Protein shakes go through your digestive tract quickly, much faster than a solid meal, in fact. In theory, though, the longer the process
of digestion takes, the more efficiently your body utilizes the nutrients from the meal. A 1991 study demonstrated that the body utilizes
the amino acids from 30 grams (g) of whey protein differently than it does 30g of casein, its milk-protein counterpart, because whey protein
is digested much more quickly than casein.
Many scientists were surprised to learn that casein may have an advantage over whey because it is digested more slowly. Casein digestion takes up to six hours (or more); whey takes three hours or less to digest. For muscle growth, casein appears to have a less desirable amino-acid profile than whey; however, casein makes up for that because it is digested more slowly and utilized more efficiently than whey.
These findings were based on consuming 30g of either protein on an empty stomach. Since Boirie examined the proteins alone, not with carbs or other protein types the data left most of us hanging. For example, popular meal-replacement drinks combine protein with carbs, and that protein is generally a mix of whey and casein. To the rescue came researchers employing the same methodology as the now famous Boirie study, but they tested milk-protein concentrate, which is 80% casein and 20% whey, instead of whey or casein alone (C. Gaudichon et aL, "Net postprandial utilization of L1SNI-labeled milk protein nitrogen is influenced by diet composition in humans," Journal of Nutrition, 129:890-95, 1999).
This study also analyzed the utilization of milk-protein concentrate with carbs and without. Researchers confirmed that a 30g dose of milk-protein concentrate is digested over a long period of time (in this case, it took up to eight hours). Another finding was that adding carbs increases absorption time, but only slightly improves utilization efficiency. Milk protein alone has a net postprandial protein utilization efficiency (NPPU) - a measurement we can use to compare proteins - of about 8O%. When you add 100g of simple carbohydrate, the NPPU goes up to only 85%.