When the rate of ATP consumption exceeds resynthesis capacity, ADP accumulates and is degraded to uric acid among others via adenylate kinase and other enzymes. Accordingly, due to the high demand of energy necessary in competitive football, free ADP and H+ accumulate in muscle fibers, and adenine degradation products in serum are higher during training and matches.8 These metabolic events compromise muscular contraction. The decrease in serum uric acid observed in our research and that of others23 as a result of CrM intake suggests that an increase in intracellular Cr concentration could confer a greater efficiency for ATP resynthesis. The increase in CK activity observed in the current study may be related to regulation of the adenylate kinase reaction,1 by preventing free ADP accumulation, resulting in a lower adenine nucleotide loss, which in turn could delay the onset of fatigue.
A noteworthy finding was that serum glucose concentration decreased following CrM supplementation. Previous studies do not fully agree on this point; some reported no changes,7 24 while others demonstrated an effect of CrM on carbohydrate metabolism.5 16 25–28
In our study, CrM induced a mild hypoglycemic effect in physically active subjects, which raises the question of how carbohydrate metabolism could be influenced by CrM supplementation. Assuming that the reduction of glycemia after CrM intake means an increase in cellular glucose uptake and since skeletal muscle is an important tissue for maintenance of glucose postprandial homeostasis (and simultaneously most Cr is accumulated in this tissue), it is reasonable to believe that these two events interact. Some authors have suggested that the Cr−CK−PCr system may be involved in signaling glucose-induced insulin secretion from beta cells.29 Therefore, the increase in ATP resynthesis rate caused by an upregulation of this system could stimulate insulin secretion.30 In other studies, CrM has not been found to increase blood insulin concentrations,5 24 28 31 in spite of the fact that effects on carbohydrate metabolism were confirmed.5 31 Thus, it is possible that the gluco-regulating effect of CrM could be related to an increase in insulin sensitivity. Unfortunately, we were unable to measure this hormone and can only speculate.
Urinary markers did not show significant changes in either group after supplementation, although an asymptomatic microhematuria32 of uncertain etiology was observed in the sediment of one player from the Cre group.
The results of our investigation indicated that 8 weeks of CrM supplementation did not have any adverse effects on the subjects’ serum and urinary clinical health markers of liver, renal, muscular and metabolic function.
CrM-supplemented players showed an increase of total CK activity. This may represent an upregulation of the Cr−CK−PCr system, which would improve the efficiency of ATP resynthesis. This was indirectly confirmed by the decreasing tendency in serum uric acid concentration. These data imply that the effects of CrM could be partly associated with an increase in cellular CK activity.
Likewise, the decrease in serum glucose found suggests that CrM could have positive effects on glucoregulation, indicating potential therapeutic applications, although further research is required to confirm this.
It is worth mentioning that, although CrM supplementation appears to be safe in football players, our sample was small and is a limitation of our findings. Additional research should examine the effects of long-term supplementation in a larger population of athletes and patients, following a crossover design. Finally, it should be remembered that Cr pharmacokinetics depends on individuals and on a set of different conditions, and it may be necessary to adjust the doses to achieve the best possible results and prevent potentially adverse effects on health.
What is already known on this topic
Creatine monohydrate (CrM) is widely used as a nutritional supplement.
Numerous studies have demonstrated that its use improves sports performance.
Although previous studies did not observe adverse effects of CrM intake at recommended doses in healthy athletes, it has been suggested that CrM may cause renal or liver damage.
What this study adds
No previous studies have been conducted to investigate the safety of CrM supplementation in football players.
In this study CrM supplementation did not have a negative effect on blood and urinary clinical health markers.
CrM seems to influence glucoregulation and adenine nucleotide homeostasis in trained athletes, which warrants further research in other groups.
We thank Mr. Martín Taborda and Darío Bravo, coaches of the Río Negro football team, for their cooperation. Creatine monohydrate and maltodextrin were kindly provided by Eng. Sergio Sgarbi (Sports Nutrition). We also thank Dr. Susana Nicola, Director of the Clinical Laboratory of the San José Hospital, Mr. Daniel Ruiz (RELAB S.R.L) and Koro Laboratories for kindly helping in the biochemical analyses. We thank Bach. Soledad Ferrer for editing our work.
Competing interests: None.
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