Protein blend ingestion following resistance exercise promotes human muscle protein synthesis

Journal Title (Medline/Pubmed accepted abbreviation): J. Nutr.
Year: 2013
Volume: 143
Page numbers: 410-416
doi (if applicable):10.3945/jn.112.168021

Summary of background and research design:
Background: Protein is recommended after resistance training (i.e. weight lifting) in order to maximize protein muscle synthesis.  Different types of protein (ex. soy, milk, beef, etc.) have different amino acid profiles and different absorption kinetics.  It is unknown what combination of protein is optimal for maximum protein synthesis.

A recent study in animal models showed that a protein blend containing soy and dairy proteins (whey and casein) contribute benefits of all the individual protein sources (whey- high in leucine, fast absorption; soy- medium rate of absorption, other nutritional benefits such as antioxidants; casein- slow digestion and therefore prolonged amino acid delivery).  This blend was tested in humans in this study.

Hypothesis: A blend of soy, whey, and casein taken after exercise will initiate and sustain muscle protein synthesis better than whey protein alone.

Experimental Design: randomized, double-blind, parallel groups

Subjects: healthy, recreationally active volunteers (n = 17 males, n = 2 females), age about 22-27 years old

Treatments: The doses were adjusted so that leucine content was equivalent (1.8 g).  The protein was dissolved in 300 mL (10 oz.) of water.  It was enriched with 8% 13C-labeled phenylalanine to monitor its utilization by skeletal muscle.
  1. Protein blend: 0.35 g/kg body weight (19.3 ± 1.1 g total protein), 50% casein, 25% whey, 25% soy
  2. Whey protein: 0.30 g/kg body weight (17.7 ± 0.9 g total protein)

The participants were first evaluated for general health parameters, body composition using dual x-ray absorptiometry (DXA), and 1 repetition maxima (1RM) for leg extension.  On the day of the experiment, the participants consumed a standardized breakfast.  Two catheters were inserted, one in their arm for administration of amino acid isotopes and one in their hand (other side of the body) for blood draws.  Muscle biopsies were acquired 2 and 4 hours after the start of the infusion of labeled amino acids to determine resting fractional synthesis rate (i.e. resting rate of protein synthesis/breakdown).  They then performed a high intensity, lower body-focused resistance exercise training session which included 8 sets of 10 repetitions of leg extensions at increasing workloads (55% 1RM for set 1, 60% 1RM for set 2, 65% 1RM for set 3, and 70% 1RM for sets 4-8).  One hour after exercise, a third muscle biopsy was acquired and then they consumed one of the two treatments.  Two more muscle biopsies were acquired 3 and 5 hours after completing exercise.  Blood samples were acquired throughout the protocol.  Blood was analyzed for free amino acid concentration, glucose, lactate, insulin.  The rate of muscle synthesis was calculated using the isotope enrichment of muscle samples (see manuscript for more details).
Summary of research findings:
  • Blood glucose levels remained steady throughout the trial.
  • Blood insulin concentrations rose after ingestion of either of the beverages.  It returned to baseline after 40 min for the protein blend and after 60 min after whey protein with no significant differences between groups.
  • Blood lactate concentrations rose after consumption of both beverages.  They returned to baseline by 60 min after consuming the protein blend and by 80 min after consuming the whey protein.
  • Blood phenylalanine concentrations (a representative amino acid) were above baseline for 180 min after consuming the protein blend and for 100 min after consuming the whey protein.  Whey protein led to a greater peak phenylalanine concentration at about 35 μM 40 min after ingestion.  At 80 min and between 120-180 min, phenylalanine concentrations were greater after consumption of the protein blend compared to the whey.
  • Blood concentrations of the branched chain amino acids (BCAAs; leucine, valine, and isoleucine) reached greater peaks after consumption of whey vs. the protein bland.  However, concentrations of valine were more sustained during the recovery period.
  • Activation of the mTOR complex (cellular signaling complex that promotes muscle protein synthesis) was apparent at 2 and 4 hours after ingestion of both beverages.  The only difference between groups was phosphorylation of S6K1, a protein downstream of mTOR, which was significantly above baseline at 4 hrs after consumption of the protein blend but not the whey protein.
  • The fractional rate of protein synthesis was elevated above rest in the early (0-2 hrs) and late (2-4 hrs) phases for the protein blend but only in the late phase for whey protein.  With that said, there were no significant difference between groups for early or late periods or for the entire period.
Key practice applications: Whey protein is often promoted for its “superior” ability to stimulate protein muscle synthesis after exercise.  However, this study shows that a protein blend with 25% soy, 25% whey, and 50% casein stimulates protein muscle synthesis to a similar degree when the same amount of leucine was consumed.
     Leucine is a stronger anabolic signal than any other amino acid.  The fact that whey protein has more leucine than many other high quality protein sources may be one of the only reasons that whey is so highly publicized for its anabolic potential.
     As has been seen in previous studies, whey protein caused a high, initial spike in blood amino acid concentrations, yet amino acids remained elevated above baseline for a longer period of time with the protein blend.  Both profiles have their own advantages and either may be superior in different circumstances.  For example, whey protein may be better if you want the initial amino acid spike and are able to consume more protein after about 2 hrs (when blood amino acids start to return to baseline).  However, if you work out before bed or before your afternoon shift at work where food is prohibited, then the protein blend may be better.
     Therefore, it is important to consume high quality protein that is high in leucine (1.8 g in this study) immediately after the meal.  Use nutritional strategies to maintain high amino acid concentrations for long after your work out to maximize muscle protein synthesis.
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