The nutritional biochemistry of creatine

Integrative metabolomics highlights gut microbiota metabolites as novel NAFLD-related candidate biomarkers in children

Altered gut microbiota and metabolites are important for non-alcoholic fatty liver disease (NAFLD) in children. We aimed to comprehensively examine the effects of gut metabolites on NAFLD progression. We performed integrative metabolomics (untargeted discovery and targeted validation) analysis of non-alcoholic fatty liver (NAFL), non-alcoholic steatohepatitis (NASH), and obesity in children. Fecal samples were collected from 75 subjects in the discovery cohort (25 NAFL, 25 NASH, and 25 obese control children) and 145 subjects in an independent validation cohort (53 NAFL, 39 NASH, and 53 obese control children). Among 2,491 metabolites, untargeted metabolomics revealed a complete NAFLD metabolic map containing 318 increased and 123 decreased metabolites. Then, machine learning selected 65 important metabolites that can distinguish the severity of the NAFLD. Furthermore, precision-targeted metabolomics selected 5 novel gut metabolites from 20 typical metabolites. The functionality of candidate metabolites was validated in hepatocyte cell lines. In the end, this study annotated two novel elevated pathogenic metabolites (dodecanoic acid and creatinine) and a relationship between depleted protective gut microbiota (Butyricicoccus and Alistipes), increased inflammation (IL-1β), lipid metabolism (TG), and liver function (ALT and AST). This study demonstrates the role of novel gut metabolites (dodecanoic acid and creatinine), as the fatty acid metabolism regulator contributing to NAFLD development through its influence on inflammation and liver function.

IMPORTANCE

Altered gut microbiota and metabolites are a major cause of non-alcoholic fatty liver disease (NAFLD) in children. This study demonstrated a complete gut metabolic map of children with NAFLD, containing 318 increased and 123 decreased metabolites by untargeted metabolomic. Multiple validation approaches (machine learning and targeted metabolomic) selected five novel gut metabolites for targeted metabolomics, which can distinguish NAFLD status and severity. The gut microbiota (Butyricicoccus and Alistipes) and metabolites (creatinine and dodecanoic acid) were novel biomarkers associated with impaired liver function and inflammation and validated by experiments of hepatocyte cell lines. The data provide a better understanding of the importance of gut microbiota and metabolite alterations in NAFLD, which implies that the altered gut microbiota and metabolites may represent a potential target to prevent NAFLD development.

Impact of Short-Term Creatine Supplementation on Muscular Performance among Breast Cancer Survivors

Breast cancer (BC) is one of the most common cancers in the United States. Advances in detection and treatment have resulted in an increased survival rate, meaning an increasing population experiencing declines in muscle mass and strength. Creatine supplementation has consistently demonstrated improvements in strength and muscle performance in older adults, though these findings have not been extended to cancer populations.

PURPOSE

The purpose of this study was to investigate the effects of short-term creatine supplementation on muscular performance in BC survivors.

METHODS

Using a double-blind, placebo-controlled, randomized design, 19 female BC survivors (mean ± SD age = 57.63 ± 10.77 years) were assigned to creatine (SUPP) (n = 9) or dextrose placebo (PLA) (n = 10) groups. The participants completed two familiarization sessions, then two test sessions, each separated by 7 days, where the participants supplemented with 5 g of SUPP or PLA 4 times/day between sessions. The testing sessions included sit-to-stand power, isometric/isokinetic peak torque, and upper/lower body strength via 10 repetition maximum (10RM) tests. The interaction between supplement (SUPP vs. PLA) and time (Pre vs. Post) was examined using a group × time ANOVA and effect sizes.

RESULTS

No significant effects were observed for sit-to-stand power (p = 0.471; ηp2 = 0.031), peak torque at 60°/second (p = 0.533; ηp2 = 0.023), peak torque at 120°/second (p = 0.944; ηp2 < 0.001), isometric peak torque (p = 0.905; ηp2 < 0.001), 10RM chest press (p = 0.407; ηp2 = 0.041), and 10RM leg extension (p = 0.932; ηp2 < 0.001). However, a large effect size for time occurred for the 10RM chest press (ηp2 = 0.531) and leg extension (ηp2 = 0.422).

CONCLUSION

Seven days of creatine supplementation does not influence muscular performance among BC survivors.

Short-Term Creatine Supplementation and Repeated Sprint Ability-A Systematic Review and Meta-Analysis

The aim of this study was to conduct a systematic review and meta-analysis of the effects of short-term creatine supplementation on repeated sprint ability. Fourteen studies met the inclusion criteria of adopting double-blind randomized placebo-controlled designs in which participants (age: 18-60 years) completed a repeated sprint test (number of sprints: 4 < n ≤ 20; sprint duration: ≤10 s; recovery duration: ≤90 s) before and after supplementing with creatine or placebo for 3-7 days in a dose of ∼20 g/day. No exclusion restrictions were placed on the mode of exercise. Meta-analyses were completed using random-effects models, with effects on measures of peak power output, mean power output, and fatigue (performance decline) during each repeated sprint test presented as standardized mean difference (δ), and with effects on body mass and posttest blood lactate concentration presented as raw mean difference (D). Relative to placebo, creatine resulted in a significant increase in body mass (D = 0.79 kg; p < .00001) and mean power output (δ = 0.61; p = .002). However, there was no effect of creatine on measures of peak power (δ = 0.41; p = .10), fatigue (δ = 0.08; p = .61), or posttest blood lactate concentration (D = 0.22 L/min; p = .60). In conclusion, creatine supplementation may increase mean power output during repeated sprint tests, although the absence of corresponding effects on peak power and fatigue means that more research, with measurements of intramuscular creatine content, is necessary to confirm.

Morning versus Evening Intake of Creatine in Elite Female Handball Players

A great deal of evidence has been gathered on the use of creatine as an ergogenic supplement. Recent studies show greater benefits when creatine ingestion is performed close in time to training, but few studies tackle the way that circadian rhythms could influence creatine consumption. The aim of this study was therefore to observe the influence circadian rhythms exert on sports performance after creatine supplementation. Our method involved randomly assigning fourteen women players of a handball team into two groups in a single-blind study: one that consumed the supplement in the morning and one that consumed it in the evening, with both groups following a specific training program. After twelve weeks, the participants exhibited a decreased fat percentage, increased body weight and body water, and improved performance, with these results being very similar in the two groups. It is therefore concluded that, although circadian rhythms may influence performance, these appear not to affect creatine supplementation, as creatine is stored intramuscularly and is available for those moments of high energy demand, regardless of the time of day.

Effects of Low Doses of L-Carnitine Tartrate and Lipid Multi-Particulate Formulated Creatine Monohydrate on Muscle Protein Synthesis in Myoblasts and Bioavailability in Humans and Rodents

The primary objective of this study was to investigate the potential synergy between low doses of L-carnitine tartrate and creatine monohydrate to induce muscle protein synthesis and anabolic pathway activation in primary human myoblasts. In addition, the effects of Lipid multi-particulates (LMP) formulation on creatine stability and bioavailability were assessed in rodents and healthy human subjects. When used individually, L-carnitine tartrate at 50 µM and creatine monohydrate at 0.5 µM did not affect myoblast protein synthesis and signaling. However, when combined, they led to a significant increase in protein synthesis. Increased AKT and RPS6 phosphorylation were observed with 50 µM L-carnitine tartrate 5 µM creatine in combination in primary human myoblasts. When Wistar rats were administered creatine with LMP formulation at either 21 or 51 mg/kg, bioavailability was increased by 27% based on the increase in the area under the curve (AUC) at a 51 mg/kg dose compared to without LMP formulation. Tmax and Cmax were unchanged. Finally, in human subjects, a combination of LMP formulated L-carnitine at 500 mg (from L-carnitine tartrate) with LMP formulated creatine at 100, 200, or 500 mg revealed a significant and dose-dependent increase in plasma creatine concentrations. Serum total L-carnitine levels rose in a similar manner in the three combinations. These results suggest that a combination of low doses of L-carnitine tartrate and creatine monohydrate may lead to a significant and synergistic enhancement of muscle protein synthesis and activation of anabolic signaling. In addition, the LMP formulation of creatine improved its bioavailability. L-carnitine at 500 mg and LMP-formulated creatine at 200 or 500 mg may be useful for future clinical trials to evaluate the effects on muscle protein synthesis.

Creatine Supplementation: An Update

ABSTRACT

Creatine is a popular and widely used ergogenic dietary supplement among athletes, for which studies have consistently shown increased lean muscle mass and exercise capacity when used with short-duration, high-intensity exercise. In addition to strength gains, research has shown that creatine supplementation may provide additional benefits including enhanced postexercise recovery, injury prevention, rehabilitation, as well as a number of potential neurologic benefits that may be relevant to sports. Studies show that short- and long-term supplementation is safe and well tolerated in healthy individuals and in a number of patient populations.

The Application of Creatine Supplementation in Medical Rehabilitation

Numerous health conditions affecting the musculoskeletal, cardiopulmonary, and nervous systems can result in physical dysfunction, impaired performance, muscle weakness, and disuse-induced atrophy. Due to its well-documented anabolic potential, creatine monohydrate has been investigated as a supplemental agent to mitigate the loss of muscle mass and function in a variety of acute and chronic conditions. A review of the literature was conducted to assess the current state of knowledge regarding the effects of creatine supplementation on rehabilitation from immobilization and injury, neurodegenerative diseases, cardiopulmonary disease, and other muscular disorders. Several of the findings are encouraging, showcasing creatine's potential efficacy as a supplemental agent via preservation of muscle mass, strength, and physical function; however, the results are not consistent. For multiple diseases, only a few creatine studies with small sample sizes have been published, making it difficult to draw definitive conclusions. Rationale for discordant findings is further complicated by differences in disease pathologies, intervention protocols, creatine dosing and duration, and patient population. While creatine supplementation demonstrates promise as a therapeutic aid, more research is needed to fill gaps in knowledge within medical rehabilitation.

The Role of Creatine in the Development and Activation of Immune Responses

The use of dietary supplements has become increasingly common over the past 20 years. Whereas supplements were formerly used mainly by elite athletes, age and fitness status no longer dictates who uses these substances. Indeed, many nutritional supplements are recommended by health care professionals to their patients. Creatine (CR) is a widely used dietary supplement that has been well-studied for its effects on performance and health. CR also aids in recovery from strenuous bouts of exercise by reducing inflammation. Although CR is considered to be very safe in recommended doses, a caveat is that a preponderance of the studies have focused upon young athletic individuals; thus there is limited knowledge regarding the effects of CR on children or the elderly. In this review, we examine the potential of CR to impact the host outside of the musculoskeletal system, specifically, the immune system, and discuss the available data demonstrating that CR can impact both innate and adaptive immune responses, together with how the effects on the immune system might be exploited to enhance human health.

The Effect of Creatine Supplementation on Muscle Function in Childhood Myositis: A Randomized, Double-blind, Placebo-controlled Feasibility Study

OBJECTIVE

To evaluate the feasibility of studying creatine in juvenile dermatomyositis (JDM). Secondary objectives were to determine the effect of creatine on muscle function and metabolism, aerobic capacity, fatigue, physical activity, and quality of life (QOL), as well as its safety.

METHODS

We conducted a 6-month, double-blind, randomized, multiple-baseline design; patients were assigned to creatine or placebo. Feasibility was assessed using attended study visits, completed study procedures, and adherence. Muscle function, aerobic capacity, and muscle strength were assessed with standardized exercise tests. Muscle metabolism was assessed using a 31-Phosphorus Magnetic Resonance Spectroscopy protocol. Fatigue, physical activity, and QOL were assessed by questionnaires. Statistical significance was estimated using a randomization (permutation) test. Changes in outcome measures taken at baseline and end-of-study were calculated using paired t-tests.

RESULTS

Median (range) adherence to the study drug was 88.5% (20.5-95.5%) and the proportion of subjects with 80% adherence or higher was 76.9%. There were no missed study visits. There were no statistically significant changes in muscle function, strength, aerobic capacity, disease activity, fatigue, physical activity, or QOL while subjects were receiving creatine compared to placebo. There were statistically significant adaptations in muscle metabolism (e.g., decrease in change in muscle pH following exercise, and decrease in phosphate/phosphocreatine ratio) at the end-of-study compared to baseline. There were no significant adverse effects.

CONCLUSION

Creatine supplementation in children with JDM is feasible to study, and is safe and well-tolerated; it may lead to improvements in muscle metabolism.

Creatine-electrolyte supplementation improves repeated sprint cycling performance: A double blind randomized control study

Background

Creatine supplementation is recommended as an ergogenic aid to improve repeated sprint cycling performance. Furthermore, creatine uptake is increased in the presence of electrolytes. Prior research examining the effect of a creatine-electrolyte (CE) supplement on repeated sprint cycling performance, however, did not show post-supplementation improvement. The purpose of this double blind randomized control study was to investigate the effect of a six-week CE supplementation intervention on overall and repeated peak and mean power output during repeated cycling sprints with recovery periods of 2 min between sprints.

Methods

Peak and mean power generated by 23 male recreational cyclists (CE group: n = 12; 24.0 ± 4.2 years; placebo (P) group: n = 11; 23.3 ± 3.1 years) were measured on a Velotron ergometer as they completed five 15-s cycling sprints, with 2 min of recovery between sprints, pre- and post-supplementation. Mixed-model ANOVAs were used for statistical analyses.

Results

A supplement-time interaction showed a 4% increase in overall peak power (pre: 734 ± 75 W; post: 765 ± 71 W; p = 0.040; η_p² = 0.187) and a 5% increase in overall mean power (pre: 586 ± 72 W; post: 615 ± 74 W; p = 0.019; η_p² = 0.234) from pre- to post-supplementation for the CE group. For the P group, no differences were observed in overall peak (pre: 768 ± 95 W; post: 772 ± 108 W; p = 0.735) and overall mean power (pre: 638 ± 77 W; post: 643 ± 92 W; p = 0.435) from pre- to post-testing. For repeated sprint analysis, peak (pre: 737 ± 88 W; post: 767 ± 92 W; p = 0.002; η_p² = 0.380) and mean (pre: 650 ± 92 W; post: 694 ± 87 W; p < 0.001; η_p² = 0.578) power output were significantly increased only in the first sprint effort in CE group from pre- to post-supplementation testing. For the P group, no differences were observed for repeated sprint performance.

Conclusion

A CE supplement improves overall and repeated short duration sprint cycling performance when sprints are interspersed with adequate recovery periods.

Beyond muscles: The untapped potential of creatine

Creatine is widely used by both elite and recreational athletes as an ergogenic aid to enhance anaerobic exercise performance. Older individuals also use creatine to prevent sarcopenia and, accordingly, may have therapeutic benefits for muscle wasting diseases. Although the effect of creatine on the musculoskeletal system has been extensively studied, less attention has been paid to its potential effects on other physiological systems. Because there is a significant pool of creatine in the brain, the utility of creatine supplementation has been examined in vitro as well as in vivo in both animal models of neurological disorders and in humans. While the data are preliminary, there is evidence to suggest that individuals with certain neurological conditions may benefit from exogenous creatine supplementation if treatment protocols can be optimized. A small number of studies that have examined the impact of creatine on the immune system have shown an alteration in soluble mediator production and the expression of molecules involved in recognizing infections, specifically toll-like receptors. Future investigations evaluating the total impact of creatine supplementation are required to better understand the benefits and risks of creatine use, particularly since there is increasing evidence that creatine may have a regulatory impact on the immune system.

Effects of amino acid derivatives on physical, mental, and physiological activities

Nutritional ergogenic aids have been in use for a long time to enhance exercise and sports performance. Dietary components that exhibit ergogenic activity are numerous and their consumption is common and popular among athletes. They often come under scrutiny by legal authorities for their claimed benefits and safety concerns. Amino acid derivatives are propagated as being effective aids to enhance physical and mental performance in many ways, even though studies have pointed out that individuals who are deficient are more likely to benefit from dietary supplementation of amino acid derivatives than normal humans. In this review, some of the most common and widely used amino acids derivatives in sports and athletics namely creatine, tyrosine, carnitine, HMB, and taurine have been discussed for their effects on exercise performance, mental activity as well as body strength and composition. Creatine, carnitine, HMB, and taurine are reported to delay the onset of fatigue, improve exercise performance, and body strength. HMB helps in increasing fat-free mass and reduce exercise induced muscle injury. Taurine has been found to reduce oxidative stress during exercise and also act as an antihypertensive agent. Although, studies have not been able to find any favorable effect of tyrosine administration on exercise performance, it has been proved to be very effective in fighting stress, improving mood and cognitive performance particularly in sleep-deprived subjects. While available data from published studies and findings are equivocal about the efficacy of creatine, tyrosine, and HMB, more comprehensive researches on carnitine and taurine are necessary to provide evidence for the theoretical basis of their ergogenic role in nutritional modification and supplementation.

Creatina e treinamento resistido: efeito na hidratação e massa corporal magra

INTRODUÇÃO: a creatina é um recurso ergogênico cuja suplementação tem sido associada ao aumento da hidratação corporal total e ao aumento da massa muscular dos consumidores. Entretanto, estudiosos questionam se o aumento da massa muscular é um ganho real.OBJETIVO: avaliar o efeito da suplementação de creatina sobre a hidratação e o aumento de massa magra em indivíduos previamente treinados e não treinados, submetidos a um programa de treinamento resistido.MÉTODOS: ensaio clínico não randomizado, constituído por três momentos, M1 - Início da suplementação com 20g/dia de creatina; M2 - 7 dias após iniciada a suplementação e redução da suplementação para 5g/dia; M3 - 28 dias de suplementação. Nos momentos propostos, foram realizadas aferições de peso, estatura e avaliação da composição corporal (massa magra, água corporal total) com a utilização do BYODINAMICS(r) Modelo 310. Para todos os testes estatísticos, foi adotado o nível de significância de 95% (p<0,05).RESULTADOS: participaram desse estudo 14 voluntários adultos do sexo masculino, com idade média de 22,57(±1,45) anos, dos quais sete eram treinados e sete não treinados. Após 28 dias de suplementação, no grupo treinado observou-se um aumento significativo no peso, água corporal total, massa magra e hidratação da massa magra, mas nenhum aumento significativo foi observado no grupo não treinado. Em relação ao ângulo de fase, este aumentou no grupo não treinado e reduziu no grupo treinado.CONCLUSÃO: a suplementação de creatina associada ao treinamento resistido é mais efetiva na hidratação de indivíduos treinados, como também é suficiente para reduzir a diferença significativa do ângulo de fase intergrupos, sugerindo assim, maior hidratação celular em ambos os grupos. Contudo, esse aumento na hidratação não revelou aumento significativo no tecido muscular.

Effects of creatine monohydrate supplementation on simulated soccer performance

PURPOSE

To determine the effects of acute short-term creatine (Cr) supplementation on physical performance during a 90-min soccer-specific performance test.

METHODS

A double-blind, placebo-controlled experimental design was adopted during which 16 male amateur soccer players were required to consume 20 g/d Cr for 7 d or a placebo. A Ball-Sport Endurance and Speed Test (BEAST) comprising measures of aerobic (circuit time), speed (12- and 20-m sprint), and explosive-power (vertical jump) abilities performed over 90 min was performed presupplementation and postsupplementation.

RESULTS

Performance measures during the BEAST deteriorated during the second half relative to the first for both Cr (1.2-2.3%) and placebo (1.0-2.2%) groups, indicating a fatigue effect associated with the BEAST. However, no significant differences existed between groups, suggesting that Cr had no performance-enhancing effect or ability to offset fatigue. When effect sizes were considered, some measures (12-m sprint, -0.53 ± 0.69; 20-m sprint, -0.39 ± 0.59) showed a negative tendency, indicating chances of harm were greater than chances of benefit.

CONCLUSIONS

Acute short-term Cr supplementation has no beneficial effect on physical measures obtained during a 90-min soccer-simulation test, thus bringing into question its potential as an effective ergogenic aid for soccer players.

Effect of creatine supplementation on muscle capacity in individuals with multiple sclerosis

There has been interest in the use of exogenous creatine (Cr) as an adjunct treatment for neurological disorders. Creatine enhances bouts of activity through augmenting phosphocreatine for increased synthesis of ATP; however, multiple sclerosis (MS) individuals suffering from muscle weakness have not been shown to improve muscle work after 5 days of Cr supplementation. This study's purpose was to determine whether the extended duration would increase muscle capacity. In a double-blind, crossover trial, with a 3-week washout period, eleven MS subjects were randomly assigned to either Cr (5 g 4/day, day 1-7: 2.5 g 2/day, day 8-14) or placebo groups for two 14-day periods. Biodex Dynamometer recorded total work and power over three bouts of 30 maximal knee extensions and flexions. Total work was nonsignificant with Cr for knee extension (pretest 1277.7 ± 214.9 J vs. posttest = 1313.14 ± 200.5 J; p = 0.81) and flexion (pretest = 1220.7 ± 200.5 J vs. posttest = 1302.10 J ± 189.64 J; p = 0.93). Creatine did not enhance muscle power in knee extension (p = 0.31; pretest = 82.1 ± 12.7 W vs. posttest = 87.7 ± 12.6 W) or flexion (p = 0.29; pretest = 75.3 ± 12.1 W vs. posttest = 81.2 ± 11.1 W). Therefore, 14 days of Cr supplementation did not improve muscle capacity or habitual fatigue in MS individuals.

Creatine supplementation does not decrease oxidative stress and inflammation in skeletal muscle after eccentric exercise

Thirty-six male rats were used; divided into 6 groups (n = 6): saline; creatine (Cr); eccentric exercise (EE) plus saline 24 h (saline + 24 h); eccentric exercise plus Cr 24 h (Cr + 24 h); eccentric exercise plus saline 48 h (saline + 48 h); and eccentric exercise plus Cr 48 h (Cr + 48 h). Cr supplementation was administered as a solution of 300 mg · kg body weight(-1) · day(-1) in 1 mL water, for two weeks, before the eccentric exercise. The animals were submitted to one downhill run session at 1.0 km · h(-1) until exhaustion. Twenty-four and forty-eight hours after the exercise, the animals were killed, and the quadriceps were removed. Creatine kinase levels, superoxide production, thiobarbituric acid reactive substances (TBARS) level, carbonyl content, total thiol content, superoxide dismutase, catalase, glutathione peroxidase, interleukin-1b (IL-1β), nuclear factor kappa B (NF-kb), and tumour necrosis factor (TNF) were analysed. Cr supplementation neither decreases Cr kinase, superoxide production, lipoperoxidation, carbonylation, total thiol, IL-1β, NF-kb, or TNF nor alters the enzyme activity of superoxide dismutase, catalase, and glutathione peroxides in relation to the saline group, respectively (P < 0.05). There are positive correlations between Cr kinase and TBARS and TNF-α 48 hours after eccentric exercise. The present study suggests that Cr supplementation does not decrease oxidative stress and inflammation after eccentric contraction.

Effects of dietary protein content on renal parameters in normal cats

This study evaluates the effect of dietary protein content on renal parameters in 23 healthy spayed female cats. The objective was to determine if cats eating diets high in protein will have higher serum urea nitrogen (UN) and creatinine values without a detectable change in kidney function, as assessed by urinalysis. A single random cross-over design was used. Cats were fed a standard maintenance diet for at least 1 month prior to the dietary trial. They were fed in two phases. For the first phase, cats were randomly assigned to receive either a high protein [HP=46% metabolizable energy (ME)] or low protein (LP=26% ME) diet. For the second phase, cats were fed whichever diet they were not fed during the phase I period. Blood and urine samples were collected at 2-week intervals for the duration of the study (10 weeks). UN, albumin, alanine aminotransferase and urine specific gravity were significantly higher, and creatinine and phosphorus were significantly lower (P<0.05) when cats were fed the HP diet as compared to when they were fed the LP diet, although none of the mean values were found to be outside of the corresponding reference interval. Dietary intake can result in clinically significant changes in UN and statistically significantly changes in several other biochemical analytes, although all analytes are likely to remain within normal reference intervals. Therefore, an accurate dietary history is necessary to help determine if renal parameters are being influenced by diet in a particular patient.

Analysis of the efficacy, safety, and regulatory status of novel forms of creatine

Creatine has become one of the most popular dietary supplements in the sports nutrition market. The form of creatine that has been most extensively studied and commonly used in dietary supplements is creatine monohydrate (CM). Studies have consistently indicated that CM supplementation increases muscle creatine and phosphocreatine concentrations by approximately 15–40%, enhances anaerobic exercise capacity, and increases training volume leading to greater gains in strength, power, and muscle mass. A number of potential therapeutic benefits have also been suggested in various clinical populations. Studies have indicated that CM is not degraded during normal digestion and that nearly 99% of orally ingested CM is either taken up by muscle or excreted in urine. Further, no medically significant side effects have been reported in literature. Nevertheless, supplement manufacturers have continually introduced newer forms of creatine into the marketplace. These newer forms have been purported to have better physical and chemical properties, bioavailability, efficacy, and/or safety profiles than CM. However, there is little to no evidence that any of the newer forms of creatine are more effective and/or safer than CM whether ingested alone and/or in combination with other nutrients. In addition, whereas the safety, efficacy, and regulatory status of CM is clearly defined in almost all global markets; the safety, efficacy, and regulatory status of other forms of creatine present in today’s marketplace as a dietary or food supplement is less clear.

The effects of supplementation with creatine and protein on muscle strength following a traditional resistance training program in middle-aged and older men

OBJECTIVES

Creatine and protein supplementation can enhance the training outcomes of young subjects, but it is not clear if there are benefits for older individuals. Therefore, the purpose of this study was to determine the effects of creatine and protein supplementation on strength gains following a traditional resistance training program for middle-aged and older men.

DESIGN, SETTING, PARTICIPANTS

This study assessed changes in strength of men aged 48-72 years following 14 weeks of resistance training supplemented with creatine and/or protein. A double-blind, randomized, placebo-controlled design placed 42 males into one of four groups: Resistance Trained Placebo (RTP, n=10); Resistance Trained Creatine (RTCr, 5g Cr, n=10); Resistance Trained Protein (RTPr, 35g whey Pr, n=11); or Resistance Trained Creatine and Protein (RTCrPr, 5g Cr and 35g Pr, n=11).

INTERVENTION

All groups trained 3 days per week for 14 weeks. The resistance training program was based on progressive overload. Training loads corresponded to 80% 1 RM (one repetition maximum strength), 3 sets of 8 repetitions for the following exercises: knee extension/knee flexion; bicep curl/tricep extension; military press; lat pull down; seated leg press; and bench press.

MEASUREMENTS

1 RM for each exercise and measures of lean body mass were assessed prior to and following the 14 week program.

RESULTS

Each group significantly (p < 0.05) increased strength and lean body mass, however, there were no significant group effects or group X trial interactions.

CONCLUSION

Resistance training in middle-aged and older men significantly increased muscular strength and added muscle mass with no additional benefits from creatine and/or protein supplementation.

1H NMR metabolomics study of age profiling in children

Metabolic profiling of urine provides a fingerprint of personalized endogenous metabolite markers that correlate to a number of factors such as gender, disease, diet, toxicity, medication, and age. It is important to study these factors individually, if possible to unravel their unique contributions. In this study, age-related metabolic changes in children of age 12 years and below were analyzed by (1)H NMR spectroscopy of urine. The effect of age on the urinary metabolite profile was observed as a distinct age-dependent clustering even from the unsupervised principal component analysis. Further analysis, using partial least squares with orthogonal signal correction regression with respect to age, resulted in the identification of an age-related metabolic profile. Metabolites that correlated with age included creatinine, creatine, glycine, betaine/TMAO, citrate, succinate, and acetone. Although creatinine increased with age, all the other metabolites decreased. These results may be potentially useful in assessing the biological age (as opposed to chronological) of young humans as well as in providing a deeper understanding of the confounding factors in the application of metabolomics.

The effects of creatine ethyl ester supplementation combined with heavy resistance training on body composition, muscle performance, and serum and muscle creatine levels

Numerous creatine formulations have been developed primarily to maximize creatine absorption. Creatine ethyl ester is alleged to increase creatine bio-availability. This study examined how a seven-week supplementation regimen combined with resistance training affected body composition, muscle mass, muscle strength and power, serum and muscle creatine levels, and serum creatinine levels in 30 non-resistance-trained males. In a double-blind manner, participants were randomly assigned to a maltodextrose placebo (PLA), creatine monohydrate (CRT), or creatine ethyl ester (CEE) group. The supplements were orally ingested at a dose of 0.30 g/kg fat-free body mass (approximately 20 g/day) for five days followed by ingestion at 0.075 g/kg fat free mass (approximately 5 g/day) for 42 days. Results showed significantly higher serum creatine concentrations in PLA (p = 0.007) and CRT (p = 0.005) compared to CEE. Serum creatinine was greater in CEE compared to the PLA (p = 0.001) and CRT (p = 0.001) and increased at days 6, 27, and 48. Total muscle creatine content was significantly higher in CRT (p = 0.026) and CEE (p = 0.041) compared to PLA, with no differences between CRT and CEE. Significant changes over time were observed for body composition, body water, muscle strength and power variables, but no significant differences were observed between groups. In conclusion, when compared to creatine monohydrate, creatine ethyl ester was not as effective at increasing serum and muscle creatine levels or in improving body composition, muscle mass, strength, and power. Therefore, the improvements in these variables can most likely be attributed to the training protocol itself, rather than the supplementation regimen.

Gender-related and age-related urinalysis of healthy subjects by NMR-based metabonomics

NMR-based metabonomic analysis is a well-established approach to characterizing healthy and diseased states. The aim of this study was to investigate inter-individual variability in the metabolic urinary profile of a healthy Greek population, not subjected to strict dietary limitations, by NMR-based metabonomics. The overall metabonomic urinalysis showed a homogeneous distribution among the population. The metabolic profile was examined in relation to gender and age, and reference intervals of major metabolites were determined. Multivariate data analysis led to the construction of two robust models that were able to predict the class membership of the subjects studied according to their gender and age. The most influential low molecular weight metabolites responsible for the differences in gender groups were citrate, creatinine, trimethylamine N-oxide, glycine, creatine and taurine, and for the differences in age groups they were citrate, creatinine, trimethylamine N-oxide and an unidentified metabolite (delta 3.78).

Creatine supplementation attenuates corticosteroid-induced muscle wasting and impairment of exercise performance in rats

The objective of the present study was to investigate whether creatine (Cr) could attenuate the deleterious effects of high doses of dexamethasone (Dexa) on body mass, exercise performance, and respiratory variables of rodents. Forty-four Wistar rats performed incremental maximal exercise tests. They were then assigned to four groups: G1: subcutaneous (SC) and intraperitoneal (IP) saline; G2: SC saline and IP Cr (250 mg·kg−1·day−1); G3: SC Dexa (7.5 mg·kg−1·day−1) and IP saline; G4: SC Dexa and IP Cr. New exercise tests and analysis of the respiratory pattern under resting conditions and after stimulation with doxapram (2 mg/kg IP) were performed after 18 days. Post- minus pretreatment differences were compared between groups. G3 and G4 showed a significant impairment in body mass gain compared with G1 and G2 ( P < 0.05) (G1: 65.3 ± 26.1, G2: 93.1 ± 27.4, G3: −18.4 ± 20.1, G4: 9.8 ± 23.1 kg × 10−3). Similar results were observed for maximal oxygen consumption (G1: 9.5 ± 8.5, G2: 25.8 ± 14.5, G3: −25.5 ± 6.0, G4: −4.8 ± 9.5 ml·kg−1·min−1) and test duration (G1: 43.0 ± 45.0, G2: 72.0 ± 59.5, G3: −165.0 ± 60.6, G4: −48.0 ± 48.5 s). Simultaneous use of Cr significantly attenuated the Dexa-induced impairment of the last two variables. Cr attenuated Dexa-induced gastrocnemius and diaphragm muscle weight losses and the atrophy of gastrocnemius type IIb fibers. Cr supplementation had only small effects on Dexa-induced respiratory changes. These results suggest that Cr may play a role in the prophylaxis or treatment of steroid-induced myopathy.

Influência da suplementação aguda e crônica de creatina sobre as concentrações sanguíneas de glicose e lactato de ratos Wistar

Estudos recentes sugerem que a suplementação de creatina pode interferir com a captação de glicose e a produção de lactato durante a atividade física. O objetivo deste estudo foi investigar os efeitos da suplementação aguda (5g.kg¹ durante uma semana) e crônica (1g.kg¹ durante oito semanas) de creatina sobre as concentrações sanguíneas de glicose e lactato de ratos sedentários e exercitados (natação a 80% da carga máxima tolerada). Setenta e dois ratos Wistar machos (240 ± 10g) foram utilizados e divididos igualmente em quatro grupos experimentais (n = 18): CON - ratos sedentários não suplementados; NAT - ratos exercitados não suplementados; CRE - ratos sedentários e suplementados; CRE + NAT - ratos exercitados e suplementados. As amostras sanguíneas foram obtidas antes e após o teste de determinação da carga máxima realizado semanalmente durante todo o experimento. Antes do teste de carga máxima, com exceção do grupo CRE-NAT (3-5 semanas), que apresentou concentrações plasmáticas de glicose inferiores em relação os demais grupos, todos os outros resultados foram semelhantes entre os grupos experimentais. Após o teste de carga máxima todos os grupos experimentais apresentaram redução das concentrações plasmáticas de glicose e aumento das concentrações plasmáticas de lactato. Contudo, em relação à glicose, esta redução foi significativamente (p < 0,05) pronunciada nos grupos CRE (1-4 semanas) e CRE + NAT (1-8 semanas) e, em relação ao lactato, o aumento foi significativamente (p < 0,05) menor nos grupos CRE (1-2 semanas) e CRE + NAT (1-8 semanas). Os achados deste estudo sugerem que o regime adotado de suplementação influenciou o perfil metabólico glicêmico, minimizou o acúmulo de lactato e potencializou a máxima carga suportada nos animais suplementados.

Dietas vegetarianas e desempenho esportivo

As evidências atuais apontam benefícios da dieta vegetariana para a saúde humana. Contudo, a partir da adoção de práticas vegetarianas mais restritivas, confirmam-se os riscos à saúde. As dietas vegetarianas são caracterizadas pelo elevado consumo de carboidratos, fibras, magnésio, potássio, folato e antioxidantes, podendo apresentar deficiências em aminoácidos e ácidos graxos essenciais, cálcio, zinco, ferro e cobalamina. Pesquisas experimentais em humanos indicam que vegetarianos e não-vegetarianos apresentam capacidade aeróbica semelhante. Em relação ao desempenho em atividades de força e potência muscular, as pesquisas são escassas, mas as existentes não apontam diferenças significativas. Situações de risco cardiovascular têm sido confirmadas, devido ao provável quadro de hiperhomocisteinemia, em decorrência da baixa ingestão de cobalamina. As dietas vegetarianas são isentas de creatina, o que resulta em estoques musculares mais baixos nessa população. Possivelmente ocorrem alterações hormonais e metabólicas em resposta às dietas vegetarianas, como baixos níveis de testosterona e androstenediona. A função imune parece não ser prejudicada. Dessa forma, a prática de dietas vegetarianas apresenta-se compatível com a prática esportiva cotidiana, desde que bem planejada para evitar deficiências nutricionais.

Stimulation of skin's energy metabolism provides multiple benefits for mature human skin

As an organism ages, there is a decline in mitochondrial function and cellular energy balance. This decline is both accelerated by and can cause the formation of reactive oxygen species (ROS) that damage nuclear and mitochondrial DNA, lipid membranes as well as structural and catalytic proteins, especially those involved in energetic pathways of cells. Further, ROS have also been linked to some of the detrimental skin changes that occur as a result of photoaging. We have previously shown that levels of Coenzyme Q10 (CoQ10), a component of the respiratory chain in mitochondria, are reduced in skin cells from aging donors, and that topical supplementation can ameliorate processes involved in skin aging. Creatine is another important component of the cellular energy system and phosphocreatine, its phosphorylated form, functions as a reservoir for high energy phosphates. Unfortunately the creatine system and thus the energy storage mechanism in skin are negatively affected by aging and conditions of oxidative stress. This article reviews some of our in vivo data about the synergistic effects of combining a stabilized form of Creatine with CoQ10 and clearly depicts their beneficial effects as active ingredients in topical formulations.

Effects of creatine supplementation on the performance and body composition of competitive swimmers

The objective of this study was to determine the effect of creatine supplementation on performance and body composition of swimmers. Eighteen swimmers were evaluated in terms of post-performance lactate accumulation, body composition, creatine and creatinine excretion, and serum creatinine concentrations before and after creatine or placebo supplementation. No significant differences were observed in the marks obtained in swimming tests after supplementation, although lactate concentrations were higher in placebo group during this period. In the creatine-supplemented group, urinary creatine, creatinine, and body mass, lean mass and body water were significantly increased, but no significant difference in muscle or bone mass was observed. These results suggest that creatine supplementation cannot be considered to be an ergogenic supplement ensuring improved performance and muscle mass gain in swimmers.

Effects of creatine supplementation in cystic fibrosis: results of a pilot study

BACKGROUND

The CF transmembrane conductance regulator (CFTR), whose mutations cause cystic fibrosis (CF), depends on ATP for activation and transport function. Availability of ATP in the cell and even more in specific cellular microcompartments often depends on a functional creatine kinase system, which provides the 'energy buffer' phosphocreatine. Creatine supplementation has been shown to increase phosphocreatine levels, thus promoting muscle growth and strength in athletes and having protective effects in neuromuscular disorders.

AIM

To test clinically, if creatine supplementation improves maximal isometric muscle strength (MIMS), lung function and CFTR channel activity in patients with CF, and to determine enzymatic activity of creatine kinase in respiratory epithelial cells.

METHODS

In an open-label pilot study 18 CF patients (8-18-year-old) with pancreatic insufficiency and mild to moderate lung disease received daily creatine supplementation during 12 weeks. Patients were monitored during 24-36 weeks. Enzymatic activity of creatine kinase was measured in primary epithelial cell cultures.

RESULTS

After creatine supplementation, there was no change in lung function and sweat electrolyte concentrations, possibly due to the very low creatine kinase activities detected in respiratory epithelia. However, the patients consistently showed significantly increased MIMS (18.4%; P < 0.0001), as well as improved general well-being, as assessed by a standardized questionnaire. Except for one patient with transient muscle pain, no side effects were reported.

CONCLUSIONS

Our pilot study suggests, that creatine supplementation should be further evaluated as a possible clinically beneficial adjuvant therapy for patients with CF to increase muscle strength, body-weight and well-being.

Oral creatine supplementation and skeletal muscle metabolism in physical exercise

Creatine is the object of growing interest in the scientific literature. This is because of the widespread use of creatine by athletes, on the one hand, and to some promising results regarding its therapeutic potential in neuromuscular disease on the other. In fact, since the late 1900s, many studies have examined the effects of creatine supplementation on exercise performance. This article reviews the literature on creatine supplementation as an ergogenic aid, including some basic aspects relating to its metabolism, pharmacokinetics and side effects. The use of creatine supplements to increase muscle creatine content above approximately 20 mmol/kg dry muscle mass leads to improvements in high-intensity, intermittent high-intensity and even endurance exercise (mainly in nonweightbearing endurance activities). An effective supplementation scheme is a dosage of 20 g/day for 4-6 days, and 5 g/day thereafter. Based on recent pharmacokinetic data, new regimens of creatine supplementation could be used. Although there are opinion statements suggesting that creatine supplementation may be implicated in carcinogenesis, data to prove this effect are lacking, and indeed, several studies showing anticarcinogenic effects of creatine and its analogues have been published. There is a shortage of scientific evidence concerning the adverse effects following creatine supplementation in healthy individuals even with long-term dosage. Therefore, creatine may be considered as a widespread, effective and safe ergogenic aid.

Caffeine is ergogenic after supplementation of oral creatine monohydrate

PURPOSE

The purpose of this investigation was to assess the acute effects of caffeine ingestion on short-term, high-intensity exercise (ST) after a period of oral creatine supplementation and caffeine abstinence.

METHODS

Fourteen trained male subjects performed treadmill running to volitional exhaustion (T(lim)) at an exercise intensity equivalent to 125% VO(2max). Three trials were performed, one before 6 d of creatine loading (0.3 g x kg x d(-1) baseline), and two further trials after the loading period. One hour before the postloading trials, caffeine (5 mg x kg(-1)) or placebo was orally ingested in a cross-over, double-blind fashion. Four measurements of rating of perceived exertion were taken, one every 30 s, during the first 120 s of the exercise. Blood samples were assayed for lactate, glucose, potassium, and catecholamines, immediately before and after exercise.

RESULTS

Body mass increased (P < 0.05) over the creatine supplementation period, and this increase was maintained for both caffeine and placebo trials. There was no increase in the maximal accumulated oxygen deficit between trials; however, total VO(2) was significantly increased in the caffeine trial in comparison with the placebo trial (13.35 +/- 3.89 L vs 11.67 +/- 3.61 L). In addition, caffeine T(lim) (222.1 +/- 48.9 s) was significantly greater (P < 0.05) than both baseline (200.8 +/- 33.4 s) and placebo (198.3 +/- 45.4 s) T(lim). RPE was also lower at 90 s in the caffeine treatment (13.8 +/- 1.8 RPE points) in comparison with baseline (14.6 +/- 1.9 RPE points).

CONCLUSION

As indicated by a greater T(lim), acute caffeine ingestion was found to be ergogenic after 6-d of creatine supplementation and caffeine abstinence.

Creatine supplementation reduces skeletal muscle degeneration and enhances mitochondrial function in mdx mice

The mdx mouse serves as animal model for Duchenne muscular dystrophy. Energy status in muscles of mdx mice is impaired and we have demonstrated recently that the energy precursor creatine exerts beneficial effects on mdx skeletal muscle cells in culture. Here we show that feeding a creatine-enriched diet to new-born mdx mice strongly reduced the first wave of muscle necrosis four weeks after birth. Necrosis of the fast-twitch muscle extensor digitorum longus was inhibited by 63+/-14% (P<0.0001) while necrosis of the slow-twitch soleus muscle was not significantly decreased. In addition, using chemically skinned muscle fibres, we found that mitochondrial respiration capacity was decreased by about 25% in mdx-derived fibres and that long-term creatine-feeding restored respiration to wild-type levels. These results provide evidence that creatine supplementation in mdx mice improves muscle health and may provide a scientific basis for its use as adjuvant therapy in Duchenne muscular dystrophy.

A creatina como suplemento ergogênico para atletas

A creatina vem sendo muito pesquisada devido ao seu potencial efeito no rendimento físico de atletas envolvidos em exercícios de alta intensidade e curta duração, intermitentes e com curtos períodos de recuperação. A creatina fosforilada é uma reserva de energia nas células musculares. Durante um exercício intenso, a sua quebra libera energia é usada para regenerar o trifosfato de adenosina. Aproximadamente 95% do pool de creatina encontra-se na musculatura esquelética e sua regeneração após o exercício é um processo dependente de oxigênio. Estudos mostram que a suplementação com este composto pode aumentar o pool orgânico em 10 a 20%, e este percentual é maior em atletas vegetarianos (até 60%). Ainda existe controvérsia com relação aos benefícios e riscos da suplementação com esta substância. Este estudo revisa alguns dos aspectos relacionados com o metabolismo da creatina e seu uso como substância ergogênica na prática desportiva.

Evaluation of creatine transport using Caco-2 monolayers as an in vitro model for intestinal absorption

Creatine is a nutraceutical that has gained popularity in both well-trained and casual athletes for its performance-enhancing or ergogenic properties. The major disadvantages of creatine monohydrate formulations are poor solubility and oral bioavailability. In the present study, creatine transport was examined using Caco-2 monolayers as an in vitro model for intestinal absorption. Confluent monolayers of Caco-2 cells (passage 25-35) were used for the permeability studies. Monolayers were placed in side-by-side diffusion chambers. (14)C-Creatine (0.1-0.5 microCi/mL) was added to either the apical or basolateral side, and the transport of the creatine across the Caco-2 monolayer was measured over a 90-min period. The apical to basolateral transport of (14)C-creatine was small, ranging from 0.2-3% of the original amount appearing on the receiver side in a 90-min period. Interestingly, the basolateral to apical permeability of radiolabeled creatine was substantially greater than that observed in the apical to basolateral direction. Studies with drug efflux transport inhibitors indicate that neither the P-glycoprotein nor multidrug resistance-associated protein is involved in the enhanced basolateral to apical transport of creatine.

Creatine and creatinine metabolism

The goal of this review is to present a comprehensive survey of the many intriguing facets of creatine (Cr) and creatinine metabolism, encompassing the pathways and regulation of Cr biosynthesis and degradation, species and tissue distribution of the enzymes and metabolites involved, and of the inherent implications for physiology and human pathology. Very recently, a series of new discoveries have been made that are bound to have distinguished implications for bioenergetics, physiology, human pathology, and clinical diagnosis and that suggest that deregulation of the creatine kinase (CK) system is associated with a variety of diseases. Disturbances of the CK system have been observed in muscle, brain, cardiac, and renal diseases as well as in cancer. On the other hand, Cr and Cr analogs such as cyclocreatine were found to have antitumor, antiviral, and antidiabetic effects and to protect tissues from hypoxic, ischemic, neurodegenerative, or muscle damage. Oral Cr ingestion is used in sports as an ergogenic aid, and some data suggest that Cr and creatinine may be precursors of food mutagens and uremic toxins. These findings are discussed in depth, the interrelationships are outlined, and all is put into a broader context to provide a more detailed understanding of the biological functions of Cr and of the CK system.

Protective effect of the energy precursor creatine against toxicity of glutamate and beta-amyloid in rat hippocampal neurons

The loss of ATP, which is needed for ionic homeostasis, is an early event in the neurotoxicity of glutamate and beta-amyloid (A(beta)). We hypothesize that cells supplemented with the precursor creatine make more phosphocreatine (PCr) and create larger energy reserves with consequent neuroprotection against stressors. In serum-free cultures, glutamate at 0.5-1 mM was toxic to embryonic hippocampal neurons. Creatine at >0.1 mM greatly reduced glutamate toxicity. Creatine (1 mM) could be added as late as 2 h after glutamate to achieve protection at 24 h. In association with neurotoxic protection by creatine during the first 4 h, PCr levels remained constant, and PCr/ATP ratios increased. Morphologically, creatine protected against glutamate-induced dendritic pruning. Toxicity in embryonic neurons exposed to A(beta) (25-35) for 48 h was partially prevented by creatine as well. During the first 6 h of treatment with A(beta) plus creatine, the molar ratio of PCr/ATP in neurons increased from 15 to 60. Neurons from adult rats were also partially protected from a 24-h exposure to A(beta) (25-35) by creatine, but protection was reduced in neurons from old animals. These results suggest that fortified energy reserves are able to protect neurons against important cytotoxic agents. The oral availability of creatine may benefit patients with neurodegenerative diseases.

Downregulation of the Na(+)-creatine cotransporter in failing human myocardium and in experimental heart failure

BACKGROUND

The failing myocardium is characterized by depletion of phosphocreatine and of total creatine content. We hypothesized that this is due to loss of creatine transporter protein.

METHODS AND RESULTS

Creatine transporter protein was quantified in nonfailing and failing human myocardium (explanted hearts with dilated cardiomyopathy [DCM; n=8] and healthy donor hearts [n=8]) as well as in experimental heart failure (residual intact left ventricular tissue, rats 2 months after left anterior descending coronary artery ligation [MI; n=8] or sham operation [sham; n=6]) by Western blotting. Total creatine content was determined by high-performance liquid chromatography. Donor and DCM hearts had total creatine contents of 136.4+/-6.1 and 68.7+/-4.6 nmol/mg protein, respectively (*P<0.05); creatine transporter protein was 25.4+/-2.2 optical density units in donor and 17.7+/-2.5 in DCM (*P<0.05). Total creatine was 87.5+/-4.2 nmol/mg protein in sham and 65.7+/-4.2 in MI rats (*P<0.05); creatine transporter protein was 139.0+/-8.7 optical density units in sham and 82.1+/-4.0 in MI (*P<0.05).

CONCLUSIONS

Both in human and in experimental heart failure, creatine transporter protein content is reduced. This mechanism may contribute to the depletion of creatine compounds and thus to the reduced energy reserve in failing myocardium. This finding may have therapeutic implications, suggesting a search for treatment strategies targeted toward creatine transport.

Creatine uptake in isolated soleus muscle: kinetics and dependence on sodium, but not on insulin

The increased use of creatine by athletes as a dietary supplement to improve their physical performance assumes that increased serum creatine levels will increase intracellular skeletal muscle creatine. Despite this common assumption, skeletal muscle creatine uptake awaits full characterization. Consequently, we have investigated 14C-labelled creatine uptake in isolated, incubated rat soleus (type I) muscle preparations at 37 degrees C. We found that the apparent Km for creatine uptake was 73 microM and the Vmax was 77 nmol h-1 gww-1. Creatine uptake was 82% inhibited by 2 mM beta-guanidinopropionic acid, the structural analogue of creatine. In addition, a decrease in buffer Na+ concentration, from 145 to 25 mM, reduced the rate of 14C-labelled creatine uptake by 77%, indicating that uptake is largely Na+-dependent in soleus muscle. Insulin had no effect on the rate of creatine uptake in vitro. The total creatine content was 34% lower, but the rate of creatine uptake in the presence of 100 microM extracellular creatine was 45% higher, in soleus than in extensor digitorum longus (type II) muscle. However, at 1 mM extracellular creatine, the maximal rate of uptake was not significantly different for the two muscle types, implying that soleus muscle has a lower Km for creatine uptake. We suggest that intracellular creatine levels may play a role in the regulation of skeletal muscle creatine uptake.