Does brain creatine content rely on exogenous creatine in healthy youth? A proof-of-principle study

Creatine supplementation research fails to support the theoretical basis for an effect on cognition: Evidence from a systematic review

Creatine supplementation has been put forward as a possible aid to cognition, particularly for vegans, vegetarians, the elderly, sleep deprived and hypoxic individuals. However, previous narrative reviews have only provided limited support for these claims. This is despite the fact that research has shown that creatine supplementation can induce increased brain concentrations of creatine, albeit to a limited extent. We carried out a systematic review to examine the current state of affairs. The review supported claims that creatine supplementation can increases brain creatine content but also demonstrated somewhat equivocal results for effects on cognition. It does, however, provide evidence to suggest that more research is required with stressed populations, as supplementation does appear to significantly affect brain content. Issues with research design, especially supplementation regimens, need to be addressed. Future research must include measurements of creatine brain content.

Protocol for a single-arm, pilot trial of creatine monohydrate supplementation in patients with Alzheimer's disease

BACKGROUND

Impaired brain bioenergetics is a pathological hallmark of Alzheimer's disease (AD) and is a compelling target for AD treatment. Patients with AD exhibit dysfunction in the brain creatine (Cr) system, which is integral in maintaining bioenergetic flux. Recent studies in AD mouse models suggest Cr supplementation improves brain mitochondrial function and may be protective of AD peptide pathology and cognition.

AIMS

The Creatine to Augment Bioenergetics in Alzheimer's disease (CABA) study is designed to primarily assess the feasibility of supplementation with 20 g/day of creatine monohydrate (CrM) in patients with cognitive impairment due to AD. Secondary aims are designed to generate preliminary data investigating changes in brain Cr levels, cognition, peripheral and brain mitochondrial function, and muscle strength and size.

METHODS

CABA is an 8-week, single-arm pilot study that will recruit 20 patients with cognitive impairment due to AD. Participants attend five in-person study visits: two visits at baseline to conduct screening and baseline assessments, a 4-week visit, and two 8-week visits. Outcomes assessment includes recruitment, retention, and compliance, cognitive testing, magnetic resonance spectroscopy of brain metabolites, platelet and lymphocyte mitochondrial function, and muscle strength and morphology at baseline and 8 weeks.

DISCUSSION

CABA is the first study to investigate CrM as a potential treatment in patients with AD. The pilot data generated by this study are pertinent to inform the design of future large-scale efficacy trials.

TRIAL REGISTRATION

ClinicalTrials.gov, NCT05383833 , registered on 20 May 2022.

"Heads Up" for Creatine Supplementation and its Potential Applications for Brain Health and Function

There is emerging interest regarding the potential beneficial effects of creatine supplementation on indices of brain health and function. Creatine supplementation can increase brain creatine stores, which may help explain some of the positive effects on measures of cognition and memory, especially in aging adults or during times of metabolic stress (i.e., sleep deprivation). Furthermore, creatine has shown promise for improving health outcome measures associated with muscular dystrophy, traumatic brain injury (including concussions in children), depression, and anxiety. However, whether any sex- or age-related differences exist in regard to creatine and indices of brain health and function is relatively unknown. The purpose of this narrative review is to: (1) provide an up-to-date summary and discussion of the current body of research focusing on creatine and indices of brain health and function and (2) discuss possible sex- and age-related differences in response to creatine supplementation on brain bioenergetics, measures of brain health and function, and neurological diseases.

Association between dietary creatine and visuospatial short-term memory in older adults

Aims: The purpose was to examine the relationship between habitual dietary creatine intake obtained in food and visuospatial short-term memory (VSSM). Methods: Forty-two participants (32 females, 10 males; > 60 yrs of age) completed a 5-day dietary recall to estimate creatine intake and performed a cognitive assessment which included a visuospatial short-term memory test (forward and reverse corsi block test) and a mini-mental state examination (MMSE). Pearson correlation coefficients were determined. Further, cohorts were derived based on the median creatine intake. Results: There was a significant correlation between the forward Corsi (r  =  0.703, P < 0.001), reverse Corsi (r  =  0.715, P < 0.001), and the memory sub-component of the MMSE (r  =  0.406, P = 0.004). A median creatine intake of 0.382 g/day was found. Participants consuming greater than the median had a significantly higher Corsi (P = 0.005) and reverse Corsi (P < 0.001) scores compared to participants ingesting less than the median. Conclusions: Dietary creatine intake is positively associated with measures of memory in older adults. Clinical Implications: Older adults should consider food sources containing creatine (i.e. red meat, seafood) due to the positive association with visuospatial short-term memory.

The effects of creatine supplementation on cognitive performance-a randomised controlled study

BACKGROUND

Creatine is an organic compound that facilitates the recycling of energy-providing adenosine triphosphate (ATP) in muscle and brain tissue. It is a safe, well-studied supplement for strength training. Previous studies have shown that supplementation increases brain creatine levels, which might increase cognitive performance. The results of studies that have tested cognitive performance differ greatly, possibly due to different populations, supplementation regimens, and cognitive tasks. This is the largest study on the effect of creatine supplementation on cognitive performance to date.

METHODS

Our trial was preregistered, cross-over, double-blind, placebo-controlled, and randomised, with daily supplementation of 5 g for 6 weeks each. We tested participants on Raven's Advanced Progressive Matrices (RAPM) and on the Backward Digit Span (BDS). In addition, we included eight exploratory cognitive tests. About half of our 123 participants were vegetarians and half were omnivores.

RESULTS

Bayesian evidence supported a small beneficial effect of creatine. The creatine effect bordered significance for BDS (p = 0.064, η2P = 0.029) but not RAPM (p = 0.327, η2P = 0.008). There was no indication that creatine improved the performance of our exploratory cognitive tasks. Side effects were reported significantly more often for creatine than for placebo supplementation (p = 0.002, RR = 4.25). Vegetarians did not benefit more from creatine than omnivores.

CONCLUSIONS

Our study, in combination with the literature, implies that creatine might have a small beneficial effect. Larger studies are needed to confirm or rule out this effect. Given the safety and broad availability of creatine, this is well worth investigating; a small effect could have large benefits when scaled over time and over many people.

TRIAL REGISTRATION

The trial was prospectively registered (drks.de identifier: DRKS00017250, https://osf.io/xpwkc/ ).

Dose-Response of Creatine Supplementation on Cognitive Function in Healthy Young Adults

To determine if creatine (Cr) supplementation could influence cognitive performance and whether any changes were related to changes in prefrontal cortex (PFC) activation during such cognitive tasks, thirty (M = 11, F = 19) participants were evenly randomized to receive supplementation with Cr (CR10:10 g/day or CR20:20 g/day) or a placebo (PLA:10 g/day) for 6 weeks. Participants completed a cognitive test battery (processing speed, episodic memory, and attention) on two separate occasions prior to and following supplementation. Functional near-infrared spectroscopy (fNIRS) was used to measure PFC oxyhemoglobin (O2Hb) during the cognitive evaluation. A two-way repeated measures ANOVA was used to determine the differences between the groups and the timepoints for the cognitive performance scores and PFC O2Hb. In addition, a one-way ANOVA of % change was used to determine pre- and post-differences between the groups. Creatine (independent of dosage) had no significant effect on the measures of cognitive performance. There was a trend for decreased relative PFC O2Hb in the CR10 group versus the PLA group in the processing speed test (p = 0.06). Overall, six weeks of Cr supplementation at a moderate or high dose does not improve cognitive performance or change PFC activation in young adults.

Creatine supplementation in the pediatric and adolescent athlete-- A literature review

Background

An increase in intra-muscular creatine through supplementation has been proposed as a strategy for improving muscle performance and recovery, with studies showing some benefit for adult athletes who rely on short, explosive movements. We reviewed and summarized the current literature on creatine supplementation in a pediatric and adolescent population.

Methods

The databases PubMed and EMBASE were queried to identity articles related to the use of creatine supplementation in a healthy pediatric and adolescent population according to the guidelines established by PRISMA. The abstracts of all articles were reviewed to determine relevancy, with those meeting the pre-defined criteria included in the final review.

Results

A combined total of 9393 articles were identified. Following application of filters and review of abstracts, 13 articles were found to meet criteria and were included in the final review. There was a total of 268 subjects across all studies, with mean age ranging from 11.5 to 18.2 years. More than 75% of the studies were randomized-controlled trials, and 85% involved either soccer players or swimmers. The overall quality of the studies was poor, and there were no consistent findings regarding creatine supplementation and improvements in athletic performance. No studies were designed to address the topic of safety.

Conclusions

There is a gap in the study of the safety and efficacy of creatine supplementation in adolescents. Additional studies are needed to evaluate the effects of alterations in muscle composition on the growth, development, and performance of the developing athlete. Orthopedic providers should counsel their pediatric and adolescent patients on the current limitations in trying to assess the true risk and benefit of creatine supplementation for the aspiring athlete.

Level of evidence

Review, III.

Effects of creatine supplementation on memory in healthy individuals: a systematic review and meta-analysis of randomized controlled trials

CONTEXT

From an energy perspective, the brain is very metabolically demanding. It is well documented that creatine plays a key role in brain bioenergetics. There is some evidence that creatine supplementation can augment brain creatine stores, which could increase memory.

OBJECTIVE

A systematic review and meta-analysis of randomized controlled trials (RCTs) was conducted to determine the effects of creatine supplementation on memory performance in healthy humans.

DATA SOURCES

The literature was searched through the PubMed, Web of Science, Cochrane Library, and Scopus databases from inception until September 2021.

DATA EXTRACTION

Twenty-three eligible RCTs were initially identified. Ten RCTs examining the effect of creatine supplementation compared with placebo on measures of memory in healthy individuals met the inclusion criteria for systematic review, 8 of which were included in the meta-analysis.

DATA ANALYSIS

Overall, creatine supplementation improved measures of memory compared with placebo (standard mean difference [SMD] = 0.29, 95%CI, 0.04-0.53; I2 = 66%; P = 0.02). Subgroup analyses revealed a significant improvement in memory in older adults (66-76 years) (SMD = 0.88; 95%CI, 0.22-1.55; I2 = 83%; P = 0.009) compared with their younger counterparts (11-31 years) (SMD = 0.03; 95%CI, -0.14 to 0.20; I2 = 0%; P = 0.72). Creatine dose (≈ 2.2-20 g/d), duration of intervention (5 days to 24 weeks), sex, or geographical origin did not influence the findings.

CONCLUSION

Creatine supplementation enhanced measures of memory performance in healthy individuals, especially in older adults (66-76 years).

SYSTEMATIC REVIEW REGISTRATION

PROSPERO registration no. 42021281027.

Effects of Creatine Supplementation on Brain Function and Health

While the vast majority of research involving creatine supplementation has focused on skeletal muscle, there is a small body of accumulating research that has focused on creatine and the brain. Preliminary studies indicate that creatine supplementation (and guanidinoacetic acid; GAA) has the ability to increase brain creatine content in humans. Furthermore, creatine has shown some promise for attenuating symptoms of concussion, mild traumatic brain injury and depression but its effect on neurodegenerative diseases appears to be lacking. The purpose of this narrative review is to summarize the current body of research pertaining to creatine supplementation on total creatine and phophorylcreatine (PCr) content, explore GAA as an alternative or adjunct to creatine supplementation on brain creatine uptake, assess the impact of creatine on cognition with a focus on sleep deprivation, discuss the effects of creatine supplementation on a variety of neurological and mental health conditions, and outline recent advances on creatine supplementation as a neuroprotective supplement following traumatic brain injury or concussion.

Neuroprotective Role of Nutritional Supplementation in Athletes

BACKGROUND

Neurodegenerative disorders belong to different classes of progressive/chronic conditions that affect the peripheral/central nervous system. It has been shown through studies that athletes who play sports involving repeated head trauma and sub-concussive impacts are more likely to experience neurological impairments and neurodegenerative disorders in the long run.

AIMS

The aim of the current narrative review article is to provide a summary of various nutraceuticals that offer promise in the prevention or management of sports-related injuries, especially concussions and mild traumatic brain injuries.

METHODS

This article reviews the various potential nutraceutical agents and their possible mechanisms in providing a beneficial effect in the injury recovery process. A thorough survey of the literature was carried out in the relevant databases to identify studies published in recent years. In the present article, we have also highlighted the major neurological disorders along with the associated nutraceutical(s) therapy in the management of disorders.

RESULTS

The exact pathological mechanism behind neurodegenerative conditions is complex as well as idiopathic. However, mitochondrial dysfunction, oxidative stress as well as intracellular calcium overload are some common reasons responsible for the progression of these neurodegenerative disorders. Owing to the multifaceted effects of nutraceuticals (complementary medicine), these supplements have gained importance as neuroprotective. These diet-based approaches inhibit different pathways in a physiological manner without eliciting adverse effects. Food habits and lifestyle of an individual also affect neurodegeneration.

CONCLUSION

Studies have shown nutraceuticals (such as resveratrol, omega-3-fatty acids) to be efficacious in terms of their neuroprotection against several neurodegenerative disorders and to be used as supplements in the management of traumatic brain injuries. Protection prior to injuries is needed since concussions or sub-concussive impacts may trigger several pathophysiological responses or cascades that can lead to long-term complications associated with CNS. Thus, the use of nutraceuticals as prophylactic treatment for neurological interventions has been proposed.

Increased oxidative stress contributes to enhance brain amyloidogenesis and blunts energy metabolism in sucrose-fed rat: effect of AMPK activation

Metabolic disturbances are linked to neurodegenerative diseases such as Alzheimer disease (AD). However, the cellular mechanisms underlying this connection are unclear. We evaluated the role of oxidative stress (OS), during early metabolic syndrome (MetS), on amyloidogenic processes in a MetS rat model induced by sucrose. MetS caused OS damage as indicated by serum and hypothalamus lipid peroxidation and elevated serum catalase activity. Tissue catalase and superoxide dismutase activity were unchanged by MetS, but gene expression of nuclear factor erythroid-derived 2-like 2 (NFE2L2), which up-regulates expression of antioxidant enzymes, was higher. Expression of amyloid-β cleaving enzyme 1 (BACE-1) and amyloid precursor protein (APP), key proteins in the amyloidogenesis pathway, were slightly increased by sucrose-intake in the hippocampus and hypothalamus. Activation and expression of protein kinase B (PKB) and AMP-dependent protein kinase (AMPK), pivotal proteins in metabolism and energy signaling, were similarly affected in the hippocampus and hypothalamus of MetS rats. Brain creatine kinase activity decreased in brain tissues from rats with MetS, mainly due to irreversible oxidation. Chronic metformin administration partially reversed oxidative damage in sucrose-fed animals, together with increased AMPK activation; probably by modulating BACE-1 and NFE2L2. AMPK activation may be considered as a preventive therapy for early MetS and associated neurodegenerative diseases.

Creatine Supplementation in Children and Adolescents

Creatine is a popular ergogenic aid among athletic populations with consistent evidence indicating that creatine supplementation also continues to be commonly used among adolescent populations. In addition, the evidence base supporting the therapeutic benefits of creatine supplementation for a plethora of clinical applications in both adults and children continues to grow. Among pediatric populations, a strong rationale exists for creatine to afford therapeutic benefits pertaining to multiple neuromuscular and metabolic disorders, with preliminary evidence for other subsets of clinical populations as well. Despite the strong evidence supporting the efficacy and safety of creatine supplementation among adult populations, less is known as to whether similar physiological benefits extend to children and adolescent populations, and in particular those adolescent populations who are regularly participating in high-intensity exercise training. While limited in scope, studies involving creatine supplementation and exercise performance in adolescent athletes generally report improvements in several ergogenic outcomes with limited evidence of ergolytic properties and consistent reports indicating no adverse events associated with supplementation. The purpose of this article is to summarize the rationale, prevalence of use, performance benefits, clinical applications, and safety of creatine use in children and adolescents.

Creatine Supplementation and Brain Health

There is a robust and compelling body of evidence supporting the ergogenic and therapeutic role of creatine supplementation in muscle. Beyond these well-described effects and mechanisms, there is literature to suggest that creatine may also be beneficial to brain health (e.g., cognitive processing, brain function, and recovery from trauma). This is a growing field of research, and the purpose of this short review is to provide an update on the effects of creatine supplementation on brain health in humans. There is a potential for creatine supplementation to improve cognitive processing, especially in conditions characterized by brain creatine deficits, which could be induced by acute stressors (e.g., exercise, sleep deprivation) or chronic, pathologic conditions (e.g., creatine synthesis enzyme deficiencies, mild traumatic brain injury, aging, Alzheimer’s disease, depression). Despite this, the optimal creatine protocol able to increase brain creatine levels is still to be determined. Similarly, supplementation studies concomitantly assessing brain creatine and cognitive function are needed. Collectively, data available are promising and future research in the area is warranted.

Creatine in Health and Disease

Although creatine has been mostly studied as an ergogenic aid for exercise, training, and sport, several health and potential therapeutic benefits have been reported. This is because creatine plays a critical role in cellular metabolism, particularly during metabolically stressed states, and limitations in the ability to transport and/or store creatine can impair metabolism. Moreover, increasing availability of creatine in tissue may enhance cellular metabolism and thereby lessen the severity of injury and/or disease conditions, particularly when oxygen availability is compromised. This systematic review assesses the peer-reviewed scientific and medical evidence related to creatine's role in promoting general health as we age and how creatine supplementation has been used as a nutritional strategy to help individuals recover from injury and/or manage chronic disease. Additionally, it provides reasonable conclusions about the role of creatine on health and disease based on current scientific evidence. Based on this analysis, it can be concluded that creatine supplementation has several health and therapeutic benefits throughout the lifespan.

Abnormal Thalamic Metabolism in Patients With Lifelong Premature Ejaculation

BACKGROUND

Although some recent neuroimaging studies have indicated the abnormal brain structure or function in patients with lifelong premature ejaculation (LPE), whether and how the abnormal thalamic function participates in processing sexual behavioral information are still unclear in patients with LPE.

AIM

The aim of this study was to assess the changes in the thalamus metabolism and structural integrity in patients with LPE.

METHODS

We performed a multimodal magnetic resonance approach in a 3.0 T system, including proton magnetic resonance spectroscopy (¹H-MRS), diffusion tensor imaging, and volumetric analysis to detect the differences in thalamic metabolism and structure between 20 patients with LPE and 15 healthy controls.

OUTCOMES

We analyzed and correlated the clinical symptoms of the subjects with significant ¹H-MRS-based features. Peak areas of N-acetylaspartate, choline, creatine (Cr), and glutamate/glutamine (Glu) were calculated with the LCModel software.

RESULTS

Diffusion tensor imaging and volumetric analysis of thalami showed no differences between the 2 groups. On the contrary, ¹H-MRS study disclosed that both Glu concentrations and Glu/Cr ratio values in the thalami of patients with LPE were remarkably increased when compared with healthy controls (P < .01 for both variables). In addition, both the intravaginal ejaculatory latency time score and Chinese Index of Sexual Function for Premature Ejaculation-5 score were negatively related to increased Glu concentrations and Glu/Cr ratio values.

CLINICAL IMPLICATIONS

Glutamatergic activity changes of thalamus may be an underlying indicator for evaluating sensory conduction efficiency in patients with LPE.

STRENGTHS & LIMITATIONS

The present study first found the abnormal thalamic metabolism in patients with LPE and contributed to a better understanding of the LPE etiology. Limitations include a cross-sectional study design with small samples and no examination of other brain areas.

CONCLUSION

Our findings show that the increase in glutamatergic activity of thalamus is related to LPE, suggesting that the increased Glu neurotransmission in the thalamus may contribute to the development of premature ejaculation. Xia J-D, Chen F, Zhang Q-J, et al. Abnormal Thalamic Metabolism in Patients With Lifelong Premature Ejaculation. J Sex Med 2021;18:275-283.

Safety of Creatine Supplementation in Active Adolescents and Youth: A Brief Review

Creatine has been extensively researched and is well-supported as one of the most effective dietary supplements available. There is overwhelming support within the literature regarding the ability of creatine to augment performance following short term (5–7 days) and long-duration supplementation periods. There is also strong support for creatine regarding its safety profile and minimal risk for adverse events or any negative influence on markers of clinical health and safety. Recent research has also highlighted the ability of creatine to confer several health-related benefits in select clinical populations in addition to offering cognitive benefits. Creatine is also a popular supplement of choice for adolescent athletes; however, research in this area is extremely limited, particularly when examining the safety and efficacy of creatine supplementation in this population. Therefore, the purpose of this review was to highlight the limited number of studies available in adolescent populations and systematically discuss the topic of safety of creatine supplementation in a younger population.

Protection Before Impact: the Potential Neuroprotective Role of Nutritional Supplementation in Sports-Related Head Trauma

Even in the presence of underreporting, sports-related concussions/mild traumatic brain injuries (mTBI) are on the rise. In the absence of proper diagnosis, an athlete may return to play prior to full recovery, increasing the risk of second-impact syndrome or protracted symptoms. Recent evidence has demonstrated that sub-concussive impacts, those sustained routinely in practice and competition, result in a quantifiable pathophysiological response and the accumulation of both concussive and sub-concussive impacts sustained over a lifetime of sports participation may lead to long-term neurological impairments and an increased risk of developing neurodegenerative diseases. The pathophysiological, neurometabolic, and neurochemical cascade that initiates subsequent to the injury is complex and involves multiple mechanisms. While pharmaceutical treatments may target one mechanism, specific nutrients and nutraceuticals have been discovered to impact several pathways, presenting a broader approach. Several studies have demonstrated the neuroprotective effect of nutritional supplementation in the treatment of mTBI. However, given that many concussions go unreported and sub-concussive impacts result in a pathophysiological response that, too, may contribute to long-term brain health, protection prior to impact is warranted. This review discusses the current literature regarding the role of nutritional supplements that, when provided before mTBI and traumatic brain injury, may provide neurological protection.

Beyond muscle: the effects of creatine supplementation on brain creatine, cognitive processing, and traumatic brain injury

The ergogenic and therapeutic effects of increasing muscle creatine by supplementation are well-recognized. It appears that similar benefits to brain function and cognitive processing may also be achieved with creatine supplementation, however research in this area is more limited, and important knowledge gaps remain. The purpose of this review is to provide a comprehensive overview of the current state of knowledge about the influence of creatine supplementation on brain function in healthy individuals. It appears that brain creatine is responsive to supplementation, however higher, or more prolonged dosing strategies than those typically used to increase muscle creatine, may be required to elicit an increase in brain creatine. The optimal dosing strategy to induce this response, is currently unknown, and there is an urgent need for studies investigating this. When considering the influence of supplementation strategies on cognitive processes, it appears that creatine is most likely to exert an influence in situations whereby cognitive processes are stressed, e.g. during sleep deprivation, experimental hypoxia, or during the performance of more complex, and thus more cognitively demanding tasks. Evidence exists indicating that increased brain creatine may be effective at reducing the severity of, or enhancing recovery from mild traumatic brain injury, however, only limited data in humans are available to verify this hypothesis, thus representing an exciting area for further research.

Effect of age, diet, and tissue type on PCr response to creatine supplementation

Creatine/phosphorylcreatine (PCr) responses to creatine supplementation may be modulated by age, diet, and tissue, but studies assessing this possibility are lacking. Therefore we aimed to determine whether PCr responses vary as a function of age, diet, and tissue. Fifteen children, 17 omnivorous and 14 vegetarian adults, and 18 elderly individuals ("elderly") participated in this study. Participants were given placebo and subsequently creatine (0.3 g·kg^-1·day^-1) for 7 days in a single-blind fashion. PCr was measured through phosphorus magnetic resonance spectroscopy (³¹P-MRS) in muscle and brain. Creatine supplementation increased muscle PCr in children (P < 0.0003) and elderly (P < 0.001), whereas the increase in omnivores did not reach statistically significant difference (P = 0.3348). Elderly had greater PCr increases than children and omnivores (P < 0.0001 for both), whereas children experienced greater PCr increases than omnivores (P = 0.0022). In relation to diet, vegetarians (P < 0.0001), but not omnivores, had significant increases in muscle PCr content. Brain PCr content was not affected by creatine supplementation in any group, and delta changes in brain PCr (-0.7 to +3.9%) were inferior to those in muscle PCr content (+10.3 to +27.6%; P < 0.0001 for all comparisons). PCr responses to a standardized creatine protocol (0.3 g·kg^-1·day^-1 for 7 days) may be affected by age, diet, and tissue. Whereas creatine supplementation was able to increase muscle PCr in all groups, although to different extents, brain PCr was shown to be unresponsive overall. These findings demonstrate the need to tailor creatine protocols to optimize creatine/PCr accumulation both in muscle and in brain, enabling a better appreciation of the pleiotropic properties of creatine.NEW & NOTEWORTHY A standardized creatine supplementation protocol (0.3 g·kg^-1·day^-1 for 7 days) effectively increased muscle, but not brain, phosphorylcreatine. Older participants responded better than younger participants whereas vegetarians responded better than omnivores. Responses to supplementation are thus dependent on age, tissue, and diet. This suggests that a single "universal" protocol, originally designed for increasing muscle creatine in young individuals, may lead to heterogeneous muscle responses in different populations or even no responses in tissues other than skeletal muscle.