Differences between BW and AWC were determined using separate 2 × 5 (group × time) mixed factorial ANOVA, followed by appropriate contrast tests when a significant F ratio (p ≤ 0.05) was observed.
The results for BW indicated that there was a significant time × group interaction (p = 0.018) and significant time effect (p = 0.001). When examining the time × group interaction, contrast tests indicated that only the CPS group experienced a significant increase in BW. Body weight at 10 days (78.9 ± 5.1 kg), 20 days (79.1 ± 5.1 kg), 30 days (78.9 ± 5.2 kg), and 10 days post-supplementation (78.5 ± 4.4 kg) was significantly greater compared with BL (77.9 ± 5.1 kg) (Table 2).
The results of the 2 × 5 ANOVA for AWC indicated that there was no time × group interaction (p = 0.438) (Table 2). In addition, there were no significant main effects for time (p = 0.226) or group (p = 0.149) (Table 1). The increase in AWC from BL to 30 days for the CPS and PL group was 4.2% and 1.7%, respectively. A comparison of the effect sizes indicated that CPS was moderately effective (Cohen's d = 0.41), whereas the PL was ineffective (d = 0.16).
Although a majority of Cr studies report using a high-dose loading phase for at least 5 days, Harris et al. (11) found that Cr uptake into muscle is greatest during the first 2 days of loading (6 × 5 g·d−1 × 7 d), with approximately 20% of the Cr taken up as CrP. Therefore, high doses of Cr for more than 2 days may actually be unwarranted to improve exercise performance, and the results of a few studies that either examined the effect of a 2-3 day loading phase alone (9,40,44), or with a maintenance dose thereafter (29), appear to support this finding.
Another dosing strategy that has scientific basis is to slowly load the muscle with Cr using low doses for longer periods of time. Hultman et al. (14) have reported that a single Cr dose of 3 g·d−1 for 28 days was sufficient to gradually increase muscle TCr concentrations by 20%, which corresponds to the increases typically observed with loading. Although the findings of the current study showed that a single 5 g·d−1 serving of CPS for 30 days resulted in a significant increase in BW when compared with PL, there was no statistically significant effect on AWC. For the CPS group, BW was significantly increased at 10 days (1.0 kg), and remained elevated throughout the duration of the study, whereas the PL group experienced no significant changes in BW at any time point (Table 2). The finding of no significant differences in AWC in the current study could be attributed, in part, to the exclusion of a loading dose, since other studies which have examined the effects of both long-term and short-term low-dose Cr supplementation without a loading phase have also reported nonsignificant findings (12,37,42,43).
Lower doses of Cr supplemented over shorter periods of time have also been shown to be ineffective for increasing anaerobic exercise performance. For example, Hoffman et al. (12) reported that a single Cr dose (Vitalstate US, Inc.; Boynton Beach, FL) of 6 g·d−1 for 6 days had no effect versus PL on BW, peak power, mean power, or total work derived from three 15-second Wingate anaerobic power tests in 40 physically active college-age men. However, the change in the rate of fatigue for the Cr group (n = 20) was significantly (p < 0.05) lower compared to the PL group (n = 20). Although the authors (12) did not directly measure muscle TCr concentrations, they suggested that Cr supplementation may positively affect changes in the rate of fatigue development before it results in beneficial effects on strength and power.
In addition to the differences in the length (6 days to 10 weeks) and amount of Cr supplementation (5-10 g·d−1) reported in the studies above, discrepancies in the findings may also be related to a number of other methodological factors, including the Cr delivery system used (i.e., chewables versus powders versus tablets), the subject characteristics (trained versus untrained), and the types of anaerobic indices used as dependent variables. In the current study, both the CPS and PL groups displayed a wide range of variability in AWC from BL to 30 days (−17.2% to 40.5% and −16.8% to 30.0%, respectively), which likely explains, in part, the finding of no significant effects. Although a familiarization trial was performed, it would appear that some of the subjects experienced a learning effect, since a few individuals in the PL group demonstrated considerable increases in AWC. In addition, the fact that subjects in both groups experienced decrements in performance also suggests that motivation during Critical Power testing may have been a factor, even though they were verbally encouraged throughout the test to provide a maximal effort.
The efficacy of a single serving of low-dose Cr administration for enhancing anaerobic exercise performance remains unclear; however, considering the results of the current study, and those of several similar studies (8,9,33), it would appear that supplementation involving a loading dose of Cr for at least 2-3 days is superior to a single dose of 5 g·d−1 for 30 days for increasing AWC. In addition, the current findings, as well as the results of several other studies that used low to moderate doses of Cr supplementation (2 to 10 g·d−1) without a loading phase (6,12,21,28,31,36,37,42), indicate that the Cr dosage for supplementation protocols lasting 1-4 weeks, and possibly up to 10 weeks, range between 8 and 10 g·d−1 to enhance anaerobic performance and FFW, and that 5 g·d−1 be used as a minimum dose for supplementation protocols lasting ≥ 10 weeks. Given the results of van Loon et al. (38) who found that 2 g·d−1 of Cr during a 6-week maintenance phase actually decreased muscle TCr and PCr concentrations, and considering that normal daily losses of Cr are ∼2-3 g·d−1 (3,23), it may seem reasonable to recommend that maintenance doses following loading also be no less than 5 g·d−1. Future studies are warranted which directly measure muscle TCr and PCr stores following Cr dosing strategies designed to more slowly load the muscles and determine their effectiveness on various measures of anaerobic performance.
This study was supported by a grant from NUMICO Research, Wageningen, The Netherlands.
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