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Clinical Trial
. 2016 Jan 1;310(1):E73-80.
doi: 10.1152/ajpendo.00365.2015. Epub 2015 Nov 3.

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

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Clinical Trial

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

Il-Young Kim et al. Am J Physiol Endocrinol Metab. .
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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 ~40 g [moderate protein (MP)], which has been reported to maximize the protein synthetic response or ~70 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 l-[(2)H5]phenylalanine and l-[(2)H2]tyrosine. Subjects were randomly assigned into an exercise (X, n = 12) or resting (R, n = 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(-1)·min(-1)). 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 < 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 < 0.0001). HP resulted in greater plasma essential amino acid responses (P < 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.

Keywords: essential amino acids; optimal protein intake; protein turnover; stable isotope tracers.

Figures

Fig. 1.
Fig. 1.
Tracer infusion protocol. X, exercise; R, resting.
Fig. 2.
Fig. 2.
Plasma enrichments of infused tracers (A: Phe; B: Tyr) before and following a meal intake containing ∼40 g [moderate protein (MP)] or ∼70 g [high protein (HP)] of dietary protein with prior resistance exercise (X) or time-matched resting (R). Values are expressed as means ± SE. TTR, tracer-to-tracee ratio.
Fig. 3.
Fig. 3.
Changes in rates of whole body protein net balance (NB), synthesis (PS), and breakdown (PB) from the fasted state in response to meal containing ∼40 g (MP) or ∼70 g (HP) of dietary protein with prior resistance exercise (X) or time-matched resting (R). *Significantly different from MP within the same activity group (P < 0.0001). Values are expressed as means ± SE.
Fig. 4.
Fig. 4.
Muscle protein fractional synthesis rate (MPS; %/h) following a meal intake containing ∼40 g (MP) or ∼70 g (HP) of dietary protein with prior resistance exercise (X) or time-matched resting (R). Values are expressed as means ± SE.
Fig. 5.
Fig. 5.
Plasma concentrations of total essential (EAA; A) and nonessential amino acids (NEAA; B) in the fasted states (time at 135 min) and following meal intake containing ∼40 g (MP) or ∼70 g (HP) of dietary protein with prior resistance exercise (X) or time-matched resting (R). For the EAA, there were a protein-by-time interaction (P < 0.00001) and a protein amount effect (P < 0.001): HP was significantly higher than MP. For NEAA, there was only a protein amount effect (P < 0.0001): HP was significantly higher than MP. Values are expressed as means ± SE.
Fig. 6.
Fig. 6.
Plasma concentrations of glucose (A) and insulin (B) in the fasted states (times at 90 and 180 min) and following intake of meals containing ∼40 g (MP) or ∼70 g (HP) of dietary protein with prior resistance exercise (X) or time-matched resting (R). Both insulin and glucose were elevated upon meal intake (P < 0.0001). However, there was no protein effect for insulin (P = 0.4053) and for glucose (P = 0.0866). Values are expressed as means ± SE.

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