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The Ergogenic Effects of Supplemental Nutritional Aids on Anaerobic Performance in Female Athletes

Glenn, Jordan M. PhD; Gray, Michelle PhD; Gualano, Bruno PhD; Roschel, Hamilton PhD

Author Information
Strength and Conditioning Journal: April 2016 - Volume 38 - Issue 2 - p 105-120
doi: 10.1519/SSC.0000000000000207

Abstract

INTRODUCTION

Athletes are constantly searching for techniques with which to increase sport performance and many reports support the use of ergogenic aids as a method to increase athletic ability (26,37,98,115). As of 2010, women represented 50.8% of the U.S. population (116) and 65% of female athletes report use of nutritional supplements throughout their college careers (54). However, women are traditionally understudied with regard to human research (14) and based on physiological and morphological differences between genders (102), it cannot be assumed that men and women experience similar responses from ergogenic aid use. As a result, it is becoming increasingly important to understand the ergogenic effects of nutritional supplemental aids on exercise performance in women. Therefore, the purpose of this review is to evaluate the effects of ergogenic aid use on performance (specifically from an anaerobic perspective) in female athletes and provide recommendations for use based on empirical support. To ensure the applicability of these ergogenic aids for sanctioned competition, all supplements discussed in this review are currently legal for competition under World Anti-Doping Agency (WADA) regulations (126). All empirical articles included in this review were published in peer-reviewed journals. No restriction was placed on the year or language in which the article was originally published. The review of literature is divided into the following sections: (a) caffeine, (b) creatine monohydrate (CrM), (c) beta-alanine, and (d) other aids. The 3 major aids reviewed in detail (caffeine, CrM, and beta-alanine) are classified as having the highest level of scientific evidence among other practical considerations, determining that the supplements are safe, legal, and effective for improving sports performance (9).

CAFFEINE

Caffeine is a widely used substance that is consumed daily by more than 90% of American adults (53). Historically used as a performance enhancing aid, WADA added caffeine to the banned substance list in 1984. Originally, WADA defined a urine concentration greater than 12 μg/mL as a disqualifying value; however, such a concentration is difficult to achieve. After analysis of more than 4,000 urine samples, the average urinary caffeine concentration was 1.47 μg/mL and less than 1% of samples had values greater than 12 μg/mL (33). Because of the small number of positive cases, WADA subsequently removed caffeine from the banned substance list in 2004 (33). Currently, caffeine remains a banned substance for the National Collegiate Athletics Association, but only when urinary concentrations exceed the threshold of 15 μg/mL (33,86), which is 20% higher than the previous threshold established by WADA.

Caffeine (1,3,7-trimethylxanthine) is a strong stimulant of the central nervous system and increases exercise performance by directly affecting the heart and skeletal muscles. Caffeine increases action potential transmission and membrane excitability leading to an increase in the number of motor units (MUs) recruited during skeletal muscle contraction. Increases in the number of MUs recruited leads to greater maximal force production translating into increases in anaerobic performance (30). This hypothesis has been tested and is well supported in studies using male subjects (6); however, the data among female participants are sparse. To date, there are limited investigations evaluating the effects of caffeine supplementation on changes in muscular parameters among women (47,76,80,85,90). Of those studies, beneficial results from caffeine supplementation are equivocal.

Gender differences have been reported with regard to the subjective and physiological effects from exogenous caffeine use (100,112,113). When consuming caffeine, women experience greater changes in blood pressure, but less change in heart rate compared with men (112). Within women, these responses are also dependent on circulating estradiol, indicating menstrual status is also an important factor. When circulating levels of estradiol were higher, women reported an increase in subjective “caffeine-related feelings” in reference to consumption sensitivity (112). Interestingly, these physiological and psychological responses do not necessarily result in performance discrepancies. When consuming the same relative amount of caffeine (6 mg/kg body weight), males and female athletes both experienced similar relative improvements in muscular power and endurance performance (25). Currently, it is unknown if alternative caffeine doses will elicit different performance effects between men and women. Until more data are available from caffeine supplementation examining sex differences, it may be important to evaluate female athletes independently from their male counterparts.

The optimal caffeine dose to elicit an ergogenic effect is debatable; however, general consensus is between 3 and 6 mg/kg of body weight (3,36,42,47,80,85,120). In the literature, caffeine dosages range widely from an absolute bolus (200, 400, or 800 mg) (13) to amounts relative to body weight (3–10 mg/kg) (85). Results of these concentrations on performance are also equivocal. Studies have reported no effects on physical performance (7), whereas others have demonstrated significant improvements during anaerobic exercise (3,42,47,120). Some of the purported benefits include enhanced focus (88), reductions in ratings of perceived exertion and pain (82,85), enhanced aerobic performance (3,120), increases in muscular strength (47), and greater power output (42).

Specifically, caffeine supplementation causes an approximately 2-fold increase in β-endorphin release (74). β-endorphins are known to reduce pain sensation in the working muscles, potentially leading to an increase in the ability to produce force or delay muscular fatigue, and ultimately increasing exercise performance. Although the correlation between β-endorphin concentration and pain sensation during exercise has not been examined, the effects of caffeine supplementation are quite robust. After completing a bout of maximal resistive exercise, delayed-onset muscle soreness was reduced by 6-fold in female subjects supplementing with 5 mg/kg caffeine (82). These reductions in pain among women may increase the ability to return to the gym after a shorter rest period and increase training volume, leading to significant increases in sport performance.

In addition to reductions in pain, one of the most commonly reported benefits from caffeine supplementation during exercise involves an increased reliance on free fatty acids (FFA) for fuel (36). This is important because fatigue is suggested to relate to the depletion of glycogen stores (36). During prolonged physical activity, caffeine supplementation (5 mg/kg) increased blood FFA concentration nearly 2-fold when compared with a placebo (36). In this same sample of women, not only was the FFA concentration higher in the blood, but their respiratory quotient was significantly lower suggesting less of a reliance on glucose as fuel during the activity (36). These results support the notion that caffeine is a beneficial aid when performing physical activity at moderate intensities.

Specifically related to muscular performance during anaerobic activities, studies are limited in women. Research suggests that maximal upper-body muscular strength increases (52.1–52.9 kg) with caffeine supplementation of 6 mg/kg (47); however, these results are contrasting with respect to submaximal performance. In the same investigation, when lifting 60% of 1 repetition maximum strength (1RM) to failure, college-aged women performed the same number of repetitions with 6 mg/kg caffeine supplementation compared with a placebo trial (47).

When observing changes in high-intensity cycling performance, caffeine has been ineffective when supplemented in women (76,80,90). Early evidence investigating the effects of caffeine on high-intensity cycling time to exhaustion revealed no differences with small (4 mg/kg), medium (7 mg/kg), or high (10 mg/kg) dosing protocols (90). These results were mirrored in a recent investigation by Mahdavi et al. (80), where no differences were found during a 30-second maximal exertion cycle sprint between caffeine and placebo trials. Another study evaluated female athletes when completing a series of repeated cycling sprints (76), indicating that caffeine supplementation did not increase peak or mean power outputs throughout the repeated sprints compared with a placebo. Inverse to the aforementioned studies (76,80,90), there are data indicating supplemental caffeine use can increase power output during a cycling time trial (42). The reason for the discrepancies in results may be because of the length of the exercise protocols. The studies demonstrating nonsignificant results (76,80,90) used protocols ≤ 30 seconds, whereas the work by Glaister et al. (42), used 20-km time trials. Therefore, based on the aforementioned studies, use of exogenous caffeine supplementation may be more beneficial during longer duration events and not necessarily during anaerobic cycling protocols.

Finally, caffeine supplementation seems to be safe and there is strong evidence to support the notion that, when consumed in moderation, caffeine has no adverse health effects (for a review see Ref. 53). In fact, most health-threatening cases involving caffeine reflect abuse of caffeine-containing medications and not generally accessible foods or beverages (55). As acute toxicity levels of caffeine are suggested around 10 g/d (equivalent to 100 cups of coffee), it seems caffeine supplementation is safe when used responsibly (87).

RECOMMENDATIONS FOR COACHES AND PRACTITIONERS

Exogenous caffeine use seems to be beneficial to women during specific facets of anaerobic exercise (Table 1). Supplementing with doses of at least 6 mg/kg (47) may elicit maximal strength increases during resistance exercise, albeit these results are independent from increases in submaximal performance. It has been previously suggested, in studies performed using male subjects, that training status of the subjects could influence results (125). Among the studies focusing on anaerobic performance among women, all were conducted using trained subjects; however, results relating to performance increases in recreational or untrained women are yet to be elucidated.

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Table 1-a:
Summary of studies examining effects of caffeine on anaerobic exercise performance in women
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Table 1-b:
Summary of studies examining effects of caffeine on anaerobic exercise performance in women

Additionally, habitual caffeine use has been suggested to also be a determinant of the effects of caffeine supplementation (8). Unfortunately, there is not enough literature evaluating female subjects to make a determination with regard to habituation. Based on the paucity of data available and equivocal results, it would be advantageous to expand the scope of research evaluating caffeine supplementation among women.

CREATINE MONOHYDRATE

Creatine monohydrate is a nitrogenous compound commonly found in animal proteins (72). Synthesized primarily in the liver, pancreas, and kidneys (12,119), creatine is used in the creatine kinase reaction that rephosphorylates adenosine triphosphate (ATP) from adenosine diphosphate (ADP) (95). Researchers first discovered the use of creatine as a primary energy source used by skeletal muscles as early as 1912 (39). Creatine is not an essential nutrient, meaning it is not necessary to intake creatine via outside sources (124). Skeletal muscle houses 95% of the creatine within the human body (12) and supplemental intake can increase intramuscular phosphocreatine levels up to 20% (62). As a result, supplementation of CrM has been a frequent component of ergogenic aid research in reference to the beneficial effects on exercise performance.

Compared with women, men are traditionally more likely to use CrM to enhance exercise performance (41) and many research articles involving CrM supplementation have used either male subjects (64,92) or mixed cohorts of men and women (49,67,106,128). Women may have higher endogenous muscle creatine (40) and it is suggested individuals with high endogenous muscle creatine do not respond as favorably to supplemental use (48). However, meta-analysis (19) indicated that female subjects have greater relative improvements from baseline (14.8%) compared with men (5.5%). As a result, previous results in male populations and/or mixed cohort studies cannot be generalized to female populations and data indicating CrM's effectiveness for increasing performance specifically in women must be individually examined.

In previously untrained women, 10 weeks of CrM supplementation (20g/d for 4 days, 5 g/d for 10 weeks) combined with resistance training, increased 1RM for the back squat (11%), leg press (10%), and leg extension (10%) exercises compared with subjects consuming a placebo (118). However, these increases in lower-body strength were independent from changes in upper-body 1RM. Total body lean muscle mass was also significantly increased in subjects consuming CrM at the 5-week (4.3%) and 10-week (5.5%) time points (118). Similar improvements in lower-body 1RM have also been observed in untrained older women after longitudinal CrM supplementation (2). Compared with a placebo group, older women supplementing with CrM (5 g/d for 12 weeks) exhibited significantly higher increases in knee extension strength (3.9%) after the intervention. In contrast to the work by Vandenberghe et al. (118), the older women also exhibited significant improvements in upper-body strength for the biceps curl (8.8%) and bench press (5.1%) exercises (2). Additionally, training volume (total amount of weight lifted) throughout the intervention was increased 2-fold for the CrM group (2). Based on the aforementioned data (2,118), longitudinal supplementation of CrM seems to be effective for increasing strength production in previously untrained women, although it must be established if these performance benefits can be paralleled in trained women.

When examining the ergogenic effects of CrM on strength increases in trained women, comparable results have been established in lacrosse (20) and soccer (73) athletes. After 5 weeks of CrM supplementation (20 g/d for 1 week, 2 g/d for 4 weeks) during a preseason conditioning program, female lacrosse athletes significantly increased bench press 1RM compared with subjects consuming a placebo (16.7 and 7.1%, respectively) (20). Similarly, CrM supplementation (15 g/d for 7 days, 5 g/d for 12 weeks) in soccer athletes (73) increased bench press and back squat 1RM compared with those consuming a placebo (7.3 and 8.0% greater, respectively).

Although most supplementation interventions use longitudinal dosing protocols, data are available indicating CrM can increase performance after short-term use. Elite female soccer athletes preparing for the Olympic games achieved faster times during sprint and agility tests after 6 days of CrM supplementation (20 g/d) compared with age-matched and training-matched controls (27). Another study investigating short-term CrM supplementation evaluated gender-specific effects of CrM on high-intensity exercise performance. Tarnopolsky (110) assessed healthy, recreationally active men and women separated into gender-specific groups using a crossover design with a 7-week washout period (it has been established that the minimal washout period for CrM is approximately 4 weeks (101)). After the supplementation intervention (20 g/d for 4 days), both men and women exhibited similar improvements in peak (4%) and relative anaerobic cycling power (5%) (110). These results are important as they indicate men and women may respond similarly to CrM with regard to performance benefits.

CrM supplementation has experienced controversy due to potential side effects and/or safety issues that may result from exogenous use. These concerns have included adverse effects on gastrointestinal distress (63), liver function (69), and potential renal issues arising from dehydration (79). However, it is important to note that most reports surrounding these adverse effects are anecdotal or poorly controlled designs in which subjects were also using other potentially harmful subjections (68,91). The only established side effect that may occur is rapid weight gain (noticeable within 5–6 days of use), which may be a positive for athletes with the exception of those competing in weight class–based events (17). To date, very little evidence indicates harmful and/or dangerous side effects from CrM, and an overwhelming body of literature suggests CrM is safe for supplemental use (17,111). As a result, CrM appears to be safe for both acute and longitudinal use, although athletes should be educated on proper use and dosing requirements for effectiveness.

RECOMMENDATIONS FOR COACHES AND PRACTITIONERS

In women (Table 2), CrM supplementation increases performance during strength and high-intensity type exercises (20,27,73). Women seem to respond similarly to men with regard to CrM supplementation (92) and longitudinally, these results seem to be achievable with (20) or without (2) a high-dose loading period (15–20 g/d for 4–7 days). Although supplementation of 2 g/d has been documented to have significant effects on performance (20), this is inconclusive (114) and supplementation of 5 g/d seems to be sufficient to increase performance after at least 4 weeks of use (20). High-dose supplementation protocols (20 g/d) may elicit significant performance benefits as soon as 4 days (110). Finally, the use of CrM seems to be safe and free from unwanted side effects (17,111) even with high-dose protocols.

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Table 2-a:
Summary of studies examining effects of creatine monohydrate on exercise performance in women
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Table 2-b:
Summary of studies examining effects of creatine monohydrate on exercise performance in women

BETA-ALANINE

Carnosine (β-alanyl-L-histidine) is an intracellular dipeptide found in high concentrations in skeletal muscle (51). Its synthesis in muscle is catalyzed by carnosine synthetase and derives from L-histidine and β-alanine amino acids, which are not produced in muscle, limiting muscle carnosine synthesis to the uptake of these amino acids from blood (5). β-alanine supplementation is considered to be the most effective way to increase endogenous synthesis of carnosine, as its availability is the rate-limiting factor of this reaction in humans (5).

The determinants of muscle carnosine content in humans include fiber type, age, diet, and gender (for a review see Ref. 52). In humans, muscle carnosine content ranges from 17.5 ± 4.8 mmol·kg-1 dm in women to 21.3 ± 4.2 mmol·kg−1 dm in men (81). Literature is consistent to show the efficacy of β-alanine supplementation on the increase of muscle carnosine content (10,34,35,51,56) within as little as 2 weeks (105); however, acute doses have proven ineffective (46). After longitudinal supplementation, carnosine levels are documented to increase up to 65% (51) and 80% (56) after 4 and 10 weeks, respectively. Additionally, the response to β-alanine supplementation seems to follow a linear dose-response relationship that is not dependent upon baseline muscle carnosine, muscle type, or the daily dose of β-alanine, but rather on the total amount of β-alanine consumed (105).

There is a growing body of evidence to suggest that β-alanine supplementation can improve exercise performance and capacity, particularly when those are limited by increased intramuscular acidosis. The ergogenic effects of β-alanine supplementation are mainly attributed to the increased muscle buffering capacity through elevated levels of carnosine. This notion is supported by strong evidence indicating β-alanine improves performance in high-intensity exercises lasting between 1 and 4 minutes in duration, as these are characterized by a greater contribution of the anaerobic energy sources with large accumulation of H+ (57). In accordance, most studies dedicated to investigate the effects of β-alanine supplementation in short-lasting exercises (i.e., less than 60 seconds) have shown no beneficial effect (35,59,65), as fatigue is more likely because of a decline in ATP resynthesis rate and ADP accumulation.

More recently, a few studies suggest that this optimal timeframe may be extended to between 1 and 7 minutes, although exercise modality may be an additional factor affecting the efficacy of β-alanine supplementation. These include, but are not limited to, events such as 800-m running, 100- and 200-m swimming and 2,000-m rowing (for a review, see Ref. 96). Furthermore, individuals engaged in a structured training regime should also consider supplementation as there is some evidence to suggest that supplementing with β-alanine could result in improvements in training volume (58), although further evidence is required. Importantly, previous work has demonstrated β-alanine supplementation can improve exercise performance to a similar extent in both trained and nontrained individuals, meaning that β-alanine can be used as an ergogenic aid for high-intensity exercise regardless of training status (31).

There is scant evidence about the influence of sex on muscle carnosine accumulation in response β-alanine supplementation in the current literature. Previously published reports on the effects of β-alanine supplementation on human muscle carnosine content included exclusively male participants (24,51,56,58,61,104). Only one study (34) has investigated both elderly men and women, but they did not diversify between sexes. Some studies have reported positive effects of β-alanine supplementation on exercise performance in women (43,44,83,99,108,121), yet without measurements of muscle carnosine content. In addition, it has been shown that carnosine metabolism becomes sex-specific in humans on sexual maturation (1,32), with muscle carnosine content being lower and serum carnosinase activity being higher in adult women than men. A previous work (38) reported that men, in comparison to women, had 36, 28, and 82% higher carnosine concentrations in the soleus, gastrocnemius, and tibialis anterior, respectively. However, it is not yet fully understood as to whether the factors inducing sexual dimorphism in baseline muscle carnosine content may also affect β-alanine–induced carnosine loading between sexes.

A recent work (103) showed that women supplemented with β-alanine (4 × 800 mg/d for 46 days) had similar increases in muscle carnosine to men. However, women experienced higher relative increases in muscle carnosine after supplementation, as baseline carnosine levels were lower than those found in men. It was also demonstrated that β-alanine supplementation (4 × 800 mg/d for 12 weeks) increased muscle carnosine content in elderly men and women alike (∼85.4%) (34). In this study, baseline carnosine content was also comparable between sexes. Taken together, these 2 studies suggest that there are no sex differences in the response to β-alanine supplementation. However, studies with “direct” biochemical measurement of muscle carnosine content are still unavailable, precluding us from drawing definitive conclusions on a putative sexual dimorphic response to β-alanine intake.

With respect to the ergogenic effects of β-alanine supplementation in women, literature is scarce and findings are inconsistent. For instance, a study investigated the effects of 6 weeks of β-alanine supplementation along with high-intensity interval training on maximal oxygen consumption and cycle ergometer workload at the ventilatory threshold in women (121). The authors found no additive effect of supplementation as cardiovascular fitness improved similarly across the 2 groups, irrespective of β-alanine intake. Similarly, no effect of β-alanine supplementation was found on maximal oxygen consumption, time to exhaustion, and ventilatory threshold in women after 28 days of supplementation (99). Both studies included young recreationally active women; however, neither of them determined muscle carnosine content, which limit the findings.

Conversely, a recent study (70) exhibited greater reduction (large effect sizes) in the rate of fatigue after each of the 2 subsequent Wingate tests following 28 days of β-alanine supplementation in young active women when compared with placebo. This study measured the change in muscle carnosine content, whose increase did not reach statistical significance. Importantly, the authors opted for an alternative dosing strategy (based on body weight), which may have incurred in suboptimal dosage to elicit positive responses, thus hampering the conclusions. Furthermore, a group of authors (107) observed increased time to exhaustion and delayed onset of ventilatory threshold during maximal cycle ergometry in young women after 28 days of β-alanine supplementation. Although the authors did not evaluate muscle carnosine levels, the improvements were attributed to a potential increase in buffering capacity as a possible consequence of increased muscle carnosine content.

There are no data suggesting acute or longitudinal β-alanine supplementation have adverse health effects or safety concerns. The only reported side effect from β-alanine use is paresthesia (numbness or tingling) which begins around 15 minutes after consumption (32) and can last up to 60–120 minutes (104) depending on the dosage. Although not dangerous, paresthesia can be uncomfortable and athletes should begin with lower doses and increase incrementally to minimize potential discomfort.

RECOMMENDATIONS FOR COACHES AND PRACTITIONERS

The limited number of studies investigating the effects of β-alanine in women has provided some evidence (not without controversy) suggesting the potential ability to enhance exercise performance, likely due to an improvement in buffering capacity (Table 3). Current use recommendations include longitudinal dosing designs (≥28 days) with intakes ranging from 3.2 to 6.4 g/d. It should be noted that, acute doses of more than 800 mg can induce the nondangerous side-effect paresthesia (78). For on-field application, blinding is not important for athletes, but to minimize potential discomfort, it is recommended coaches and practitioners begin with lower amounts and increase acute doses incrementally.

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Table 3-a:
Summary of studies examining effects of beta-alanine supplementation on exercise performance in women
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Table 3-b:
Summary of studies examining effects of beta-alanine supplementation on exercise performance in women

It is important to take into account that previous studies present inconsistent supplementation protocols, varying experimental designs, and no muscle carnosine measurements (78). Future studies directly comparing men and women in a variety sports and with concomitant measurements of performance and muscle carnosine are highly warranted.

OTHER AIDS

Although caffeine, CrM, and β-alanine have optimistic empirical support as to their efficacy as performance aids in women, there are other, less-investigated supplements that may potentially play an ergogenic role during anaerobic exercise. Often coaches and/or practitioners are questioned about the use of certain aids that have little to no scientific merit or empirical support. The reason for including the following ergogenic aids is to introduce certain supplements that are becoming increasingly popular, but may not yet have the scientific evidence to support use in athletes. Discussed within this section are supplements that may have beneficial effects on anaerobic performance in women, but require further investigation to ultimately determine effectiveness.

BRANCHED-CHAIN AMINO ACIDS

Branched-chain amino acids (BCAAs; leucine, isoleucine, valine) account for 14–18% of the total amino acids and nearly 35% of the essential amino acids in muscle proteins (75,93). Primarily catabolized in muscles (50), BCAA catabolism is primarily promoted by exercise (127). In untrained women, BCAAs have been used acutely as an aid to increase muscle force, with associated decreases in muscle soreness and muscle damage (97). Longitudinal data (6 weeks) in untrained female subjects have demonstrated increases in time to exhaustion during a graded exercise test, but no increases in muscular strength (4). With perspective to previously trained subjects, mixed-gender data collected in competitive rowers have suggested significant improvements in rowing time to exhaustion and upper-body power after 6 weeks of leucine supplementation alone (28). Important to note is that 10 of the 13 subjects from this mixed-gender cohort were women (28) and although the data were not analyzed independently with respect to the female subjects, it provides a promising perspective for future research involving the use of BCAA and/or leucine alone in trained women. Based on these results, future investigations should focus on similar longitudinal designs in trained female cohorts to confirm that performance increases are still present after supplementation. In addition, as the study by Crowe et al. (28) only used leucine, it is important to evaluate the effectiveness of the combination of leucine, isoleucine, and valine to evaluate if supplementing with all 3 BCAAs will elicit further increases over leucine alone.

NITRIC OXIDE INCREASING SUPPLEMENTS

The use of ergogenic aids to increase nitric oxide production has been studied extensively with regard to exercise performance (16). During exercise, nitric oxide plays an important role in the modulation of blood flow along with mitochondrial respiration (16). Additionally, increases in blood flow from elevated nitric oxide synthesis are suggested to improve glucose uptake, muscle contractility, muscle blood flow, and tissue recovery processes (16,18).

L-arginine

L-arginine is an essential amino acid traditionally found in seafood, watermelon juice, meat, and soy proteins (66). In vivo, excess L-arginine is taken up by endothelial cells and converted to nitric oxide, which has been theorized to result in performance increases (16). However, to date, these hypotheses have not been empirically supported in the literature.

In trained subjects, L-arginine failed to elicit increases in maximal aerobic capacity (15,109) or cycling power (77). Perhaps more importantly, these investigations also indicated that supplementation of exogenous L-arginine did not elicit increases in nitric oxide production (15,77). The reason for the lack of performance benefits may be resultant of the fact that L-arginine, when supplemented orally, cannot bypass hepatic metabolism, preventing potential increases in nitric oxide production (117).

Citrulline malate

Citrulline malate (CM) is made from the combination of L-citrulline and malate and functions as the precursor to L-arginine in vivo (94). To be effective, L-citrulline must be combined with the amino acid malate, which acts as an intermediate of the tricarboxylic acid cycle and may provide additional ergogenic effects (16). L-citrulline is a nonessential amino acid produced in the body and unlike L-arginine, it can avoid liver metabolism and is transported to the kidneys where it can be directly converted into L-arginine (16). Because of this mechanism, CM has recently experienced more interest for its effectiveness as an ergogenic aid.

As of this review, most data evaluating CM supplementation have been collected in men (11,89,122,123) with 1 investigation using a mixed cohort of men and women (29), and a single study using only female subjects (45). Initial results in men indicated that an acute dose of 8 g CM increased weightlifting performance in trained men for the upper-body (89) and lower-body (123) musculature when lifting at a submaximal percentage of 1RM to failure. Recently, these results were mimicked in women as 8 g CM increased submaximal performance during repeated bouts of upper-body and lower-body resistance exercise (45). However, another investigation suggested no differences in a similar resistance training protocol with an acute dose of 6 g (29), indicating 8 g may be the lower threshold with which to increase resistance exercise performance. With acute doses, significant results are also documented during body weight exercises (chin-ups, reverse chin-ups, push-ups) (122), indicating CM supplementation may benefit strength performance from a multitude of intensities, which can have attractive implications for a variety of sports. Only 1 investigation (11) has used a semilongitudinal design (7 days). After supplementation, subjects consuming CM increased peak power and total work completed during a 60-second sprint cycle test (9 and 7%, respectively).

Initial data evaluating CM supplementation suggest gastrointestinal discomfort may be a potential side effect from an acute 8 g dose (89). This is an isolated case as more recent literature report no side effects from the same acute dose (45,123). Based on the data available, recommendations for safety and/or potential side effects cannot be concluded, but it appears that an acute dose (up to 8 g) is physiologically safe and free from potential gastrointestinal discomfort.

Although positive results are demonstrated with CM supplementation in men, only 1 study has used women and it is important that additional research designs are implemented in women to determine if sex discrepancies exist with regard to performance increases.

Colostrum

Recently, colostrum has gained popularity as an ergogenic aid based on the potential to increase insulin-like growth factor 1 concentrations (84); however, these results have been equivocal (23,71). With regard to exercise performance, men have experienced significant increases in peak anaerobic power (22), whereas a mixed cohort study observed increases in sprint performance (60). To date, the only gender-specific data available in women indicate significant increases in buffering capacity; however, this was independent of improvements on submaximal or maximal performance in elite rowers (21). Based on previous results, colostrum may be beneficial for exercise performance, but further investigation into types of training and performance measures are required before proper recommendations can be established.

CONCLUSION

Although there are data available supporting the use and effectiveness of supplemental ergogenic aid use in women, these results are not without controversy. Women are traditionally understudied with regard to human research (14) and as a result data examining these ergogenic effects specifically in female subjects are minimal. As of this review, the results from most studies examining the effects of supplemental ergogenic aids have been completed in male subjects and extrapolated to other populations. However, based on physiological differences between genders (102), it cannot be assumed that men and women will experience similar responses from ergogenic aid use. As a result it is becoming increasingly important to understand the ergogenic effects of supplemental aids on exercise performance in women. Although we can make some recommendations for use and effectiveness in women with regard to increasing anaerobic sport performance, the body of knowledge needs to be expanded to further elucidate these effects.

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Keywords:

women; creatine monohydrate; beta-alanine; caffeine; sport nutrition

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