It is likely that digestion/absorption rates are critical for the enhanced protein response with peri-exercise protein ingestion because hydrolysed protein is superior to intact protein, suggesting that amino acid availability after exercise could be the critical stimulus (Koopman et al., 2009). However, some data indicate that casein stimulates muscle hypertrophy more than whey, and because whey is absorbed more quickly than casein (Cribb et al., 2007; Mahe et al., 1996) prolonged amino acid availability could also be important. Limited data suggest that vegetable protein may be suboptimal relative to protein metabolism because men aged 51–69 years on a vegetarian diet during a 12-week strength study experienced reduced gains in body density, lean mass and creatinine excretion (index of total muscle mass) and a 50% reduced increase in type 2 muscle fibre area compared with a group of meat eaters (Campbell et al., 1999).
A number of possibilities might explain these findings, such as altered digestibility, lower amino acid score or even enhanced muscle catabolism caused by increased cortisol release associated with vegan diets (Henley & Kuster, 1994; Baglieri et al., 1995; Lohrke et al., 2001). Consistent with this is the observation that milk (18 g protein, 750 kJ) ingested immediately post exercise produced a greater MPS response compared with isonitrogenous, isoenergetic and macronutrientmatched soy beverages (Wilkinson et al., 2007) (Figure 5.5). This occurred despite no observed treatment differences in blood flow, insulin, glucose or indispensable amino acid concentrations. A possible explanation could involve greater amino acid availability over time with the milk treatment as soy caused a much more rapid but transient increase in circulating amino acid concentration (Bos et al., 2003). In contrast, Brown et al. (2004) found no difference in the observed increase in lean mass with strength training in young men consuming isoenergetic bars containing soy or whey protein. These soy versus milk results are intriguing but far from definitive.
Apparently, protein type does play a role in the muscle response to exercise but much more study is needed before it is possible to determine the best approach.