Does oral creatine supplementation improve strength? A meta-analysis

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Original Research

Does oral creatine supplementation improve strength? A meta-analysis

J Fam Pract. 2002 November;51(11):945-952

By

Rania L. Dempsey, MD

OBJECTIVES: Oral creatine is the most widely used nutritional supplement among athletes. Our purpose was to investigate whether creatine supplementation increases maximal strength and power in healthy adults.
STUDY DESIGN: Meta-analysis of existing literature.
DATA SOURCES: We searched MEDLINE (1966–2000) and the Cochrane Controlled Trials Register (through June 2001) to locate relevant articles. We reviewed conference proceedings and bibliographies of identified studies. An expert in the field was contacted for sources of unpublished data. Randomized or matched placebo controlled trials comparing creatine supplementation with placebo in healthy adults were considered.
OUTCOMES MEASURED: Presupplementation and postsupplementation change in maximal weight lifted, cycle ergometry sprint peak power, and isokinetic dynamometer peak torque were measured.
RESULTS: Sixteen studies were identified for inclusion. The summary difference in maximum weight lifted was 6.85 kg ( 95% confidence interval [CI], 5.24–8.47) greater after creatine than placebo for bench press and 9.76 kg (95% CI, 3.37–16.15) greater for squats; there was no difference for arm curls. In 7 of 10 studies evaluating maximal weight lifted, subjects were young men (younger than 36 years) engaged in resistance training. There was no difference in cycle ergometer or isokinetic dynamometer performance.
CONCLUSIONS: Oral creatine supplementation combined with resistance training increases maximal weight lifted in young men. There is no evidence for improved performance in older individuals or women or for other types of strength and power exercises. Also, the safety of creatine remains unproven. Therefore, until these issues are addressed, its use cannot be universally recommended.

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KEY POINTS FOR CLINICIANS

Oral creatine supplementation combined with resistance training increases maximal weight young men can lift.
It is unknown whether this increase in strength translates into improvement in sports performance.
Evidence in the existing literature is insufficient to draw conclusions about the effect of creatine in women or older individuals.
Because no long-term studies have been performed on the safety of creatine supplementation, its use should not be universally recommended.

Creatine has gained widespread popularity during the past decade as a possible performance-enhancing agent among professional and recreational athletes. It is the most widely used performance-enhancing supplement among youth aged 10 to 17 years,¹ with 15% to 30% of high school athletes^2,3and 48% of male Division I college athletes⁴ reporting creatine use. Considered a nutritional supplement, it is not regulated by the United States Food and Drug Administration nor is it banned by the International Olympic Committee or National Collegiate Athletic Association. Because of the widespread use of creatine, primary care providers must be knowledgeable about its effectiveness and safety.

Oral creatine monohydrate increases skeletal muscle creatine concentration by 16% to 50%,^5-7 but whether it is an effective ergogenic aid remains controversial. Multiple studies have investigated this question, but many have been small, often including fewer than 10 subjects, and results have been conflicting. Several reviews^8-14 have addressed the effectiveness of creatine, but there has not been a systematic and comprehensive meta-analysis to resolve the uncertainties in the literature or to quantify the magnitude of the effect of creatine. To evaluate whether oral creatine supplementation improves strength and power in healthy adults, and further to quantify the effect, we performed a meta-analysis of randomized and matched controlled trials investigating creatine supplementation and strength.

Methods

Search strategy

To identify possible studies for inclusion, 1 author (M.F.M.) searched the MEDLINE electronic database (1966–2000) using the terms “creatine supplementation” or “creatine” combined with “strength” or “power.” Another MEDLINE search (1966–2000) was independently conducted by another author (R.L.D.) using the term “creatine not kinase” combined with a previously published search strategy to comprehensively identify randomized clinical trials.¹⁵ We searched the Cochrane Controlled Trials Register using the term “creatine not kinase.” We manually reviewed bibliographies of identified studies, abstracts from American College of Sports Medicine annual meetings (1999 and 2000), and a reference list distributed by an expert on the subject at the annual meeting of the American Medical Society for Sports Medicine (2000). Titles and available abstracts were screened and relevant articles retrieved. An expert in the field was contacted for sources of unpublished data.

Inclusion and exclusion criteria

Two reviewers independently assessed articles for inclusion. A third reviewer was consulted to resolve discrepancies. We used the following inclusion criteria: (1) the articles reported results of randomized or matched placebo-controlled trials investigating the effect of oral creatine supplementation on strength or power with or without concomitant resistance training; (2) the study subjects were healthy men or women older than 16 years with or without previous athletic training; and (3) the studies were published in any language. Given the general disagreement regarding the time required for muscle creatine concentration to return to presupplementation levels after discontinuing oral creatine,^16-18 studies using a crossover design were excluded from the statistical analysis unless data from the first arm, before crossover, could be abstracted or obtained from the original investigator. Outcomes were measures of strength or power of any muscle group, including maximal weight lifted; peak power achieved in maximal (sprint) cycle ergometry; and peak knee flexion/extension torque in isokinetic dynamometer testing. Measurements of endurance, such as time to fatigue on cycle ergometer and number of repetitions achieved in submaximal weight lifting, were excluded. For studies reporting outcome per kilogram of body weight, we contacted investigators to obtain absolute outcome values and excluded studies if uncorrected data were not received. We also excluded articles that evaluated outcomes not investigated in at least 2 other studies. Finally, if we could not extract data in a usable form, we contacted investigators to obtain adequate data.

Quality assessment

Two independent reviewers appraised articles to determine methodological quality with respect to risk of bias under the following categories: method of randomization, allocation concealment, blinding, similarity of study groups, withdrawals and dropouts, and intention-to-treat analysis. Each study that met inclusion criteria was given a quality score, with a maximum possible score of 10, using a tool adapted from the Cochrane Handbook.¹⁹ The quality assessment data are presented but were not used to exclude or rank any study.