A moderate acute increase in physical activity enhances nutritive flow and the muscle protein anabolic response to mixed nutrient intake in older adults

Journal Title (Medline/Pubmed accepted abbreviation):Am. J. Clin. Nutr.
Year: 2012
Volume: 95
Page Numbers :1403-1412
doi :10.3945/ajcn.111.020800

Summary of Background and Research Design

Background: Muscle tissue in older adults exhibits a weaker anabolic response to amino acids and insulin in the blood, leading to sarcopenia, or a reduction in muscle mass and strength. It has been shown that: 1) a supplement with essential amino acids (EAA) and sugar and 2) aerobic exercise can independently stimulate anabolism in muscle.

Hypothesis: Aerobic activity will augment the response of muscle tissue to an EAA+sucrose supplement in older adults.

Subjects: Sedentary adults, age 70 ± 3 y, 3 male and 3 female

Experimental design:randomized, cross-over

Treatments : Aerobic exercise or no exercise the evening before consumption of an EAA+sucrose supplement

Protocol : The participants reported to the laboratory on two evenings, 4-6 weeks apart.  On one day, they followed an aerobic exercise routine, and on the other day they did not exercise.  Meals around the exercise session were controlled.  The next morning, the participants consumed a beverage with 20 g EAA and 35 g sucrose over the course of 3 hours.  Using controlled intravenous infusions of isotopically labeled amino acids, the rate of protein synthesis and protein breakdown was monitored.  During the evaluation, blood flow, plasma glucose concentration, and plasma insulin concentration were also monitored.  Muscle biopsies were acquired to assess protein concentration and concentration of enzymes involved in protein synthesis and breakdown.

Summary of Research Findings
  • During consumption of the EAA+sucrose beverage, blood flow and microvascular perfusion were significantly greater when exercise was performed the night before.
  • Blood insulin and glucose concentrations increased upon consumption of the beverage in both groups, with no difference between groups.
  • In the morning, before beverage consumption, net protein balance (protein synthesis minus protein breakdown) was negative in both groups.  During/after consumption of the beverage, net protein synthesis remained negative when no exercise was performed, but shifted to positive when exercise was performed the evening before.
  • Some enzymes were up-regulated in response to beverage consumption, but there were no statistically significant differences between treatments.

Key practice applications

The anabolic nature of amino acids was indeed observed when exercise was performed the evening before consumption of the amino acid beverage.  An increase in blood flow and amino acid delivery to muscle cells was key in the mechanism by which exercise enhanced these anabolic effects.  Routine aerobic exercise can help older adults build and maintain muscle mass, thereby preventing debilitating age-related muscle loss.

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