Basal muscle amino acid kinetics and protein synthesis in healthy young and older men

Abstract

Context: Sarcopenia is associated with loss of strength and function, eventually leading to loss of independence. Some studies suggest that basal muscle protein turnover is reduced with aging, but other studies do not confirm this finding.

Objective: To determine if aging per se affects basal muscle protein turnover in men.

Design and setting: Cross-sectional study conducted from June 1997 to July 2000 in a general US community.

Participants: Twenty-six young (mean [SE] age, 28 [2] years) and 22 older (mean [SE] age, 70 [1] years) men, who were healthy and independent based on activities of daily living, physical examinations, and screening tests. Subjects were excluded if they had cardiac, pulmonary, liver, or kidney disease; any impairment in activities of daily living; or steroid use.

Main outcome measures: We measured basal muscle protein and amino acid kinetics, based on stable isotope techniques with femoral arteriovenous catheterization and muscle biopsies. Three models (arteriovenous balance, three-pool, and fractional synthesis rate) were used to estimate the metabolic parameters.

Results: Mean (SE) total leg volume was 9.60 (0.32) L in older men vs 10.83 (0.43) L in younger men, which suggests muscle loss in the older men. Net muscle protein balance was similar in both groups (older men, - 19 [2] nmol/min per 100 mL of leg volume vs younger men, - 21 [2] nmol/min per 100 mL of leg volume; P =.51). Small differences were found in mean (SE) muscle protein synthesis in comparisons of older vs younger men: arteriovenous balance, 48 (5) nmol/min per 100 mL of leg volume vs 32 (3) nmol/min per 100 mL of leg volume; P =.004; three-pool, 58 (5) nmol/min per 100 mL of leg volume vs 43 (4) nmol/min per 100 mL of leg volume; P =.04; and fractional synthesis rate, 0.0601 (0.0046) %/h vs 0.0578 (0.0047) %/h; P =.73. Small differences were also found in mean (SE) muscle protein breakdown: arteriovenous balance, 66 (5) nmol/min per 100 mL of leg volume in older vs 53 (4) nmol/min per 100 mL of leg volume in younger men, P =.045; and three-pool, 76 (6) nmol/min per 100 mL of leg volume vs 64 (5) nmol/min per 100 mL of leg volume, P =.14.

Conclusion: Differences in basal muscle protein turnover between older and younger men do not appear to explain muscle loss that occurs with age.

Figure 1

Infusion Protocol After the insertion of the femoral artery and venous catheters, l-[ring-² H₅]-phenylalanine was infused for 5 hours. Indocyanine green (ICG) was infused from 230 to 270 minutes to measure rate of blood flow. At the time indicated by the arrows, muscle biopsy samples were taken and blood samples from femoral artery, femoral vein, and wrist vein were collected.

Figure 2

Relationship Between Total Leg Volume and Leg Muscle Volume Muscle volume measured by magnetic resonance imaging in 7 young (gray circles) and 10 older (black circles) subjects was significantly correlated to the leg volume of the same subjects measured using an anthropometric method.

Figure 3

Mixed Muscle Protein Fractional Synthesis Rate (FSR) Mixed muscle protein FSR was similar in 26 healthy young and 22 healthy older men, with a power of β=0.06. The least significant number of subjects to be studied to achieve an α=.05 with β=.80 should have been 3208 (sum of both groups). Error bars indicate SE.

Figure 4

Relationship Between Total Testosterone and Mixed Muscle Protein Fractional Synthesis Rate (FSR) Basal total testosterone concentration was not significantly correlated with basal mixed muscle protein FSR in 19 young (gray circles) and 21 older (black circles) subjects.