Whey and casein labeled with l-[1-13C]leucine and muscle protein synthesis: effect of resistance exercise and protein ingestion


Journal Title (Medline/Pubmed accepted abbreviation):  Am. J. Physiol. Endocrinol. Metab.
Year: 2011
Issue: 300
Page numbers: E231-242
doi (if applicable): 10.1152/ajpendo.00513.2010
Summary of Background and Research Design

Background: The combination of repeated training sessions and at least 20 g of post-workout protein has been shown to stimulate muscle protein synthesis (MPS). Protein from cow’s milk is composed mostly of whey and casein; whey is known to be absorbed and utilized more rapidly while casein is known to exhibit slower yet more long-lasting effects.

Isotopes are atoms that have a slightly different subatomic composition yet have practically identical properties within a molecule. They are present at predictable proportions in nature, and therefore can be incorporated into “enriched” compounds and subsequently be traced through the experiment. For example, 13C composes 1% of all carbon atoms in nature. Its low natural abundance is useful in that the scientist can administer 13C-labeled leucine, for example, into a human and then monitor its fate.

α-ketoisocaproate (KIC)- a leucine metabolite


Hypotheses: 1) Cow’s milk can be effectively labeled with 13C leucine in both casein and whey. 2) MPS would be stimulated more rapidly yet transiently with 13C-labeled whey and more moderately yet long-lasting with 13C-labeled casein.
 
Subjects: Healthy, moderately active, male Americans that were not involved in regular participation of aerobic or resistance training in the last 6 mos. n = 17, experimental group: n=9, age = 28 ± 2 yrs old, control group: n=8, 26 ± 2 yrs old.
 
Experimental design:randomized into an experimental group (whey one time and casein one time after exercise) and a control group (water after exercise one time).
 
Treatments and protocol: Before the experiment, baseline parameters including 1RM for one-legged, seated leg extensions were obtained. On the day of the experiment, time t=0 signified the end of the exercise session. At t= -180 min, a 15 µmol/kg primer was administered followed by a 15 µmol/kg/hr continuous intravenous infusion of l-[1-13C] leucine. The leucine was administered with 0.9% sodium chloride. After initiation of the tracer infusion, a femoral artery and vein of the exercise leg were cannulated (i.e. a small tube was inserted). Muscle biopsies were obtained from the lateral portion of the vastus lateralis muscle in the exercise leg (outer muscle in the quadriceps). Muscle tissue was assayed for myofibrillar protein synthesis and other physiological markers for protein synthesis.
                Acute heavy resistance training consisted of 10 sets of 8 repetitions of leg extensions at 80% of the participant’s 1RM. Immediately post-workout, participants imbibed water, 0.30 g/kg lean body mass (LBM) of [13C]whey, or the same amount of [13C]casein.
                Intrinsically-labeled milk was produced by administering l-[1-13C]leucine into cows. Milk was collected and separated into casein and whey.
                Physiological markers in the participants’ muscle included the following: 1) myofibrillar fractional synthesis rate, which was calculated based on incorporation of 13C-labeled leucine into muscle protein, and 2) phosphorylation of proteins downstream on the insulin receptor (reports on the activities of muscle synthesizing enzymes).
 
Summary of research findings:
  • Percent enrichment was determined to be 10.0% in both casein and whey.
  • Peak insulin concentrations as well as the total insulin response [area under the curve (AUC) for insulin concentration vs. time (15-360 min)] were higher in the whey protein group than the casein group.
  • Plasma peak concentrations of leucine, valine, isoleucine, arginine, total essential amino acids, and total amino acids were higher after whey ingestion compared to casein ingestion. However, the plasma concentration of all these molecules remains elevated for a longer amount of time after casein ingestion compared to whey, at various extents. These biomarkers, in general, did not change after water consumption.
  • The proportion of [1-13C]-labeled to unlabeled α-ketoisocaproate (KIC) was higher than baseline following exercise and did change notably between 60 to 210 to 360 min post-exercise. The proportion was slightly higher following casein than whey and both protein beverages resulted in higher proportions compared to water.
  • Whey protein stimulated a rapid and high increase in fractional synthesis rate (FSR). The peak FSR after casein ingestion was more moderate but persisted into the 3.5 to 6 hr post-exercise sample collection period.
  • Phosphorylation of various cell signaling proteins was observed in addition to rates of protein synthesis. Infusion of leucine induced protein synthesis before exercise began. Whey exhibited a higher amount of total 4E-BP1 than casein. The ratio of phosphorylated 4E-BP1 (inactive) to total 4E-BP1 was lower for whey than it was for casein. This would tend to support an increased effect of casein vs. whey on protein synthesis, although actual measures of protein synthesis only found this to be true in the later half of the 6-h postprandial period.

Interpretation of findings/Key practice applications:

 The whey protein induced a transiently greater increase in plasma amino acids and muscle protein synthesis in the first 3.5 hours postprandial versus casein. However, the casein treatment promoted more sustained plasma amino acid levels and higher protein synthesis in the later half of the study (3.5 to 6 h postprandial. In the end, both proteins resulted in comparable amounts of protein synthesis by the end of the 6-h study. Thus, it appears that whey and casein complement each other well for promoting muscle protein synthesis.
 
Limitations:

Exercise causes duress on muscles and can stimulate protein degradation as well as protein synthesis. Protein degradation was not measured. Therefore the net rate of protein synthesis could not be calculated. This is an important consideration, because casein tends to reduce protein breakdown more so than does whey. Age of the subjects is also a significant factor. Casein tends to increase overall net protein balance more than whey in younger adults, whereas whey stimulates higher net protein balance in older adults.
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