Dietary Supplements and Sports Performance: Metabolites, Constituents, and Extracts

Numerous metabolites, constituents, and extracts of plants and animals have been marketed as sports supplements for physically active individuals. In general, these products are theorized to enhance various physiological processes involved in energy production for sport performance. Many of these products consist of single ingredients, while others may be combinations of several substances. This article will focus on two single products, creatine and HMB, which primarily have been marketed as a means to enhance muscle growth, strength and power.

Creatine is a nitrogen-containing substance found naturally in small amounts in animal foods. In recent years, creatine has been synthesized, mainly as creatine monohydrate, and has been marketed to athletes at all levels. Creatine supplements come in various forms (powder, pills, candy, chews, gels, serum, micronized) for both strength and endurance athletes, including products marketed specifically for males, females, and adolescents. Creatine is a very popular sports supplement. Studies of various athletic groups report usage ranging from 17–75 percent [1]. Relative to the adolescents, in a survey of middle and high school athletes age 10–18 in a New York City suburb found that creatine is being used by athletes at all grades [2]. Use increases with grade level and the prevalence of use by athletes in grades 11 and 12 approaches levels reported among college athletes. The most cited reasons for taking creatine were enhanced performance and appearance. Indeed, creatine continues to be one of the most popular sports supplements of all time.

Oral creatine supplementation, usually as creatine monohydrate, has been reported to increase muscle supplies of free creatine and creatine phosphate (phosphocreatine; PCr), a high-energy phosphagen. A typical creatine loading protocol involves the ingestion of 20 grams of creatine monohydrate over the course of a day, usually in four separate doses of 5 grams each, for 4–6 consecutive days. Casey and Greenhaff [3] note such a loading protocol will increase muscle total creatine and PCr levels, but doses beyond 20–30 grams provide no additional benefits. Once loaded, a daily dose of 2–5 grams helps maintain elevated muscle creatine levels. Consuming carbohydrate with creatine appears to increase muscle creatine stores significantly more than creatine supplementation alone [4]. Some individuals, particularly vegetarians with initially low levels of intramuscular creatine, may respond more effectively to creatine supplementation [5], while individuals with higher intramuscular creatine and phosphocreatine levels and those with fewer type II muscle fibers may be less responsive to supplementation [6].

Casey and Greenhaff [3] suggest that the improvements in performance associated with creatine supplementation are due to parallel improvements in ATP resynthesis during exercise as a consequence of increased PCr availability, particularly within the type II muscle fibers. Yquel and others [7], using magnetic resonance to evaluate PCr levels during repeated bouts of exercise, indicated that the ergogenic effects also could be attributed to a higher rate of phosphocreatine resynthesis, which would provide more PCr for ATP resynthesis. Creatine is one of the most researched sports supplements, as literally hundreds of studies have evaluated its effects on various types of sport performance. Most of the research has focused on the ability of creatine supplementation to increase muscle mass and related muscular strength and power applicable to performance in very-high-intensity sports, such as sprinting in track events.

Creatine supplementation appears to increase total body and lean body mass; however, short-term gains in total body mass may be primarily water, but long-term gains associated with resistance training appear to be lean muscle mass. Most studies document increases in muscle mass when creatine is supplemented during resistance training [8, 9]. The increase in muscle mass may be associated with a creatine supplementation-induced ability to do more repetitions during training, which may induce favorable genetic adaptations in the muscle. For example, Deldicque and others [10] suggest that the increase in lean body mass often reported after creatine supplementation could be mediated by signaling pathways, such as those involving insulin-like growth factor. In a related vein, Willoughby and Rosene [11] studied the effect of 12 weeks of creatine supplementation and resistance training on muscular strength and myosin heavy chain (MHC) mRNA and protein expression. Compared to both a control and placebo group, the creatine group significantly increased fat-free mass and strength. Additionally, they found that in general the MHC mRNA and protein expression were significantly higher in the creatine group compared to the other groups, and suggested that the increased strength and muscle size associated with creatine supplementation may be attributed to increase MHC synthesis. Research using muscle biopsy techniques has also shown increases in muscle cell diameter associated with 12 weeks of creatine supplementation and resistance training [12], which may be associated with the potential for creatine supplementation to amplify the resistance training-induced increase in satellite cell number and myonuclei concentration in skeletal muscle [13].

Critical reviews of the scientific literature [1, 14‐17], including a meta-analysis [8] and a monograph [9] generally indicate that creatine supplementation may increase muscular strength and endurance as documented by enhanced performance in 1-repetition maximum strength tests, increased number of repetitions in various isotonic and isokinetic resistance exercise tasks, and increased work output during maximal short term (6–30 seconds) cycle ergometer tasks. In general, activities that involve sprinting, jumping or cycling performance show improved performance following creatine supplementation [15], but the beneficial effects appear to be less consistent [18]. For example, Skare and others [19], using a standard creatine loading protocol with well-trained male sprinters as subjects, reported significant improvements in 100-meter sprint velocity and time to complete 6 intermittent 60-meter sprints. Preen and others [20] reported significant increases in peak power and total work production in 10 sets of multiple 6-second bike sprints with varying periods of recovery in an 80-minute time frame following creatine supplementation. Conversely, Op 'T Eijnde and others [21] reported no significant improvement in a 70-meter shuttle run sprint power test by well-trained tennis players while Cornish and others [22] found no beneficial effects on repeated 10-second skating sprints in ice-hockey players.

Some research findings have a direct application to sports competition, such as an increased 1-RM performance in weight lifting and faster 100-meter sprint run times. The laboratory findings for other types of exercise performance are also rather strong, and do support a possible application to actual field competitions. For example, findings of increased muscle power output during intermittent sprint exercises may be applicable to football (soccer) [23] and other sports associated with high-intensity intermittent sprinting [20]. In one such study, Cox and others [24] studied the effects of creatine supplementation on an exercise test protocol designed to simulate match play in soccer (football). The test involved 5 blocks of 11-minute exercise involving sprint running, agility runs, and a precision ball-kicking drill interspersed with recovery walks, jogs and runs. Creatine supplementation improved performance in some repeated sprint and agility tasks even though the subjects increased body mass, but the creatine had no effect on ball kicking accuracy. Thus, creatine supplementation might improve speed in repetitive sprints, important for many sports, but may not necessarily enhance sports skills. In support of this latter point, several studies reported no significant effects of creatine loading on tennis skill performance as measured by power and precision of their serves [21, 25].

Although some forms of creatine have been marketed to endurance athletes, there is limited evidence that creatine supplementation can enhance performance during exercise lasting longer than 90 seconds [1, 18]. For example, recent research found that supplementation with Runners Advantage creatine serum, which is marketed to distance runners, had no effect on 5-kilometer run time [26] and oxidative capacity, substrate utilization, and endurance cycling time trial performance were not enhanced by creatine supplementation [27].

In summary, Bemben and Lamont [15] indicate that it generally appears that creatine supplementation does significantly impact force production regardless of sport, sex or age. Kreider [16] makes the interesting observation that although not all studies report significant results, no studies report ergolytic effects, i.e., that creatine supplementation impairs exercise performance.

This brief review suggests that the efficacy of various metabolites, constituents and extracts marketed as sports supplements to athletes may vary. Research findings for creatine supplementation are very supportive, while those for HMB supplementation are not. As we shall see in the next and final article of this series, several other dietary supplements may also provide some performance-enhancing effects to certain athletes, but most have not been shown to be effective or have been inadequately studied