Abstract
Short-term (5–10 days) calorie restriction (CR) downregulates muscle protein synthesis, with consumption of a high protein-based diet attenuating this decline. Benefit of increase protein intake is believed to be due to maintenance of amino acid-mediated anabolic signaling through the mechanistic target of rapamycin complex 1 (mTORC1), however, there is limited evidence to support this contention. The purpose of this investigation was to determine the effects of prolonged CR and high protein diets on skeletal muscle mTORC1 signaling and expression of associated microRNA (miR). Twelve-week old male Sprague Dawley rats consumed ad libitum (AL) or calorie restricted (CR; 40%) adequate (10%, AIN-93M) or high (32%) protein milk-based diets for 16 weeks. Body composition was determined using dual energy X-ray absorptiometry and muscle protein content was calculated from muscle homogenate protein concentrations expressed relative to fat-free mass to estimate protein content. Western blot and RT-qPCR were used to determine mTORC1 signaling and mRNA and miR expression in fasted mixed gastrocnemius. Independent of dietary protein intake, muscle protein content was 38% lower (P < 0.05) in CR compared to AL. Phosphorylation and total Akt, mTOR, rpS6, and p70S6K were lower (P < 0.05) in CR vs. AL, and total rpS6 was associated with muscle protein content (r = 0.64, r2 = 0.36). Skeletal muscle miR expression was not altered by either energy or protein intake. This study provides evidence that chronic CR attenuates muscle protein content by downregulating mTORC1 signaling. This response is independent of skeletal muscle miR and dietary protein.
Highlights
Calorie restriction (CR) is a strategy to lose body fat and reduce total body mass
Though fatfree mass was statistically similar between calorie restricted (CR) and ad libitum (AL) at the conclusion of the 16-week feeding intervention, muscle protein content was 38% lower (7 ± 2 g; P < 0.05) in CR than AL (Figure 1D)
Phosphorylation status of AktSer473, mTORSer2448, p70S6KThr389, and rpS6Ser235/236 were (P < 0.05) 1.72 ± 0.11, 1.41 ± 0.08, 1.95 ± 0.05, and 3.47 ± 0.05 fold lower, respectively, in CR compared to AL rats, regardless of protein intake (Figure 2A)
Summary
Calorie restriction (CR) is a strategy to lose body fat and reduce total body mass. skeletal muscle is lost during CR, which may compromise successful weight management and negatively impact physical function. Energy Balance and Anabolic Signaling that CR, short-term CR (≤21 days), downregulates fasting MPS and blunts anabolic sensitivity to a protein-containing meal (Pasiakos et al, 2010; Areta et al, 2014; Hector et al, 2015; Murphy et al, 2015). Increasing dietary protein intake above the recommended dietary allowance (RDA; 0.8 g·kg−1·d−1) may attenuate declines in MPS, preserve anabolic sensitivity to a protein-containing meal, and spare skeletal muscle mass during short-term CR (Pasiakos et al, 2013; Areta et al, 2014). Whether prolonged CR and dietary protein level affect mTORC1 signaling, and result in modifications in fat-free mass has not been determined
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