Casein is a slow protein as well. Fast vs slow refers to digestion and release of amino acids into the blood. Whey when ingested by itself is in and out in a couple of hours. Casein takes about 3x longer. Gives a more prolonged release of essential amino acids into the blood thus keeping protein synthesis turned on overnight. The more I read about soy, the less convinced I am that it is a good thing. I'd suggest a whey/casein blend. See below abstract from, Am J Physiology Vol. 280, Issue 2, E340-E348, February 2001
The digestion rate of protein is an independent regulating factor of postprandial protein retention
Martial Dangin1,3, Yves Boirie1, Clara Garcia-Rodenas3, Pierre Gachon1, Jacques Fauquant2, Philippe Callier3, Olivier Ballèvre3, and Bernard Beaufrère1
1 Laboratoire de Nutrition Humaine, Centre de Recherche en Nutrition Humaine, Université Clermont Auvergne, 63009 Clermont-Ferrand; 2 Laboratoire de Technologie Laitière, Institut National de la Recherche Agronomique, 35042 Rennes, France; and 3 Nestec, Nestlé Research Center, CH 1000 Lausanne 26 Switzerland
To evaluate the importance of protein digestion rate on protein deposition, we characterized leucine kinetics after ingestion of "protein" meals of identical amino acid composition and nitrogen contents but of different digestion rates. Four groups of five or six young men received an L-[1-13C]leucine infusion and one of the following 30-g protein meals: a single meal of slowly digested casein (CAS), a single meal of free amino acid mimicking casein composition (AA), a single meal of rapidly digested whey proteins (WP), or repeated meals of whey proteins (RPT-WP) mimicking slow digestion rate. Comparisons were made between "fast" (AA, WP) and "slow" (CAS, RPT-WP) meals of identical amino acid composition (AA vs. CAS, and WP vs. RPT-WP). The fast meals induced a strong, rapid, and transient increase of aminoacidemia, leucine flux, and oxidation. After slow meals, these parameters increased moderately but durably. Postprandial leucine balance over 7 h was higher after the slow than after the fast meals (CAS: 38 ± 13 vs. AA: 12 ± 11, P < 0.01; RPT-WP: 87 ± 25 vs. WP: 6 ± 19 µmol/kg, P < 0.05). Protein digestion rate is an independent factor modulating postprandial protein deposition.
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