However, significantly, casein appears to have greater effects on chronic protein deposition, perhaps because it inhibits MPB more than whey. This mechanism could explain the observed greater gains in lean mass and strength during 12 weeks of strength training in men when the dietary protein source was primarily casein versus whey. Interestingly, differences in circulating insulin are likely not responsible because insulin concentrations were similar in the experiments of Boirie and colleagues. However, it is possible that changes in insulin sensitivity are important. Some protein sources, especially deep-ocean fish, have powerful effects on insulin sensitivity due to their omega-3 fatty acid content, but others, such as cod, which have a low omega-3 content, have significant positive effects on insulin sensitivity, presumably due to their AA profile.61,62 This appears to be the result of normalizing insulin activation of the phosphatidylinositol 3-kinase/Akt and by selectively improving GLUT 4 translocation to the T-tubules in skeletal muscle. Such differing effects of protein type could be critical to the resulting protein anabolic effect and need to be investigated more fully. In addition, the post-exercise ingestion of milk as a source of protein can result in acute increases in MPSand more chronic increases in lean body mass versus soy protein.55 Cow’s milk contains about 4.8% CHO, 3.4% protein (of this 2.8% is casein and 0.6 is whey), 3.7% fat, 0.8% minerals (of this .3% is sodium and potassium), and 87.3% water. Of course, for skim milk, the fat is removed during the skimming process so the percentages of everything else increase proportionately.
Essentially then, milk is a CHO-protein-electrolyte fluid whose composition is quite appropriate as a post-exercise nutritional supplement to enhance protein balance. Others have noted post-exercise ingestion of chocolate milk (immediately and at 2 hours post) can enhance recovery following exhaustive endurance exercise bouts.
This should not be surprising because, although the CHO content of chocolate milk varies (typically, it is approximately double that of regular milk), the additional CHO content means that glycogen resynthesis would also be enhanced. Hence, it would appear that chocolate milk is a very effective post-exercise drink. Other potential explanations for the observed effects of pre-, during-, or postexercise protein ingestion on protein metabolism could involve physical or chemical differences that exist among dietary protein sources, as these affect digestion or absorption rates and, therefore, AA availability to muscle. For example, it is well known that whey protein is absorbed much more quickly than casein and this, not the specific AA content, is likely responsible for the differing effects on protein metabolism.
Vegetarian diets are now very popular to reduce saturated fat intake for health reasons but this approach may not be optimal relative to protein balance because 51- to 69-year-old men who consumed a vegetarian diet over a 12-week strength study obtained lower (P <0.05) gains in body density, fat-free mass, and creatinine excretion, as well as nonsignificant and less than 50% increase in fast twitch muscle fiber area than their counterparts who followed an identical training program but ate meat. Several possibilities could explain these observations, including the lower amino score, the lower digestibility, or the catabolic effects of postingestion increases in circulating cortisol associated with the consumption of vegetarian diets.
