Age-Related anabolic resistance of myofibrillar protein synthesis is exacerbated in obese inactive individuals

Context:
A diminished muscle anabolic response to protein nutrition may underpin age-associated muscle loss.
Objective:
To determine how chronological and biological ageing influence myofibrillar protein synthesis (MyoPS).
Design:
Cross-sectional comparison.
Setting:
Clinical Research Facility.
Participants:
Ten older lean (OL: 71.7 ± 6 yrs; ≤25 kg⋅m^-2), 7 older obese (OO: 69.1 ± 2 yrs, BMI ≥30 kg⋅m^-2) and 18 young lean individuals (YL: 25.5 ± 4 yrs, BMI ≤25 kg⋅m^-2).
Intervention:
Skeletal muscle biopsies obtained during a primed-continuous infusion of L-[ring-¹³C6]-phenylalanine.
Main outcome measures:
Anthropometrics, insulin resistance, inflammatory markers, habitual diet, physical activity, MyoPS rates and fibre-type characteristics.
Results:
Fat mass, insulin resistance, inflammation and Type II fibre intramyocellular lipid were greater, and daily step-count lower, in OO compared with YL and OL. Postprandial MyoPS rates rose above postabsorptive values by ∼81% in YL (P < 0.001) and ∼38% in OL (P = 0.002, not different from YL) and ∼9% in OO (P = 0.11). Delta change in postprandial MyoPS from postabsorptive values was greater in YL compared with OL (P = 0.032) and OO (P < 0.001). Absolute postprandial MyoPS rates and delta postprandial MyoPS change were associated with step-count (r² = 0.33, P = 0.015) and leg fat mass (r² = 0.4, P = 0.006), respectively, in older individuals. Paradoxically, lean mass was similar between groups and muscle fibre area was greater in OO vs. OL (P = 0.002).
Conclusion:
Age-related muscle anabolic resistance is exacerbated in obese inactive individuals, with no apparent detriment to muscle mass.