Nowadays, strong evidence exists that ingestion of dietary protein increases muscle protein synthesis (MPS), while preventing muscle breakdown (Figure 3) (Cermak et al., 2012; Pasiakos et al., 2015; Phillips, 2013). When protein is taken in appropriate amounts, this allows for muscle building which could promote recovery and training adaptation (Campbell et al., 2007; Pasiakos et al., 2014). The evidence behind protein supplementation has been substantiated in a meta-analysis by Cermak et al. (2012) on the effect of long-term protein supplementation and resistance-type exercise training on muscle adaptation. This study included 22 randomised control trials containing a total of 680 subjects. Cermak et al. (2012) concluded that prolonged resistance-type exercise training (> 6 weeks) combined with protein supplementation or a high protein diet (> 1.2 g/kg/day) increased FFM and maximum strength for one repetition in both younger and older subjects. Adding to that, a systematic review by Pasiakos et al. (2015) indicated strong evidence for increased gains in FFM and strength due to protein supplementation combined with resistance training. The same review also indicates that protein supplementation in addition to exercise results in increased VO2max, a measurement of aerobic fitness (Ferguson-Stegall et al., 2011; Knuiman et al., 2009), lean mass (Knuiman et al., 2019) and anaerobic powers (Crowe et al., 2006; Laskowski & Antosiewicz, 2003). However, they also argue the limitation of available research and mechanistic information on the effects of protein supplementation on aerobic and anaerobic training (Pasiakos et al., 2015). In addition, it should be kept in mind that increases in lean mass and strength in untrained individuals in the first weeks of training are not influenced by protein supplementation (Antonio et al., 2000; Erskine et al., 2012).
Increased post-exercise anabolism assists muscle adaptation and could, in theory, therefore also support recovery (Saunders, 2007). However, the data on the effects of post-exercise protein supplementation on recovery is limited (Stefan M. Pasiakos et al., 2014). These results are hard to interpret due to inconsistent outcomes, possibly resulting from subjective measurements on muscle soreness scores. However, when protein supplementation is prolonged (> 2 weeks), muscle soreness and muscle damage markers do seem to decrease. Yet, this does not always reflect in exercise performance and is often in combination with high carbohydrate consumption (Flakoll et al., 2004; Skillen et al., 1996; Thomson et al., 2011).