The purpose of this study was to compare changes in muscle insulin-like growth factor-I (IGF-I) content resulting from resistance-exercise training (RET) and creatine supplementation (CR). Male (
n= 24) and female ( n= 18) participants with minimal resistance-exercise-training experience (≥1 year) who were participating in at least 30 min of structured physical activity (i.e., walking, jogging, cycling) 3–5 ×/wk volunteered for the study. Participants were randomly assigned in blocks (gender) to supplement with creatine (CR: 0.25 g/kg lean-tissue mass for 7 days; 0.06 g/kg lean-tissue mass for 49 days; n= 22, 12 males, 10 female) or isocaloric placebo (PL: n= 20, 12 male, 8 female) and engage in a whole-body RET program for 8 wk. Eighteen participants were classified as vegetarian (lacto-ovo or vegan; CR: 5 male, 5 female; PL: 3 male, 5 female). Muscle biopsies (vastus lateralis) were taken before and after the intervention and analyzed for IGF-I using standard immunohistochemical procedures. Stained muscle cross-sections were examined microscopically and IGF-I content quantified using image-analysis software. Results showed that RET increased intramuscular IGF-I content by 67%, with greater accumulation from CR (+78%) than PL (+54%; p= .06). There were no differences in IGF-I between vegetarians and nonvegetarians. These findings indicate that creatine supplementation during resistance-exercise training increases intramuscular IGF-I concentration in healthy men and women, independent of habitual dietary routine.