Why whey is fast and casein is slow

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This study demonstrates that dietary amino acid absorption is faster with WP than with CAS. Our methodology does not allow identification of the rate limiting step(s) that might be gastric emptying and/or luminal hydrolysis and/or amino acid mucosal absorption. It is very likely, however, that a slower gastric emptying was mostly responsible for the slower appearance of amino acids into the plasma. Indeed, CAS clots into the stomach whereas WP is rapidly emptied from the stomach into the duodenum (14). Thus, after WP ingestion, large amounts of dietary amino acids (≈25 mmol of leucine) flood the small body pool (≈5 mmol) in a short time, resulting in a dramatic increase in amino acid concentrations. This is probably responsible for the stimulation of leucine oxidation and protein synthesis, as recently suggested by Giordano et al. (8), who artificially elevated plasma amino acid concentrations by i.v. infusions of amino acids. This dramatic stimulation of protein synthesis and absence of protein breakdown inhibition is quite different from the pattern observed with classic feeding studies (22). By contrast, with CAS, plasma amino acid concentrations are lower, resulting in a lower oxidation and in a lesser increase of protein synthesis but also in an inhibition of protein breakdown. This metabolic response is similar to what is usually demonstrated during steady-state studies (2–7, 17, 23–26), and it is noticeable that postprandial plasma amino acid concentrations were actually at a near steady state. Explanation for the difference of inhibition of protein breakdown between WP and CAS is unclear; protein breakdown is classically inhibited by insulin (27–29), but plasma insulin concentrations were not different between the two meals. Although their effects are less powerful than insulin, amino acids also inhibit protein breakdown (27). The absence of any change of protein breakdown with WP, despite very high amino acid concentrations, suggests that a prolonged enough time of hyperaminoacidemia would be needed to obtain a significant protein breakdown inhibition.


Slow and fast dietary proteins differently modulate postprandial protein?accretion

 

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