Meta-Analysis of the Placebo Effect in Nutritional Supplement Studies of Muscular Performance

This paper is a meta-analysis of the role of nutritional supplements in strength training focusing on the effects of placebo treatments. We address specifically the results from meta-analysis of 334 fi.ndings from 37 studies of the effect of nutritional supplements and physical fitness interventions on strength, stamina, and endurance outcomes, controlling for main effects of the group on which the results were obtained (placebo, treatment, control, for pretest or posttest), with covariates for age, gender, randomization, double-blind procedures, study duration, training load, training frequency, and training status. Finding show that there are significant placebo effects accounting for a substantial portion of the effect size typically associated with treatment interventions. In addition to produce the best evaluations of treatment effects, both control and placebo groups should be included in a double-blind research design using participants who are well familiarized with the study procedures.