par pat g » 29 Oct 2009 16:12
non, car avec les tableau la mise en page va être nulle et illisible, en plus c'est une revue de 24 pages
voilà le chapitre
Absorption rates of amino acids estimated in this review (summarized in
Table 2) are crude, yet serve as sufficient approximates given the absence of
direct data pertaining specifically to amino acid absorption (i.e., grams per hour
per kilogram of body weight). The absorption rate (measured as g/h), of free
amino acids (AA), casein isolate (CAS), and whey protein isolate (WP) were
greater than that of raw and
cooked egg white, pea flour,
and slightly greater than milk
protein. Free amino acids with
the same amino acid profile
as casein protein elicits a fast
transient peak of plasma amino
acids, while casein releases
amino acids slowly over many
hours after consumption. This
is consistent with other studies
that show free amino acid
mixtures induce a more rapid
absorption than intact proteins
(49, 50). The two milk protein
fractions, micellar casein
and the soluble whey protein
have been synonymous with
the concept of “slow,” and
“fast” digestibility of protein.
A detailed discussion of these
two milk protein fractions is
beyond the scope of this article and provided elsewhere (51). It is however worth
mentioning that WP is soluble, allowing faster gastric emptying, whereas casein
clots in the stomach delaying gastric emptying, resulting in a slower release of
amino acids (52).
An important question then must be posed: “Does a more rapidly absorbable
protein result in greater in vivo protein synthesis?” This is a central issue of large
protein consumption with fitness enthusiasts, athletes, and bodybuilders.
Early findings suggest that rapidly absorbed proteins such as free amino acids
and WP, transiently and moderately inhibit protein breakdown (39, 53), yet stimulate
protein synthesis by 68% [using nonoxidative leucine disposal (NOLD) as an
index of protein synthesis] (54). Casein protein has been shown to inhibit protein
breakdown by 30% for a 7-h postprandial period, and only slightly increase protein
synthesis (38, 54). Rapidly absorbed amino acids despite stimulating greater
protein synthesis, also stimulate greater amino acid oxidation, and hence results
in a lower net protein gain, than slowly absorbed protein (54). Leucine balance,
a measurable endpoint for protein balance, is indicated in Figure 1, which shows
slowly absorbed amino acids (~ 6 to 7 g/h), such as CAS and 2.3 g of WP repeatedly
taken orally every 20 min (RPT-WP), provide significantly better protein
balance than rapidly absorbed amino acids (39, 54).This “slow” and “fast” protein concept provides some clearer evidence that
although human physiology may allow for rapid and increased absorption rate of
amino acids, as in the case of WP (8 to 10 g/h), this fast absorption is not strongly
correlated with a “maximal protein balance,” as incorrectly interpreted by fitness
enthusiasts, athletes, and bodybuilders. Using the findings of amino acid absorption
rates shown in Table 2 (using leucine balance as a measurable endpoint for protein
balance), a maximal amino acid intake measured by the inhibition of proteolysis
and increase in postprandial protein gain, may only be ~ 6 to 7 g/h (as described
by RPT-WP, and casein) (38), which corresponds to a maximal protein intake of
144 to 168 g/d.
The rate of amino acid absorption from protein is quite slow (~ 5 to 8 g/h,
from Table 2) when compared to that of other macronutrients, with fatty acids at
~ 0.175 g ∙ kg-1 ∙ h-1 (~ 14 g/h) (55) and glucose 60 to 100 g/h (0.8 to 1.2 g carbohydrate
∙ kg-1 ∙ h-1) for an 80 kg individual (56). From our earlier calculations
elucidating the maximal amounts of protein intake from MRUS, an 80 kg subject
could theoretically tolerate up to 301 to 365 g of protein per day, but this would
require an absorption rate of 12.5 to 15 g/h, an unlikely level given the results of
the studies reported above. However, some support for this level of absorption of
amino acids is found when amino acids are infused intravenously at 50, 100, 150
and 250 mg ∙ kg-1 ∙ h-1 (57). This protocol investigated the relationship between
the rate of infusion of amino acids and the muscle protein synthetic rate, which
peaked at 150 mg ∙ kg-1 ∙ h-1, corresponding to an absorption rate of 12 g/h for an
80 kg individual (57).
