The popularity of dietary supplements has steadily increased. Among dietary supplements, carbohydrate supplementation is the main source of energy for muscle contraction, whereas amino acid supplementation not only provides energy but also affects muscle protein synthesis and muscle recovery after resistance exercise. The aim of this study was to investigate the effect of whey protein supplementation on changes in muscle mass and isokinetic muscle function following resistance exercise while controlling the participants’ normal diet by providing three meals a day during the entire study period This study was a randomised, double-blind, placebo-controlled trial which recruited thirty-six men in their 20s and 30s. Participants were randomly assigned (1:1) to a whey protein supplement intake group (PSG) and a placebo intake group (CON). Results show that after four weeks of resistance exercise: - the change in muscle mass was significantly different between the two groups. The PSG showed significantly increased muscle mass, whereas the CON did not. - changes in body weight, body fat mass, and percent body fat were not significantly different between the two groups. - the PSG group showed significantly increased strength of both the dominant and non-dominant knees and shoulders, whereas the CON group did not. - muscular strength did not increase even after resistance exercise in the CON. - the PSG showed significantly increased endurance of the dominant knee and shoulder, whereas the endurance of the CON did not change. Authors conclude that whey protein supplementation promotes muscle mass increase and selective increases in muscular strength and endurance from resistance exercise. Thus, whey protein supplementation is practically effective independently of the normal diet.