Abstract
BackgroundMuscle protein synthesis and muscle net balance plateau after moderate protein ingestion in adults. However, it has been suggested that there is no practical limit to the anabolic response of whole-body net balance to dietary protein. Moreover, limited research has addressed the anabolic response to dietary protein in adolescents. The present study determined whether whole-body net balance plateaued in response to increasing protein intakes during post-exercise recovery and whether there were age- and/or sex-related dimorphisms in the anabolic response.MethodsThirteen adults [7 males (M), 6 females (F)] and 14 adolescents [7 males (AM), 7 females (AF) within ~ 0.4 y from peak height velocity] performed ~ 1 h variable intensity exercise (i.e., Loughborough Intermittent Shuttle Test) prior to ingesting hourly mixed meals that provided a variable amount of protein (0.02–0.25 g·kg− 1·h− 1) as crystalline amino acids modeled after egg protein. Steady-state protein kinetics were modeled noninvasively with oral L-[1-13C]phenylalanine. Breath and urine samples were taken at plateau to determine phenylalanine oxidation and flux (estimate of protein breakdown), respectively. Whole-body net balance was determined by the difference between protein synthesis (flux – oxidation) and protein breakdown. Total amino acid oxidation was estimated from the ratio of urinary urea/creatinine.ResultsMixed model biphasic linear regression explained a greater proportion of net balance variance than linear regression (all, r2 ≥ 0.56; P < 0.01), indicating an anabolic plateau. Net balance was maximized at ~ 0.15, 0.12, 0.12, and 0.11 g protein·kg− 1·h− 1 in M, F, AM, and AF, respectively. When collapsed across age, the y-intercept (net balance at very low protein intake) was greater (overlapping CI did not contain zero) in adolescents vs. adults. Urea/creatinine excretion increased linearly (all, r ≥ 0.76; P < 0.01) across the range of protein intakes. At plateau, net balance was greater (P < 0.05) in AM vs. M.ConclusionsOur data suggest there is a practical limit to the anabolic response to protein ingestion within a mixed meal and that higher intakes lead to deamination and oxidation of excess amino acids. Consistent with a need to support lean mass growth, adolescents appear to have greater anabolic sensitivity and a greater capacity to assimilate dietary amino acids than adults.
Highlights
Muscle protein synthesis and muscle net balance plateau after moderate protein ingestion in adults
When normalized to Fat-free mass (FFM), whole-body net balance increased up to a plateau in all groups at a protein intake corresponding to 0.17 ± 0.05, 0.14 ± 0.04, 0.14 ± 0.04, and 0.12 ± 0.02 g·kgFFM− 1·h− 1 in M, F, adolescents [7 males (AM), and adolescent females (AF), respectively, with no differences in the breakpoint intake
While the differences in feeding pattern may have influenced the maximal relative protein intake at the breakpoint, we believe that the present data, as well as others that demonstrate plateaus in muscle [6,7,8], non-muscle [7], and whole-body protein synthesis [24] are supportive of a maximal anabolic response to mixed meal ingestion
Summary
Muscle protein synthesis and muscle net balance plateau after moderate protein ingestion in adults. It has been suggested that there is no practical limit to the anabolic response of whole-body net balance to dietary protein. The capacity to assimilate dietary amino acids into new proteins is greater when considering all body protein pools such as the labile pool within the splanchnic bed [9]. This has led to the suggestion that there is no practical limit to the anabolic response to protein ingestion such that increasing protein intake at each meal is the most efficient way to maximize whole-body net balance over a 24-h time period [10,11,12]. To optimize dietary protein efficiency during post-exercise recovery, it is important to establish a protein intake that maximizes net balance while simultaneously minimizing oxidation
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