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

Abstract

Background: Nutrient stimulation of muscle protein anabolism is blunted with aging and may contribute to the development and progression of sarcopenia in older adults. This is likely due to insulin resistance of protein metabolism and/or endothelial dysfunction with a reduction in nutritive flow, both of which can be improved by aerobic exercise.

Objective: Our objective was to determine whether increasing physical activity can enhance the muscle protein anabolic effect of essential amino acid (EAA) + sucrose intake in older subjects by improving nutritive flow and/or insulin signaling.

Design: Using a randomized crossover design, we measured in older subjects [n = 6, 70 ± 3 y of age, BMI (in kg/m2) of 25 ± 1] the acute effects of increasing physical activity with aerobic exercise, as compared with normal sedentary lifestyle, on the response of blood flow, microvascular perfusion, insulin signaling, and muscle protein kinetics to EAA+sucrose intake.

Results: No differences between treatment groups were found in the basal state. The change from the basal state in blood flow, muscle perfusion, phenylalanine delivery, net balance, and muscle protein synthesis during the consumption of EAA+sucrose was significantly higher after the exercise than after the control treatment (P < 0.05). Insulin signaling increased during EAA+sucrose ingestion in both groups (P < 0.05).

Conclusions: Our data indicate that a prior bout of aerobic exercise increases the anabolic effect of nutrient intake in older adults. This effect appears to be mediated by an exercise-induced improvement in nutrient-stimulated vasodilation and nutrient delivery to muscle rather than to improved insulin signaling. This trial was registered at clinicaltrials.gov as NCT00690534.

FIGURE 1.

Study design. EAA, essential amino acids.

FIGURE 2.

Blood flow, microvascular perfusion, phenylalanine net balance across the leg, and mixed-muscle-protein fractional synthetic rate in the basal period and during the consumption of EAA+sucrose in healthy older adults after a control or aerobic exercise treatment. n = 6 for both treatments by using a randomized crossover design. Paired t tests were used to compare basal with EAA+sucrose within treatments (control and exercise) and delta scores between treatments. ^*Significantly different from basal, P < 0.05. ^†Exercise:EAA+sucrose delta score > control:EAA+sucrose delta score (P < 0.05 unless otherwise noted). EAA, essential amino acids.

FIGURE 3.

Representative contrast ultrasound images of Definity microspheres infused into the vastus lateralis of one subject during each of the 4 conditions: control:basal, control:EAA+sucrose, exercise:basal, and exercise:EAA+sucrose. Each image was captured from the 10-s pulsing interval. EAA, essential amino acids.

FIGURE 4.

Skeletal muscle Akt, mTOR, and 4E-BP1 phosphorylation (relative to total protein expression) in the basal state and at 1.5 and 3.0 h of EAA+sucrose consumption in healthy older adults after a control (▪) or aerobic exercise (□) treatment. n = 6 for each treatment by using a randomized crossover design. Representative basal images are included for comparison. Paired t tests were used to compare the basal state with EAA+sucrose at 1.5 and 3.0 h within treatments (control and exercise) and delta scores between treatments.^*Significantly different from basal. P < 0.05. Akt, protein kinase B; AU, arbitrary units; EAA, essential amino acids; mTor, mammalian target of rapamycin; 4E-BP1, 4E-binding protein 1.