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Endurance Exercise Attenuates Postprandial Whole-Body Leucine Balance in Trained Men

MAZZULLA, MICHAEL1; PAREL, JUSTIN T.2; BEALS, JOSEPH W.3; VAN VLIET, STEPHAN2; ABOU SAWAN, SIDNEY1; WEST, DANIEL W. D.1; PALUSKA, SCOTT A.4; ULANOV, ALEXANDER V.5; MOORE, DANIEL R.1; BURD, NICHOLAS A.2,3

Author Information
Medicine & Science in Sports & Exercise: December 2017 - Volume 49 - Issue 12 - p 2585-2592
doi: 10.1249/MSS.0000000000001394

Abstract

Dietary protein ingestion is necessary for the maintenance of whole-body protein mass throughout adult life. Specifically, the ingestion of protein at rest is used to replenish postabsorptive amino acid losses, which subsequently increases postprandial protein balance (8). It is well known that the increased metabolic demands associated with endurance exercise can contribute to the depletion of muscle and liver glycogen (7) and stimulate the loss of body fluids and electrolytes through sweat (16). However, endurance exercise can also significantly affect whole-body protein metabolism. For example, endurance exercise can stimulate the release of amino acids from muscle and body proteins through the stimulation of protein breakdown rates and induce a marked increase in amino acid oxidation rates (4,12,13,17). Moreover, endurance exercise shifts metabolic processes such that energy and substrates are diverted away from protein synthesis and toward events supporting muscle contraction (e.g., energy production via amino acid oxidation) (11,20,38). Thus, nutritional intake is required to compensate for the metabolic demands of prolonged endurance exercise, which includes exercise-induced amino acid oxidative losses and the resultant negative net protein balance that occurs during postexercise recovery (18,19).

In the resting state, 60%–70% of the ingested dietary protein-derived amino acids (e.g., leucine) are released into circulation (2), where the dietary amino acids can be used to support extrasplanchnic (e.g., muscle) protein metabolic needs. However, it has been demonstrated that an acute bout of exercise can increase markers of small intestinal damage such as plasma intestinal fatty acid binding protein (I-FABP) concentrations (33). The presumed compromise in epithelial integrity may negatively affect intestinal absorption and/or splanchnic sequestration of dietary amino acids (34). This altered splanchnic metabolism may limit plasma amino acid availability for the replacement of amino acid oxidative losses (primarily from skeletal muscle), which may subsequently attenuate the feeding-induced improvement in whole-body protein balance during recovery from exercise. Despite the well-established role of dietary protein ingestion for enhancing whole-body protein anabolism (22), no studies, to our knowledge, have compared the whole-body protein metabolic responses to the ingestion of a single mixed macronutrient meal with and without prior endurance exercise.

Therefore, the purpose of this study was to examine the effect of 60 min of treadmill running at 70% of V˙O2peak on postprandial leucine kinetics and net leucine balance in response to a practical mixed macronutrient meal, which provided ~18 g whole egg protein (~0.25 g·kg−1) to enhance muscle protein synthesis and 60 g of carbohydrate to facilitate postexercise glycogen resynthesis (21). To address the fate of dietary protein-derived leucine, we combined primed constant infusions of stable isotope amino acids with specifically produced intrinsically labeled eggs (31). We hypothesized that i) mixed meal ingestion would enhance postprandial leucine balance at rest and during postexercise recovery in trained young men, ii) the magnitude of increase in leucine balance during recovery would likely be attenuated by exercise-induced oxidative losses, and iii) given the potential for exercise-induced alterations in gut function, endurance exercise would attenuate the postprandial availability of dietary protein-derived leucine in circulation.

METHODS

Participants and ethical approval

Seven endurance-trained young men (age = 25.6 ± 2.3 yr, weight = 72.4 ± 2.9 kg, lean body mass = 60.4 ± 2.1 kg, V˙O2peak = 61.4 ± 2.9 mL·kg−1·min−1; mean ± SEM) were included in the present study. Participants were active runners who engaged in endurance exercise three to six times per week and were recruited from various athletic clubs at the University of Illinois at Urbana–Champaign. All participants were considered healthy based on responses to a physical activity readiness questionnaire and a medical history form. Participants were informed of the study purpose, experimental protocol, and potential risks before providing written consent to participate. The study was approved by the Institutional Review Board at the University of Illinois at Urbana–Champaign and the University of Toronto Research Ethics Board and was conducted in accordance to standards for the use of human participants in research as outlined in the Declaration of Helsinki.

Experimental design

A randomized crossover design was used for the present study. Before the infusion protocol, participants reported to the laboratory for familiarization with the exercise equipment and for V˙O2peak assessment as determined by graded intensity treadmill exercise testing and gas flow analysis. In addition, body composition was measured by dual-energy x-ray absorptiometry (Discovery QDR Series Bone Densitometer, version 12.7.9 for Windows; Hologic, Bedford, MA). On a subsequent visit, participants reported to the laboratory to perform a habituation trial. During this trial, participants performed 60 min of treadmill running to determine the treadmill settings for the experimental exercise trial. Treadmill settings were adjusted during the first 5 min to elicit an intensity of 70% V˙O2peak as measured by indirect calorimetry while running at 1% grade and maintained for 60 min. After the familiarization trial, participants were randomly assigned to either rest or exercise for their first infusion trial. Participants were also instructed to refrain from any form of vigorous physical exercise for 72 h before the infusion trial and to consume their habitual diet. Dietary intake was evaluated for 2 d before the first infusion trial using food records that were administered via the Automated Self-Administered 24-h recall system (ASA24, version 2014 for Windows; National Cancer Institute, Bethesda, MD), which has been demonstrated to minimize estimation errors compared with other methods of dietary evaluation (15). Habitual dietary intake included a balance of protein (1.66 ± 0.17 g·kg−1·d−1), carbohydrate (3.74 ± 0.37 g·kg−1·d−1), and fat (1.35 ± 0.17 g·kg−1·d−1) and an energy intake of 34.7 ± 3.4 kcal·kg−1·d−1 (mean ± SEM). The participants were instructed to maintain their previously recorded dietary habits in the 2 d leading into their second infusion trial.

Infusion protocol

Crossover trials were separated by ≥7 d (Fig. 1). Upon completion of the first infusion trial, a copy of the food diary was returned to the participants, and they were subsequently instructed to maintain their previously recorded dietary habits in the 2 d leading into the second infusion trial. The infusion trials differed only in the inclusion of a 60-min bout of treadmill running at 70% V˙O2peak (Table 1). On the morning of the infusion trials, participants reported to the laboratory at ~0700 h after an overnight fast. A Teflon catheter was inserted in a dorsal hand vein and placed in a heated blanket (60°C) for repeated arterialized blood sampling and remained patent with a 0.9% saline drip. At t = −180 min, a baseline blood sample was collected in a prechilled 10-mL EDTA Vacutainer. In addition, a baseline breath sample was collected in a 10-mL Vacutainer and stored at 20°C. Subsequently, a second Teflon catheter was inserted in an antecubital vein of the contralateral arm for stable isotope infusion. Participants received priming doses of NaH13CO2 (2.4 μmol·kg−1) and L-[1-13C]leucine (7.6 μmol·kg−1) before initiating a constant infusion of L-[1-13C]leucine (0.13 μmol·kg−1·min−1). Blood sampling occurred before (t = −120, −90, −60, −40, −20, −5, and 0 min) and after (t = 30, 60, 90, 120, 150, 180, 210, 240, and 300 min) mixed meal ingestion (see Fig. 1). At similar time points as blood was drawn, breath samples were collected to determine 13CO2 enrichment by isotope ratio mass spectrometry (ID Microbreath; Compact Science Systems Ltd., Newcastle-Under-Lyme, United Kingdom). For the resting trial, fewer blood and breath samples were collected during the period corresponding with exercise (i.e., single sampling point at t = −30 min during the resting trial). The blood samples were immediately analyzed for whole blood glucose concentrations (2300 Stat Plus; YSI Life Sciences, Springs, OH) and subsequently centrifuged at 10,000 rpm for 10 min at 4°C. Aliquots of plasma were frozen and stored at −80°C until subsequent analysis. Total CO2 production was measured for 20 min by indirect calorimetry (TrueOne 2400; ParvoMedics, Sandy, UT) at t = −85, −40, 35, 95, 165, and 275 min.

T1-24
TABLE 1:
Exercise responses (n = 7).
F1-24
FIGURE 1:
Schematic representation of the resting and exercise infusion protocols. Exercise denotes a 60-min bout of treadmill running at 70% V˙O2peak that was performed only during the exercise trial. Arrows indicate blood and breath sampling time points. V˙CO2 refers to collection of total CO2 production rates. Drink refers to ingestion of a mixed meal containing 18 g intrinsically labeled egg protein, 17 g fat, and 60 g carbohydrate.

Meal composition

The intrinsically L-[5,5,5-2H3]leucine-labeled eggs were produced by supplementing the diet of laying hens (Lohmann LSL Whites) with 0.3% L-[5,5,5-2H3]leucine as described previously (31). The eggs had the yolks and whites completely mixed, cooked, and freeze dried before storage in aliquots of 156 g (approximately three whole eggs). The L-[5,5,5-2H3]leucine enrichment of the egg aliquots was determined by gas chromatography–mass spectrometry (GC-MS) and averaged 21.2 mole percent excess (31). During the infusion trials, participants ingested a mixed meal at t = 0 min containing approximately three whole eggs (18 g protein, 17 g fat) and 60 g carbohydrate (blend of 50% sucrose, 50% maltodextrin) dissolved in 400 mL of water that was flavored with vanilla. This amount of protein (~0.25 g·kg−1) was selected as we have shown that this amount of protein intake per meal maximally stimulates postprandial muscle protein synthesis rates in young men (23). Moreover, previous work has shown that this amount of ingested protein is sufficient to stimulate muscle protein synthesis rates during recovery from resistance- (24) and endurance-based exercise (6).

Blood analyses

Plasma insulin concentrations were determined by solid two-site enzyme immunoassay (Mercodia Diagnostics, Uppsala, Sweden). Plasma I-FABP concentrations were assessed by enzyme-linked immunosorbent assay according to manufacturer’s instructions (Hycult Biotechnology, Uden, Netherlands). Plasma leucine concentration was determined by liquid chromatography tandem mass spectrometry (1290 HPLC, Agilent Technologies, Santa Clara, CA; 5500 Q-Trap MS, Sciex, Framingham, MA) according to previously described methods (10).

Plasma L-[1-13C]leucine and L-[5,5,5-2H3]leucine enrichments were determined by GC-MS (5975C MSD, Agilent Technologies) as described previously (1,31). Briefly, 200 μL of plasma was extracted using 1 mL of a single phase mixture of isopropanol–acetonitrile–water (3:2:2, v/v) and centrifuged for 10 min at 10,000 rpm and 4°C. The supernatant was dried under N2, and the amino acids were converted to their tert-butyldimethylsilyl derivative before GC-MS analysis. Plasma leucine enrichments were determined by ion monitoring at m/z 302 and 303 for unlabeled and labeled [1-13C]leucine, respectively, and m/z 302 and 305 for unlabeled and labeled [5,5,5-2H3]leucine, respectively.

Plasma enrichments of α-[1-13C]ketoisocaproate (α-KIC) were measured by GC-MS (7890B GC, 5977A MSD, Agilent Technologies) as a surrogate for intramuscular leucyl-transfer RNA labeling (35,36) according to previously described methods (24). Briefly, 100 μL of plasma was deproteinized and centrifuged for 10 min at 10,000 rpm and 4°C. The supernatant was dried under N2 and the remaining residue was extracted using o-phenylenediamine and dichloromethane. α-KIC was converted to its tert-butyldimethylsilyl derivative before GC-MS analysis. Plasma α-KIC enrichments were determined by selective ion monitoring at m/z 232 and 233 for unlabeled and labeled α-KIC, respectively.

Calculations

Whole-body leucine kinetics were estimated in non–steady-state conditions by the ingestion of intrinsically labeled egg protein combined with intravenous infusion of L-[1-13C]leucine as described previously (9). Briefly, leucine oxidation was calculated using α-KIC as the precursor pool with a fractional bicarbonate retention factor of 0.8 as previously described (3,9). For the other fluxes, calculations were performed using plasma [1-13C] and [5,5,5-2H3]leucine mole percent excess as precursors (9). Total, exogenous, and endogenous leucine rate of appearance (Ra), plasma protein-derived leucine availability, total leucine rates of disappearance (Rd), total leucine oxidation, and nonoxidative leucine disposal (NOLD) were calculated using modified Steele’s equations (29). Postabsorptive leucine balance was calculated directly as the difference between the intravenously infused leucine and the amount of leucine oxidized over the 3-h postabsorptive period. Postprandial leucine balance was calculated as the difference between dietary leucine intake (estimated as 8.9% of egg protein as described previously) (31) and the amount of leucine oxidized over the 5-h postprandial period. Total leucine balance was calculated as the sum of postabsorptive and postprandial leucine balance.

Statistical analyses

A within-subject repeated-measures design was used for the present study. Differences in plasma and blood concentrations, leucine kinetics, and net leucine balance were tested using a two-factor (treatment–time) repeated-measures ANOVA. For the resting trials, the concentration and kinetic variables at t = −40, −20, and −5 min were estimated by linear interpolation using values at t = −60, −30, and 0 min; this was done after verifying that each variable was not changing over time using linear regression. Where significant interactions were identified in the ANOVA, a Bonferroni post hoc test was performed to determine differences between means for all significant main effects and interactions. A t-test was used to determine whether net leucine balance was significantly different from zero. All data are expressed as mean ± SEM. Statistical analyses were performed using GraphPad Prism (version 6.02 for Windows, San Diego, CA). For all analyses, statistical significance was set at P < 0.05.

RESULTS

Plasma metabolites

Blood glucose concentrations increased above baseline during exercise (P < 0.01) and declined toward baseline values immediately after exercise (i.e., at 0 min; Fig. 2A). Blood glucose concentrations increased above baseline at 30 min after meal ingestion (P < 0.01) and returned to baseline values at 60 and 90 min in the exercise and rest conditions, respectively. Blood glucose concentrations reached higher peak values after meal ingestion in the resting state versus the exercise state (P < 0.01). Plasma insulin concentrations increased after meal ingestion (P < 0.01) and returned to baseline values at 120 min of the postprandial period in both the resting and the exercise conditions (Fig. 2B). Plasma insulin concentrations reached higher peak values after meal ingestion in the resting state versus the exercise state (P < 0.05). Plasma leucine concentrations rapidly increased after meal ingestion at 30 min (P < 0.01) and returned to baseline values at 120 min in both the resting and the exercise conditions (Fig. 2C).

F2-24
FIGURE 2:
Mean ± SEM blood glucose (mg·dL−1; A), plasma insulin (mU·L−1; B), and plasma leucine (μmol·L−1; C) concentrations during the rest and exercise trials in young men (n = 7). The gray area corresponds to the exercise bout; the dashed line refers to meal ingestion. Data were analyzed using a two-factor (treatment–time) repeated-measures ANOVA. A Bonferroni post hoc test was used to identify differences between means for all statistically significant interactions. Glucose: time effect, P < 0.01; treatment–time, P < 0.01. Insulin: time effect, P < 0.01; treatment–time, P < 0.01. Leucine: time effect, P < 0.01; treatment–time, P = 0.62. †Different from baseline in the rest condition (P < 0.01). #Different from baseline in the exercise condition (P < 0.01). *Different between rest and exercise conditions (P < 0.01).

Plasma I-FABP concentrations

At the onset of exercise, plasma I-FABP concentrations (surrogate marker of small intestinal damage) (33) increased to a peak that was ~2.5-fold above baseline at −5 min (P < 0.01) and returned to baseline values by 60 min of recovery (Fig. 3). When compared with rest, plasma I-FABP concentrations were greater during exercise and for 30 min of recovery (P < 0.01). Plasma I-FABP concentrations were unaltered in the rest condition across time (P > 0.05).

F3-24
FIGURE 3:
Mean ± SEM plasma intestinal fatty acid binding protein (I-FABP; pg·mL−1) concentrations during the rest and exercise trials in young men (n = 7). The gray area corresponds to the exercise bout; the dashed line refers to meal ingestion. Data were analyzed using a two-factor (treatment–time) repeated-measures ANOVA. A Bonferroni post hoc test was used to identify differences between means for all statistically significant interactions. I-FABP: time effect, P < 0.01; treatment–time, P < 0.01. #Different from baseline in the exercise condition (P < 0.01). *Different between rest and exercise conditions (P < 0.01).

Plasma tracer enrichments and whole-body leucine kinetics

Plasma L-[1-13C]leucine and α-[1-13C]KIC enrichments did not differ between the resting and the exercise conditions throughout the infusion trials (both, P > 0.05; see Figure, Supplemental Digital Content 1, Plasma enrichments over time, https://links.lww.com/MSS/B13). Plasma L-[5,5,5-2H3]leucine enrichments rapidly increased after meal ingestion and did not differ between the resting and the exercise states (P = 0.71; see Figure, Supplemental Digital Content 1, Plasma enrichments over time, https://links.lww.com/MSS/B13). Exogenous leucine Ra (representing the appearance of dietary protein-derived leucine into the circulation) increased after meal ingestion (P < 0.01) in both the resting and the exercise conditions (Fig. 4A). The amount of dietary protein-derived leucine that appeared into circulation over the 5-h postprandial period was similar at rest and after exercise (62% ± 2% and 63% ± 2%, respectively; P = 0.82). Endogenous leucine Ra (representing the appearance of leucine derived from whole-body protein breakdown into the circulation) decreased below baseline values after meal ingestion (P < 0.01) and returned to baseline by 240 min in both conditions (Fig. 4B). At the onset of exercise, NOLD (i.e., whole-body protein synthesis; Fig. 4C) decreased below baseline values (P < 0.01) and returned back to baseline at 0 min. NOLD was suppressed during the exercise period when compared with the resting state (P < 0.01). Leucine oxidation increased at the onset of exercise to a peak that was ~2.5-fold above baseline at −5 min (P < 0.01) and returned to baseline values at 0 min (Fig. 4D). Leucine oxidation was greater during the exercise period when compared with the resting state (P < 0.01).

F4-24
FIGURE 4:
Mean ± SEM exogenous leucine R a (μmol·kg−1·min−1; A), endogenous leucine R a (μmol·kg−1·min−1; B), NOLD (μmol·kg−1·min−1; C), and leucine oxidation rates (μmol·kg−1·min−1; D) during the rest and exercise trials in young men (n = 7). The gray area corresponds to the exercise bout; the dashed line refers to meal ingestion. Data were analyzed using a two-factor (treatment–time) repeated-measures ANOVA. A Bonferroni post hoc test was used to identify differences between means for all statistically significant interactions. Exogenous leucine R a: time effect, P < 0.01; treatment–time, P = 0.96. Endogenous leucine R a: time effect, P < 0.01; treatment–time, P = 0.91. NOLD: time effect, P < 0.01; treatment–time, P < 0.01. Leucine oxidation: time effect, P < 0.01; treatment–time, P < 0.01. †Different from baseline in the rest condition (P < 0.01). #Different from baseline in the exercise condition (P < 0.01). *Different between rest and exercise conditions (P < 0.01).

Leucine balance

Postabsorptive, postprandial, and total leucine balances were significantly different from zero during the rest and exercise trials (all, P < 0.01; Fig. 5). When compared with rest, postabsorptive leucine balance was more negative in the exercise condition (P < 0.01). After meal ingestion, postprandial leucine balance increased above postabsorptive values at rest (40.8 ± 3.9 vs −24.2 ± 0.8 μmol·kg−1·h−1; P < 0.01) and after exercise (47.4 ± 3.9 vs −71.4 ± 3.4 μmol·kg−1·h−1; P < 0.01) with no differences between conditions (P > 0.05). Total leucine balance (i.e., the sum of postabsorptive and postprandial leucine balance) was lower in the exercise state when compared with the resting state (−24.0 ± 5.2 vs 16.6 ± 3.7 μmol·kg−1·h−1, respectively; P < 0.01).

F5-24
FIGURE 5:
Mean ± SEM postabsorptive, postprandial, and total leucine balances (μmol·kg−1·h−1) during the rest and exercise trials in young men (n = 7). Data were analyzed using a two-factor (treatment–time) repeated-measures ANOVA. A Bonferroni post hoc test was used to identify differences between means for all statistically significant interactions. **Different from postabsorptive leucine balance in the same condition (P < 0.01). *Different between rest and exercise conditions (P < 0.01).

DISCUSSION

The ingestion of protein functions to replenish postabsorptive and/or exercise-induced amino acid losses to maintain whole-body protein stores (28). We demonstrate that the bolus ingestion of a mixed macronutrient meal containing a moderate amount of protein enhances postprandial leucine balance both at rest and after an acute bout of endurance exercise. However, the amount of protein consumed (~0.25 g·kg−1), which has been shown previously to enhance postexercise muscle protein synthesis rates (6,24), was not sufficient to fully replace exercise-induced whole-body oxidative leucine losses. Consequently, this resulted in a negative total leucine balance over the entire exercise and postexercise recovery period. In addition, we report for the first time that, despite exercise-induced elevations in plasma I-FABP concentrations, the postprandial release of dietary protein-derived leucine into circulation from a mixed meal was not modulated in trained men. Moreover, endurance exercise-induced intestinal damage did not further increase leucine oxidation rates or reduce net leucine balance during postexercise recovery. Thus, a mixed meal can be consumed immediately after cessation of exercise without the perceived exercise-induced impairments in GI function interfering with protein metabolic processes important for recovery and subsequent adaptations.

In the present study, the exercise bout did not augment whole-body protein breakdown rates, which is in agreement with previous findings (5). Whole-body protein synthesis rates were suppressed, and leucine oxidation rates were increased during treadmill running at 70% V˙O2peak (Figs. 4C and 4D), a finding that is consistent with previous data (5). Hence, our data align with previous research, suggesting that amino acids are diverted away from events supporting protein synthesis and toward events supplying fuel for muscle contraction during exercise (e.g., energy production via leucine oxidation) (11,20,38). Notably, the increased use of leucine as an alternative fuel source during the exercise bout led to a more negative leucine balance during postabsorptive exercise when compared with rest. Consumption of a mixed meal both at rest and after exercise elicited an expected increase in whole-body net leucine balance that was mediated, in part, via an insulin-induced suppression of endogenous leucine rate of appearance (i.e., body protein breakdown) (8,9). The lack of an appreciable effect on whole-body protein synthesis (i.e., NOLD) is consistent with previous studies at rest (9) and after exercise (18,19) with the ingestion of mixed macronutrient beverages. However, whole-body net leucine balance, although similarly positive during the 5-h postprandial period, was negative during the exercise trial when accounting for the exercise-induced oxidative losses. Therefore, our data suggest that targeted nutritional strategies for endurance exercise recovery should supply dietary protein in greater amounts than currently recommended to maximize muscle protein synthesis at rest or after resistance exercise (21,24,37) to counterbalance the endurance exercise-induced oxidative leucine losses and shift whole-body leucine balance into the positive.

It has been reported that endurance exercise can compromise GI function during and immediately after exercise (26,27). This apparent impairment in GI function is thought to be due to intestinal injury that results from a redistribution of blood flow away from the splanchnic region and toward exercising muscle, which subsequently may induce intestinal damage (25). Circulating I-FABP concentrations are often used as a biomarker of exercise-induced intestinal injury as it correlates with the extent of exercise-induced splanchnic hypoperfusion (33) and increased small intestinal permeability (32). Here, we demonstrate that treadmill running resulted in a marked increase in plasma I-FABP concentrations that returned to baseline within 60 min of postexercise recovery. Resistance exercise-induced increases in I-FABP have been associated with blunted appearance rates of dietary protein-derived phenylalanine into circulation after the ingestion of isolated dairy protein in recreationally active young men (34). Here, the increase in plasma I-FABP concentrations during and after endurance exercise did not reduce the appearance rates of dietary protein-derived leucine into circulation or plasma leucine availability from a mixed meal, which was similar during the postprandial period at rest and after exercise (62% ± 2% and 63% ± 2%, respectively). Moreover, this perceived intestinal injury did not influence postexercise leucine oxidative losses or postexercise net leucine balance. Thus, in the context of endurance exercise in trained males, the increase in plasma I-FABP concentrations has less effect on the dietary protein digestion and absorption kinetics of a mixed meal when compared with the ingestion of an isolated dairy-based protein source consumed after resistance exercise in untrained males (34) and does not upregulate amino acid oxidative capacity.

From an optimal performance nutrition perspective, it is recommended that endurance athletes consume a mixed meal to replenish endogenous fuel stores and support muscle remodeling during recovery from exercise (30). Therefore, the presence of carbohydrate and fat in our meal likely reduced gastric emptying rates and subsequently delayed amino acid absorption rates, giving the gut an opportunity to recover from possible exercise-induced intestinal injury. Moreover, it is possible that this recovery effect may have been aided by the training status of our population. For example, it is possible that exercise-induced adaptations occur in the intestine, the so-called “train the gut” concept (14), which may allow endurance-trained individuals to better handle dietary protein during recovery from exercise despite what appears to be increased gut damage as indicated by exercise-induced elevations in plasma I-FABP concentrations. Overall, given the importance of exogenous amino acids to facilitate postexercise recovery (22), our data suggest that the effect of exercise-induced intestinal injury on the availability of dietary protein-derived amino acids during recovery may be diminished when a mixed meal is consumed.

In summary, we demonstrate that the ingestion of a moderate amount of protein (~0.25 g·kg−1) within a mixed macronutrient meal enhances postprandial leucine balance in the resting state in healthy young males. However, the amount of protein ingested in this study was not sufficient to fully replace exercise-induced oxidative leucine losses, which ultimately resulted in a negative total leucine balance during recovery from endurance exercise. Therefore, a greater protein dose or more frequent feedings may be required to counterbalance the exercise-induced oxidative losses and fully restore net leucine balance after a bout of endurance exercise. Importantly, any potential exercise-induced alterations in GI function do not appear to influence protein metabolism or dietary amino acid–mediated postexercise recovery in trained men.

Funding for this research was provided by grants from the Faculty of Kinesiology and Physical Education at the University of Toronto, the Natural Sciences and Engineering Research Council of Canada, and the University of Illinois Center on Health, Aging, and Disability. Dr. Kim Volterman is thanked for her laboratory assistance.

No authors have any conflicts of interest, financial or otherwise, to declare. The results of the present study do not constitute endorsement by the American College of Sports Medicine and are presented clearly, honestly, and without fabrication, falsification, or inappropriate data manipulation.

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Keywords:

DIETARY AMINO ACIDS; AEROBIC EXERCISE; RECOVERY; ATHLETE; PROTEIN SYNTHESIS

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