4. Discussion
muscle mass, muscular endurance, weight loss, muscle hypertrophy
In this study, we investigated the effects of whey protein supplementation on improvements in muscle mass and isokinetic muscular function, followed by resistance exercise. We designed this study as a double-blind, randomized, placebo-controlled trial, and to eliminate dietary effects, we provided all study participants with three meals a day based on the calculated individual estimated calorie intake during the intervention period.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. Although the increase in muscle mass was relatively small (effect size, 0.56), considering that body weight was also significantly decreased, this increase in muscle mass could be meaningful. These results suggest that whey protein supplementation has an additional effect on resistance exercise-induced increases in muscle mass. Although the additional effects of protein supplementation during resistance exercise appear to be controversial, previous studies support our results. A recent meta-analysis reported that protein supplementation had a slight positive effect during resistance exercise [Changes in body weight, body fat mass, and percent body fat were not significantly different between the two groups in this study. Similar results have been reported in previous studies. The most recent meta-analysis reported no significant effect of whey protein supplementation on body mass, body fat mass, or body fat percentage [Interestingly, body weight significantly decreased after resistance exercise in both groups in this study. This could be mainly due to the significant decrease in body fat mass of approximately 20% in the PSG and 12% in the CON. Because the changes in muscle mass were much smaller than the changes in body fat mass, overall weight loss could have occurred. In fact, in the PSG, the change in body fat mass was −3.2 kg, while the change in muscle mass was +0.3 kg. In the CON, the change in body fat mass was −2.0 kg, while there were no changes in muscle mass. Another possibility for weight loss in this study may be that the actual calorie intake was lower than the estimated caloric intake (approximately 4.6% in the PSG and 4.4% in the CON). We attempted to accurately calculate the daily calorie intake required for each participant and provided the same number of calories. However, each person’s meal menu was diverse; therefore, it was difficult to match the exact calories in the food preparation process. Nevertheless, these unintentionally reduced calorie intakes could be considered to be within acceptable variability for a typical diet because even in previous studies with calorie restriction, the variation in calorie intake was approximately 6% to 10% [We measured isokinetic muscular function in parallel with muscle mass in this study because it is possible that muscular strength changes due to adaptations that occur within the muscle, regardless of muscle hypertrophy [Interestingly, in this study, muscular strength did not increase even after resistance exercise in the CON. This is probably because muscle hypertrophy did not occur or because the exercise period in this study was shorter than that in the previous studies. In fact, it has been reported that muscular strength gains can be induced by resistance exercise after at least six weeks [The difference in muscular strength increases between the two groups was found only in the flexors for knees and extensors for shoulders. These results may be related to the type of exercise program. Among the exercises in our program, the high foot position leg press was suitable for increasing the strength of the knee flexors [Similar to the results for muscular strength, there were also significant differences between the two groups in selective muscular endurance of the knee and shoulder. The PSG showed significantly increased endurance of the dominant knee and shoulder, whereas the endurance of the CON did not change. Although there was no interaction between the groups, trunk endurance significantly increased in the PSG. Thus, these results suggest that whey protein supplementation may also have substantial overall effects on the increase in muscular endurance induced by resistance exercise. Although there are differences in the combinations of protein supplements, previous studies also support our findings. Jang et al. [Interestingly, differences in muscular endurance gains between the two groups were found only in the dominant extensors for both knees and shoulders. These results may also be related to our resistance exercise program. In this study, exercises related to knee joint muscular function were squats, lunges, and leg presses, which mainly used extensors. Knee joint extensors are continuously trained during lower-body exercises, which seems to improve muscular endurance. The major muscles involved in the extension of the glenohumeral joint are the latissimus dorsi and teres major [