The anabolic response to a meal containing different amounts of protein is not limited by the maximal stimulation of protein synthesis in healthy young adults

Abstract

We have determined whole body protein kinetics, i.e., protein synthesis (PS), breakdown (PB), and net balance (NB) in human subjects in the fasted state and following ingestion of ϳ40 g [moderate protein (MP)], which has been reported to maximize the protein synthetic response or ϳ70 g [higher protein (HP)] protein, more representative of the amount of protein in the dinner of an average American diet. Twenty-three healthy young adults who had performed prior resistance exercise (X-MP or X-HP) or time-matched resting (R-MP or R-HP) were studied during a primed continuous infusion of]phenylalanine and L-[ 2 H2]tyrosine. Subjects were randomly assigned into an exercise (X, n ϭ 12) or resting (R, n ϭ 11) group, and each group was studied at the two levels of dietary protein intake in random order. PS, PB, and NB were expressed as increases above the basal, fasting values (mg·kg lean body mass Ϫ1 ·min Ϫ1 ). Exercise did not significantly affect protein kinetics and blood chemistry. Feeding resulted in positive NB at both levels of protein intake: NB was greater in response to the meal containing HP vs. MP (P Ͻ 0.00001). The greater NB with HP was achieved primarily through a greater reduction in PB and to a lesser extent stimulation of protein synthesis (for all, P Ͻ 0.0001). HP resulted in greater plasma essential amino acid responses (P Ͻ 0.01) vs. MP, with no differences in insulin and glucose responses. In conclusion, whole body net protein balance improves with greater protein intake above that previously suggested to maximally stimulating muscle protein synthesis because of a simultaneous reduction in protein breakdown. essential amino acids; optimal protein intake; protein turnover; stable isotope tracers.THE PRINCIPAL NUTRITIONAL goal of a protein-rich meal is to induce an anabolic state in which protein synthesis exceeds breakdown. Several recent studies, including our own (25), indicate that the maximum acute stimulation of muscle protein synthesis (MPS) occurs with ingestion of ϳ 20 -35 g of high-quality protein (20,25,31) or more specifically 0.24 g·kg body wt Ϫ1 ·meal Ϫ1 in healthy young adults (19). The maximal dose in terms of stimulation of MPS is less than that typically consumed with the dinner meal in the average American diet, which generates a hypothesis that distributing the amount of protein intake throughout the day can more effectively stimulate anabolic response. However, the assertion that there is limited effectiveness of the conventional protein intake with dinner is based on incomplete assessment of the metabolic response of muscle protein. Importantly, the extent of muscle protein anabolism (the anabolic response) is not simply the response of MPS but rather the net balance between the response of protein synthesis and protein breakdown. We recently demonstrated the potential importance of suppression of protein breakdown in response to dietary intake of meals containing two levels of protein totaling either 0.8 or 1.5 g protein·kg Ϫ1 ·day Ϫ1 . We found that at both levels of dietary p...

Optimizing Protein Intake in Adults: Interpretation and Application of the Recommended Dietary Allowance Compared with the Acceptable Macronutrient Distribution Range

“…NB progressively increases as nitrogen intake increases above the RDA, as shown in Figure 1B. Whereas many discount the validity of those NB results for reflecting actual changes in LBM, the NB results are consistent with the results of stable isotope tracer studies that show that net protein synthesis increases linearly with increasing protein or amino acid intake (13–15). …”

Reduced plasma concentration of branched-chain amino acids in sarcopenic older subjects: a cross-sectional study

“…At present, no optimal or minimum plasma concentrations of the BCAA required for maintenance of SM mass have been defined. Increased plasma and intramuscular concentration of leucine, in particular, stimulates the muscle protein synthesis by activation of the mammalian target of rapamycin (mTOR) signalling pathway, as well as decreasing muscle protein breakdown (17,19,21) . Older adults may be less responsive to activation of the mTOR signalling pathway by amino acids than younger adults (anabolic resistance) (41)(42)(43) .…”

“…A postprandial increase in BCAA has been observed, leading to a rapid increase of plasma BCAA and uptake into muscle tissue (12)(13)(14)(15) . Intramuscular concentration of BCAA, particularly leucine, stimulates the muscle protein synthesis, as well as decreases muscle protein breakdown, and an anabolic response occurs when the synthesis exceeds the breakdown (16)(17)(18)(19)(20)(21)(22) . Increased availability of amino acids when ingested, infused or from increased protein intake has been shown to stimulate the muscle protein synthesis, and a positive linear relationship between essential amino acids intake and the anabolic response has been suggested (20,(22)(23)(24) .…”

Post‐intensive care syndrome: its pathophysiology, prevention, and future directions

“…Nutritional therapy is vital for the prevention of PICS, especially ICU‐AW. Adequate energy delivery and protein intake are the most important factors for muscle synthesis; moreover, energy debt is covered by catabolism mainly of the muscle, which is associated with lean body mass loss related to risk mortality . Previous studies on nutrition therapy targeted mortality and infectious complications as outcomes.…”