Absorption of Whey Casein Milk and Soy Proteins

The amount of time that a protein takes to enter circulation is partly dependent upon the form of the protein. Whole foods are absorbed slowly, and hydrolyzed proteins, in the form of small peptides, are absorbed the fastest. It is generally thought that whey protein is a quickly absorbed protein due in part to the fact that it remains soluble in solution.9 Casein is absorbed very slowly, most likely due to the fact that it tends to clot in the stomach, delaying gastric emptying.918 Most other protein sources fall somewhere between whey and casein in their rate of appearance in systemic circulation, with a tendency to be slightly slower than whey. Milk proteins consist of approximately 80% casein and 20% whey and appear in a pattern similar to that of casein alone. Soy protein is mostly soluble and appears in circulation more like whey proteins than milk or casein proteins.19 The intestinal half transit time of soy proteins is shorter than that of milk proteins, potentially playing a role in the reduced bioavailability of soy proteins.12 It has also been shown that soy proteins are less digestible on a nitrogen basis.12 The pattern of amino acids reaching the liver is more unbalanced in soy proteins than milk proteins due to the lower digestibility of the soy proteins.12,20 Also, soy proteins tend to produce more urea than casein intake, indicating an increased oxidation of soy proteins and reduced availability of their amino acids for protein synthesis.21 Soy proteins arrive quickly and in large amounts in the splanchnic region, most likely contributing to the increased urea production as the splanchnic region uses these amino acids.17,21 The appearance of amino acids in the peripheral circulation are, for the most part, dependent upon the amino acids that have been ingested. However, this is not the case for amino acids that are highly metabolized in the splanchnic regions. There is a paucity of data directly comparing whey vs. soy regarding these postprandial properties. To fully elucidate the differences, it is necessary to directly compare whey and soy proteins considering that both whey and soy proteins exit the stomach quickly. Whey appears to have a slow and efficient intestinal absorption.22

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