Myofibrillar protein synthesis following ingestion of soy protein isolate at rest and after resistance exercise in elderly men

Journal Title (Medline/Pubmed accepted abbreviation): Nutr. Metab.
Year: 2012
Volume: 9
First Page :57
doi : 10.1186/1743-7075-9-57

Summary of Background and Research Design

Background: Muscle tissue in older adults exhibits a weaker anabolic response to ingestion of amino acids, leading to age-induced muscle loss.  Several studies have shown that the source of protein (e.g. soy, whey, casein) stimulates different responses in muscle.

Research goals: To compare the response of muscle to ingestion of 0, 20, or 40 g of soy protein to the response of muscle after ingestion of the same amounts of whey protein after resistance training in healthy, elderly men.

Subjects: Men age 71 ± 5 y, n = 30

Experimental design: randomized, parallel groups

Treatments : Beverage containing 20 or 40 g soy protein isolate.  The data were then compared to a previous study that looked at 0, 20, and 40 g whey protein isolate.

Protocol : The participants were first evaluated for height, weight, body composition (DEXA), blood pressure, and physical ability using a short physical performance battery (walk, standing from sitting on a chair, and balance tests).  A blood sample was also evaluated for fasting blood glucose, triglycerides, and cholesterol.  They were also evaluated for their 10 repetition maxima on unilateral knee extensor exercises.  They reported to the laboratory on two subsequent occasions after an overnight (10 hour) fast.  Upon arrival, CO2 from their breath was measured and an IV was inserted into their vein.  They performed 3 sets of unilateral knee-extensor exercises. Immediately after exercise, one of the two treatment beverages was consumed.  Using a controlled infusion of isotopically labeled amino acids, the rate of protein synthesis and protein breakdown could be calculated.  Blood and breath samples were acquired regularly for 240 min.  Muscle biopsies were also acquired for analysis of amino acid amount and composition.

Summary of Research Findings
  • Plasma insulin increased upon beverage consumption about 2.2 fold for both 20 and 40 g soy beverages.
  • The maximum concentration of leucine (a branched chain amino acid, BCAA) in the blood occurred at 1.0-1.5 hours after consumption of 20 g soy, 20 g whey, or 40 g whey and at 1.5-2.0 hours for 40 g soy.  Peak concentrations were higher for consumption with whey then consumption of soy.
  • The rate of leucine breakdown was similar for 20 g soy, 40 g soy, and 40 g whey, but was lesser for 20 g whey.
  • Myofibrillar protein fractional synthetic rate (FSR; the rate of protein synthesis) was similar between the 20 and 40 g soy groups.  For both doses, soy myofibrullar protein FSR was greater in response to whey then soy.
  • Net muscle protein synthesis was not increased after the post-workout beverage containing 20 g soy protein, but it was in response to the 40 g soy protein post-workout beverage.  In contrast, whey protein at both 20 and 40 g doses stimulated net muscle protein synthesis after exercise.

Key practice applications

Elderly muscle exhibits a hampered anabolic response to post-workout protein beverages.  Therefore, a post-workout beverage must be optimized so that elderly people can benefit maximally from exercise.  This study showed that whey protein is more effective at stimulating net muscle protein synthesis than soy protein, perhaps due to the higher amount of leucine in whey. 

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