PURPOSE: Creatine monohydrate (CM) is shown to augment resistance training (RT) increases in lean body mass (LBM). CM may increase intramuscular water volume which may influence LBM measurements. Changes in fluid distribution with CM have mainly been explored in males, and it may be different across the menstrual cycle (MC) in females. The acute effects of CM on LBM, especially in females, are therefore unclear. Thus, the purpose of this study was to investigate if LBM measured by dual X-ray absorptiometry (DXA) was affected by 7 days of CM prior to a RT intervention, and if this was influenced by sex. We then aimed to assess if changes in LBM after the loading phase would alter the interpretation of changes in LBM following RT. 

 

METHODS: Healthy, untrained adults were randomly assigned to a CM (n=17, 8 female/9 male, 28.5±8.0 years) or control (CO) (n=17, 12 female/5 male, 28.9±10.9 years) group. DXA scans were conducted in a fasted and hydrated state at 3 time points: 1) Baseline, 2) Post 7-day CM loading, and 3) Post 12-week RT intervention. CM group subjects consumed 5 g of CM daily at any mealtime after the first scan. The CO group received no supplement. The RT program included 4 sets of 5 exercises at a 6 to 12 repetition maximum on 3 days a week. Both groups began RT after the second scan. Subjects were instructed to maintain habitual diet and physical activity. Analyses of covariance (ANCOVA) were used to analyse group differences in LBM changes with baseline LBM as the covariate. This was done with sexes combined and then repeated with sexes separated. 

 

RESULTS: After 7 days of loading, CM group LBM changed by 0.51 ± 0.23 kg more (p = 0.03) than the CO group. With sexes separated, LBM in CM group females changed by 0.59 ± 0.28 kg more than those in the CO group (p = 0.04), while there were no changes in males. There were no differences in LBM changes between groups for the combined cohort, or with sexes separated, between the end of loading and the end of RT. Between baseline and the end of RT, CM group LBM changed by 0.74 ± 0.37 kg more than the CO group (p = 0.05). With sexes separated, LBM in CM group females changed by 1.2 ± 0.45 kg more than those in the CO group (p = 0.01), and there were no changes in males.

 

CONCLUSION: CM significantly increased LBM measurements in females after a 7-day non-exercise loading phase. However, CM had no effect on LBM in either sex after the ensuing RT intervention. Past research has not used a non-exercise loading phase. Therefore, previously reported added LBM gains from CM when combined with RT may be due to methodological design. The LBM changes from acute CM supplementation has implications for the dosing rationale and strategies in females, particularly in weight category sports. Given that MC was not controlled for in this study, future research should investigate the physiology underlying these sex-based differences in DXA measures of LBM with CM supplementation at different stages of the MC.