Protein Blends, Not Isolates Promote Maximal Skeletal Muscle Protein Retention(!) - It's Not About How Much You Pump into the Muscle, It's About How Much You Retain

Scientific evidence suggests: There is not one optimal protein to build muscle - it's the mix of fast to slow proteins that's key.

For someone like yourself, who's making sure to get his daily dose of SuppVersity Science News, the results Reidy et al. present in their latest paper in the Journal of Applied Physiology can hardly be surprising. I have, after all, written about the superiority of whey + casein blends as potential muscle builders only recently ("When Whey & Casein Unite in the Spirit of True Physique Improvements, BCAAs & Glutamine Better Shut the F*** Up"  | (re-)read the article). It was thus only to be expected that a study in which the scientists from the University of Texas Medical Branch compared the effects of the prolonged hyperaminoacidemia that's associated with the ingestion of a blend of plant (25% soy) and dairy (50% casein, 25% whey) proteins (with varying digestion rates) to that of a pure rapidly digested whey would yield a definite points win for the "time-released" formula.
The reasons why it's still well worth taking a closer look at the study results are (a) the fact that the f**** up supplement industry is still trying to tell you that protein blends would be inferior to overpriced isolates and (b) the educative value of the post-workout + post-supplementation serum amino acid profiles Reidy et al. observed the 16 healthy, young subjects (age range: 19 –30 yr) who participated in their double-blind, randomized clinical trial (with body fat levels of >24% those were certainly no physical culturists, though ;-)

Figure 1: Graphical overview of the study design (Reidy. 2014)

As you can see in Figure 1 the study protocol involved a standardized resistance training session in the course of which the subjects who had been kept on a diet containing 20% protein, 60% carbohydrate, and 20% fat at 12 kcal/kg for 72h, performed leg extensions on a Cybex-VR2 (Medway, MA), i.e. 8 sets of 10 repetitions at 55% (set 1), 60% (set 2), 65% (set 3), and 70% (sets 4 – 8) of the participants previously determined 1 RM with 3-min rest between sets, before they consumed the protein beverages (Whey or Blend) exactly 1 h postexercise.

Figure 2: Net phenylalanine enrichment (left) and inward and outward transport (right)

The ingestion of the beverages of which the blend and the whey protein contained of 20.1 g total protein (providing 1.9 g leucine, 1.0 g phenylalanine, 1.3 g valine, and 9.0 g EAA; 50% protein from sodium caseinate, 25% protein from whey protein isolate, and 25% protein from soy protein isolate) and 17.3 g of protein (providing 1.9 g leucine, 0.6 g phenylalanine, 1.1 g valine, and 8.7 g EAA; 100% whey protein isolate), respectively, lead to significant increases in amino acid transporter activity (2/SLC38A2, proton-assisted amino acid transporter 1/SLC36A1, cationic amino acid transporter 1/SLC7A1).

"However, the ingestion of the protein blend resulted in a prolonged and positive net phenylalanine balance during postexercise recovery compared with whey protein (P 0.05)." (Reidy)

In view of identical postexercise myofibrillar protein synthesis in both groups this difference may appear negligible. If you've been following my articles about the often oversimplified protein synthesis and increases in skeletal muscle mass, you should be aware that net retention and not fractional synthesis is the term you have to look for, when you're analyzing corresponding studies.
Reference: 

• Reidy, Paul T., et al. "Soy-dairy protein blend and whey protein ingestion after resistance exercise increases amino acid transport and transporter expression in human skeletal muscle." Journal of Applied Physiology 116.11 (2014): 1353-1364.