Medicine & Science in Sports & Exercise:
BASIC SCIENCES: Original Investigations
Effects of Increasing Insulin Secretion on Acute Postexercise Blood Glucose Disposal
KAASTRA, BRECHJE1; MANDERS, RALPH J. F.1; VAN BREDA, ERIC2; KIES, ARIE3; JEUKENDRUP, ASKER E.4; KEIZER, HANS A.2; KUIPERS, HARM2; VAN LOON, LUC J. C.1,2
1Department of Human Biology, Nutrition and Toxicology Research Institute Maastricht (NUTRIM), Maastricht University, Maastricht, THE NETHERLANDS; 2Department of Movement Sciences, NUTRIM, Maastricht University, THE NETHERLANDS; 3DSM Food Specialties, Delft, THE NETHERLANDS; and 4School of Sport and Exercise Sciences, University of Birmingham, Birmingham, UNITED KINGDOM
Address for correspondence: Dr. L. J. C. van Loon, Department of Human Biology, Maastricht University, PO Box 616, 6200 MD Maastricht, The Netherlands; E-mail: L.vanLoon@HB.Unimaas.nl.
Submitted for publication March 2005.
Accepted for publication August 2005.
Background: Coingestion of protein and/or free amino acids with carbohydrate has been reported to accelerate postexercise muscle glycogen synthesis due to an increase in the insulin response.
Purpose: To determine the extent to which the combined ingestion of carbohydrate and a casein protein hydrolysate with or without additional free leucine can increase insulin levels during postexercise recovery in endurance-trained athletes. To determine how this affects whole-body plasma glucose disposal during postexercise recovery.
Methods: Fourteen male athletes (age: 24.3 ± 0.8 yr; V̇O2max: 62.9 ± 1.4 mL·kg−1·min−1) were subjected to three randomized crossover trials in which they performed 2 h of exercise (55% Wmax). Thereafter, subjects were studied for 3.5 h during which they ingested carbohydrate (CHO: 0.8 g·kg−1·h−1), carbohydrate and a protein hydrolysate (CHO-PRO: 0.8 and 0.4 g·kg−1·h−1, respectively), or carbohydrate, a protein hydrolysate, and free leucine (CHO-PRO-LEU: 0.8, 0.4, and 0.1 g·kg−1·h−1, respectively) in a double-blind fashion. Continuous infusions with [6,6-2H2] glucose were applied to quantify plasma glucose appearance (Ra) and disappearance rates (Rd).
Results: Plasma insulin responses were 108 ± 17 and 190 ± 33% greater in the CHO-PRO and CHO-PRO-LEU trial, respectively, compared with the CHO-trial (P < 0.01). Plasma glucose responses were lower in the CHO-PRO and CHO-PRO-LEU trial compared with the CHO-trial (35 ± 5 and 42 ± 11% lower, respectively; P < 0.01). Plasma glucose Ra and Rd were greater in the CHO versus the CHO-PRO and CHO-PRO-LEU trials (P < 0.05). Glucose Rd represented 100 ± 0.03% of Ra in all trials.
Conclusions: The combined ingestion of a protein hydrolysate and/or free leucine with carbohydrate (0.8 g·kg−1·h−1) substantially augments insulin secretion, but does not affect plasma glucose disposal during the first 3.5 h of postexercise recovery in trained athletes.
From a quantitative point of view, muscle glycogen represents the most important fuel source during prolonged moderate- to high-intensity exercise. The reliance on muscle glycogen increases with increasing exercise intensity and a direct relationship between fatigue and muscle glycogen depletion has been described (3). Consequently, postexercise muscle glycogen repletion forms the most important factor in determining the time needed to recover. The latter is important to maintain performance capacity in athletes who train twice daily and/or those who need to compete on several consecutive days. Muscle glycogen synthesis is not only affected by the extent of glycogen depletion but also in a more direct manner by the type, duration and intensity of the preceding exercise as these factors will differentially influence the acute enzymatic changes as well as recovery from the acute changes induced by strenuous exercise (7). Skeletal muscle glycogen synthesis is a relatively slow process and complete restoration of the glycogen stores after exercise generally takes up to 24 h (5). The latter depends on the extent of glycogen depletion and can be optimized by ingesting adequate amounts of carbohydrate (12,13,22). To further accelerate muscle glycogen replenishment, numerous studies have been performed to assess the effects of timing (17), mode (15), amount (4,7,9,14,27) and/or type of carbohydrate supplementation (4,19,22).
Postexercise muscle glycogen synthesis appears to occur in two distinct phases (20). Early postexercise muscle glycogen repletion seems to be independent of circulating insulin levels and persists for approximately 30-60 min (13). This first stage is characterized by a high glycogensynthesis rate, which is generally attributed to the contraction-induced translocation of glucose transporter-4 (or GLUT-4) to the sarcolemma, resulting in an increased permeability of the muscle membrane to glucose. Combined with the activation of glycogen synthase (30), this leads to a high initial muscle glycogen synthesis rate. In contrast, during the second phase muscle glycogen synthesis rate depends on circulating insulin levels and is about 10-30% lower compared to the first stage (13,20). The latter might explain the observations by various groups showing co-ingestion of protein and/or free amino acids with carbohydrate (ranging from 0.4 to 1.0 g of carbohydrate per kilogram per hour) to accelerate postexercise muscle glycogen synthesis when compared to the ingestion of carbohydrate only (10,23,24,27,31). These observations are generally attributed to the greater plasma insulin response following the combined ingestion of carbohydrate and protein (27,31). The insulinotropic properties of carbohydrate/protein mixtures are not new, and have been described for many years (18,21). The subsequent rise in circulating insulin has been suggested to accelerate plasma glucose disposal (11) and/or to augment glycogen synthase activity (2,16), thereby accelerating glycogen synthesis (6,31). We aimed to establish whether plasma glucose disposal represents the limiting factor during postexercise recovery when ingesting 0.8 g of carbohydrate per kilogram per hour. Therefore, we measured glucose appearance and disappearance rates during postexercise recovery while ingesting carbohydrate with or without an insulinotropic protein hydrolysate.
Over the last few years we have experimented with the in vivo insulinotropic response to the ingestion of free amino acids (25-28). The latter has shown that leucine and phenylalanine coingestion can substantially increase the insulin response following the ingestion of a carbohydrate/ protein hydrolysate mixture. Aside from its insulinotropic potential, leucine has received much attention because of its proposed anabolic properties (1). As such, we also aimed to determine the insulinotropic properties of the addition of free leucine (0.1 g·kg−1·h−1) to the combined ingestion of carbohydrate (0.8 g·kg−1·h−1) and a protein hydrolysate (0.4 g·kg−1·h−1). As we hypothesized that coingestion of leucine further increases the plasma insulin response, the subsequent effects on plasma glucose kinetics were also evaluated.
In the present study, we investigated the extent to which the combined ingestion of carbohydrate and a protein hydrolysate with and without additional leucine can elevate plasma insulin concentrations during postexercise recovery in highly trained endurance athletes. Furthermore, we determine whether such an increase in circulating insulin can modulate plasma glucose disposal when ingesting 0.8 g of carbohydrate per kilogram per hour. Therefore, continuous infusions of [6,6-2H2] glucose were applied to determine plasma glucose appearance and disappearance rates during 3.5 h of postexercise recovery under each of these three nutritional conditions.
Fourteen highly trained male cyclists were selected to participate in this study. Subjects had a training history of more than 4 yr and trained at least three times per week for more than 2 h·d−1. Subjects' characteristics are presented in Table 1. All subjects were informed about the nature and risks of the experimental procedures before they gave their written informed consent. This study was approved of by the medical ethical committee of the Academic Hospital Maastricht.
At least 1 wk before the first trial, an incremental exhaustive exercise test was performed on an electronically braked cycle ergometer (Lode Excalibur, Groningen, The Netherlands) to determine maximal workload capacity (Wmax), maximal oxygen uptake (V̇O2max; Oxycon, Mijnhart, Bunnik, The Netherlands) and maximal heart rate (Polar Electro, Kempele, Finland). Body mass was measured with a digital balance with an accuracy of 0.001 kg (E1200, August Sauter GmbH, Albstadt, Germany). Body composition was determined by the hydrostatic weighing method in the morning after an overnight fast. Simultaneously, residual lung volume was measured by the helium-dilution technique using a spirometer (Volugraph 2000, Mijnhart, Bunnik, The Netherlands). Fat-free mass (FFM) was calculated by subtracting fat mass (FM) from total body mass. Before selection into the study, all subjects performed a standard oral glucose tolerance test (OGTT). Following an overnight fast, subjects arrived at the laboratory at 8:00 a.m. by car or public transportation. A fasting blood sample was collected, after which a bolus of 75 g of glucose (dissolved in 250 mL of water) was ingested (t = 0 min). After 120 min a second blood sample was obtained. Plasma glucose concentrations were measured to screen for glucose intolerance and/or type 2 diabetes.
Diet and activity before testing.
All subjects maintained normal dietary and physical activity patterns throughout the entire experimental period. In addition, subjects refrained from heavy physical labor and exercise training for at least 3 d before each trial and filled out a food intake diary for 2 d before the first trial to keep dietary intake identical before the other trials. Energy intake did not differ between trials and averaged 12.8 ± 0.7 MJ·d−1; consisting of 55 ± 2 energy percent (En%) carbohydrate, 30 ± 2 En % fat, and 15 ± 1 En % protein. The evening before each trial, subjects received a standardized meal (consisting of 52 En % carbohydrate, 30En % fat, and 18 En % protein).
All subjects participated in three randomized crossover exercise trials, separated by a 1-wk period, after which plasma insulin responses and subsequent plasma glucose disposal rates were determined following ingestion of three different beverage compositions during postexercise recovery (CHO: carbohydrate only; CHO-PRO: carbohydrate with a protein hydrolysate; CHO-PRO-LEU: carbohydrate, protein hydrolysate, and free leucine). The main aim was to determine the effects of coingestion of a protein hydrolysate (CHO+PRO) or a protein hydrolysate with free leucine (CHO+PRO+LEU) on plasma glucose disposal when compared with the ingestion of CHO. Following exercise, subjects were placed in a supine position and remained inactive for a period of 3.5 h. All trials were performed in a randomized order and drinks were provided in a double-blind fashion.
Following an overnight fast, subjects arrived at the laboratory at 7:30 a.m. by car or public transportation. A Teflon catheter (Baxter BV, Utrecht, The Netherlands) was inserted into a dorsal hand vein for arterialised blood sampling using a hot-box and another catheter was inserted in the contralateral arm for isotope infusion. Background blood samples were taken from both the hand and the antecubital vein (t = −120). Thereafter, subjects started cycling for 120 min at a 55% Wmax workload. During the first exercise trial, water was provided ad libitum. In the 2 other trials subjects drank the same amount of water. After 1 h of exercise, subjects were administered a [6,6-2H2] glucose prime (13.5 μmol·kg−1), followed by a continuous infusion of [6,6-2H2] glucose (0.3 μmol·kg−1·min−1) via a calibrated infusion pump (IVAC 560, San Diego, CA). Glucose tracer infusion was continued for 270 min until the end of the postexercise recovery period. During exercise, V̇O2 and carbon dioxide production (V̇CO2) were measured for 5 min every 30 min. During postexercise recovery, V̇O2 and V̇CO2 were measured at t = 60 min for a period of 2 h (Oxycon β). Immediately after cessation of exercise, a postexercise blood sample was collected (t=0 min), followed by the administration of the first bolus of test drink. Thereafter, blood samples were collected at t=15, 30, 45, 60, 75, 90, 120, 150, 180, and 210 min for the measurement of plasma glucose, insulin, and glucose enrichment. Test drinks were administered every 30 min after blood sample collection (t = 0, 30, 60, 90, 120, 150, and 180 min).
During postexercise recovery, subjects received 321 ± 8 mL of a carbohydrate (CHO), carbohydrate plus a casein hydrolysate (CHO-PRO) or a carbohydrate plus casein hydrolysate and leucine containing drink (CHO-PRO-LEU) in a double-blind fashion every 30 min, to ensure the intake of 0.8 g·kg−1·h−1 carbohydrate with or without 0.4 g·kg−1·h−1 casein hydrolysate and/or 0.1 g·kg−1·h−1 leucine. Carbohydrates were provided by AVEBE (maltodextrin; Veendam, The Netherlands), Roquette (maltodextrin; Cedex, France), and Cerestar (maltose; Bergen op Zoom, The Netherlands). The casein protein hydrolysate (PeptoPro Sports) was prepared by DSM Food Specialties (Delft, The Netherlands), and the free leucine was provided by Sigma-Aldrich (Zwijndrecht, The Netherlands). Beverages were flavored to make the taste comparable in all trials.
From respiratory measurements, total fat and carbohydrate oxidation rates during exercise were calculated using the nonprotein respiratory quotient.
with V̇O2 and V̇CO2 in liters per minute and oxidation rates in grams per minute. The glucose tracer (99% enriched, Cambridge Isotope laboratories, Andover, MA) was dissolved in 0.9% saline. Glucose tracer concentration in the infusates averaged 24.5 ± 0.5 mmol·L−1. The [6,6-2H2] glucose infusion rate averaged 251 ± 1nmol·kg−1·min−1. Plasma glucose enrichments are expressed as tracer/tracee ratios (TTR). Rate of appearance (Ra) and rate of disappearance (Rd) of glucose were calculated using nonsteady-state Steele equations adapted for stable isotope methodology.
where F is the infusion rate (μmol·kg−1·min−1); V is the distribution volume for glucose (160 mL·kg−1); C1 and C2 are the glucose concentrations (mmol·L−1) at time 1 (t1) and 2 (t2), respectively; and E1 and E2 are the plasma glucose enrichments (TTR) at times 1 and 2, respectively.
Blood sample analysis.
Blood (10 mL) was collected in EDTA containing tubes and centrifuged at 1000 × g and 4°C for 10 min. Aliquots of plasma were immediately frozen in liquid nitrogen and stored at −80°C until analyses. Glucose concentrations (Uni Kit III, Roche, Basel) were analyzed with the COBAS FARA semiautomatic analyzer (Roche). Plasma insulin was analyzed by radioimmunoassay (human insulin-specific RIA KIT, Nuclilab B.V., Ede, The Netherlands). For determination of plasma [6,6-2H2] glucose enrichment, deproteinized plasma samples were derivatized with heptafluorobutyric acid (HFB). Briefly, 100 mL of plasma was deproteinized with 1 mL of aceton. The samples were then derivatized with 160 μL of a HFB/ethyl acetate mixture (1:1). Thereafter, the enrichment of the derivative was measured by GC/MS by injecting 1 μL of the derivative into an Agilent 6890N gas chromatograph equipped with a split/splitless injector and 7683 autosampler (Agilent Technologies, Stockport, UK). Mass spectrometric detection was obtained with an Agilent 5973N mass-selective detector. Glucose data were acquired using selected ion monitoring for masses m/z 519 and 521.
Subjects filled out a questionnaire immediately after ingestion of the first bolus of test drink (t = 0), at t = 90 min, and after the last bolus of test drink (t = 180 min). This questionnaire contained questions regarding the presence of gastrointestinal (GI) distress and addressed the following complaints: nausea, bloated feeling, belching, stomach problems and GI cramping, vomiting, diarrhea, the urge to urinate and/or defecate, headache, and dizziness. The items were scored on a 10-point scale (1= not at all, 10 = very, very much). One question regarding the taste of the test drink was also conducted (1 = horrible, 10 = very tasty).
Data are expressed as means ± SEM. The plasma responses were calculated as area under the curve minus baseline values. To compare plasma metabolite concentrations and tracer kinetics over time between trials, a two-way repeated-measures analysis of variance (ANOVA) was applied. In case of a significant time by treatment interaction, a Scheffé's post hoc test was applied to locate specific differences in time between trials. Changes in time within each group were checked for statistical significance using one-way repeated-measures ANOVA. For non-time-dependent variables, a multiway ANOVA was applied. Statistical significance was set at the 0.05 level of confidence. All calculations were performed using StatView 5.0 (SAS, Cary, NC).
The exercise trials were performed at a 55% Wmax workload, which resulted in a mean absolute workload of 220 ± 6 W. The latter corresponded with a relative workload of 66 ± 1, 67 ± 1, and 66 ± 1% V̇O2max as measured in the CHO, CHO-PRO, and CHO-PRO-LEU trials, respectively. During exercise, heart rates averaged 149 ± 2, 147 ± 4, and 149 ± 3 bpm in the CHO, CHO-PRO, and CHO-PRO-LEU trials, respectively, which corresponded with 77 ± 1% of the maximal individual heart rate. Oxygen uptake during exercise averaged 3.03 ± 0.08, 3.05 ± 0.08, and 3.03 ± 0.07 L·min−1, respectively. Total fat oxidation rates averaged 0.57 ± 0.05, 0.59 ± 0.07, and 0.54 ± 0.03 g·min−1 in the CHO, CHO-PRO, and CHO-PRO-LEU trials, respectively (NS). Total carbohydrate oxidation rates averaged 2.51 ± 0.12, 2.48 ± 0.11, and 2.58± 0.12 g·min−1 in the CHO, CHO-PRO, and CHO-PRO-LEU trials, respectively (NS).
Plasma insulin and glucose concentrations
Basal plasma insulin concentrations averaged 9.33 ± 0.52, 9.70 ± 0.51, and 9.24 ± 0.51 mU·L−1 in the CHO, CHO-PRO, and CHO-PRO-LEU trials, respectively (NS). Baseline plasma glucose concentrations, measured in both venous and arterialized blood samples, averaged 5.05 ± 037, 5.20 ± 0.10, and 5.17 ± 0.11 mmol·L−1 (venous) and 5.14 ± 0.11, 5.23 ± 0.12, and 5.18 ± 0.12 mmol·L−1 (arterialized), respectively (NS).
Plasma insulin concentrations over time during postexercise recovery and calculated insulin responses are provided in Figure 1. Plasma insulin concentrations immediately after cessation of exercise averaged 5.66 ± 0.48, 7.04 ± 0.83, and 6.07 ± 0.84 mU·L−1 in the CHO, CHO-PRO, and CHO-PRO-LEU trials, respectively (NS). Plasma insulin concentrations increased during the first hour of recovery in all trials, after which levels plateaued in the CHO trial but continued to increase in the CHO-PRO and CHO-PRO-LEU trials. The insulin responses were 108 ± 17 and 190 ± 33% greater in the CHO-PRO and CHO-PRO-LEU compared with the CHO trial, respectively (P<0.01). In addition, the insulin response in the CHO-PRO-LEU trial was increased by an additional 37 ± 8% when compared with the CHO-PRO trial separately (P<0.05).
FIGURE 1-Plasma insu...Image Tools
Plasma glucose concentrations over time during postexercise recovery and calculated glucose responses are provided in Figure 2. Plasma glucose concentrations immediately after cessation of exercise averaged 4.95 ± 0.18, 5.03 ± 0.11, and 4.89 ± 0.23 mmol·L−1 in the CHO, CHO-PRO, and CHO-PRO-LEU trials, respectively (NS). Plasma glucose concentrations increased during the first hour of recovery in all trials, after which levels gradually declined. Plasma glucose levels reached significantly higher values in the CHO compared with the CHO-PRO and CHO-PRO-LEU trials (P < 0.05). Plasma glucose responses were 35 ± 5 and 42 ± 11% lower in the CHO-PRO and CHO-PRO-LEU trials compared with the CHO trial, respectively (P < 0.01).
FIGURE 2-Plasma gluc...Image Tools
Plasma glucose tracer kinetics are provided in Table 2. Rate of appearance and disappearance of plasma glucose over time during postexericse recovery are illustrated in Figure 3. Average plasma glucose Ra and Rd were significantly greater in the CHO versus the CHO-PRO and CHO-PRO-LEU trials (Table 2: P < 0.05). Whole-body glucose disposal (Rd expressed as a percentage of Ra) did not differ between trials. Ra fully matched Rd within 30 min after onset of carbohydrate ingestion in all trials (Table 2).
The outcome of the questionnaires was scored to determine the presence of GI complaints and taste of the recovery drinks. Subjects reported no gastrointestinal complaints and no significant differences in the questionnaire scores were observed between trials (P > 0.05).
Respiratory measurements during recovery.
Gas exchange measurements and energy expenditure during recovery are provided in Table 3. Oxygen uptake showed significantly higher values in the CHO-PRO and CHO-PRO-LEU compared with the CHO trials (P < 0.05). As such, estimated energy expenditure was also significantly greater in the CHO-PRO and CHO-PRO-LEU versus the CHO trial (P < 0.05).
Increasing plasma insulin secretion during postexercise recovery has been proposed as an effective strategy to accelerate muscle glycogen synthesis. The latter has been attributed to the stimulating effects of insulin on glucose uptake and/or glycogen synthase activity. In the present study, we show that when trained athletes ingest 0.8 g of carbohydrate per kilogram of body mass per hour during postexercise recovery, plasma glucose appearance is closely matched by its disposal rate. Coingestion of a casein protein hydrolysate with or without additional leucine substantially increases the insulin response, but this does not augment plasma glucose disposal in endurance athletes during postexercise recovery.
Because of the regulatory role of insulin in postexercise protein anabolism, plasma glucose disposal and muscle glycogen synthesis, there is much interest in practical interventions that elevate postexercise insulin levels. An effective nutritional strategy to increase postprandial insulin secretion is to coingest protein with carbohydrate. The synergistically stimulating effect of the combined intake of carbohydrate and protein on plasma insulin levels has already been reported in the 1960s (21), and was later confirmed by Nuttall et al. (18). In more recent studies, we investigated the insulinotropic potential of various free amino acids and/or protein (hydrolysate) mixtures when ingested in combination with carbohydrate in both clinical as well as sport settings (25-28). In these studies strong positive correlations were observed between plasma leucine and phenylalanine concentrations and the concomitant plasma insulin response. In accordance, in vitro studies using isolated β-cells and insulin-secreting pancreatic β-cell lines have described various pathways by which leucine, phenylalanine and its derivatives could affect β-cell function and stimulate insulin secretion. We reported that coingestion of a mixture containing a protein hydrolysate, free leucine, and phenylalanine can maximize the postexercise insulin response in vivo (26).
In the present study, we investigated the insulinotropic properties of a protein hydrolysate with or without additional free leucine in the postexercise phase in highly trained endurance athletes. The combined ingestion of a casein hydrolysate and carbohydrate resulted in a 108 ± 17% greater insulin response when compared with the ingestion of carbohydrate only. These findings are in line with earlier reports, but the insulin response seems considerably higher in the present study (26-28). The latter suggests that the casein protein hydrolysate has a greater insulinotropic potential compared with the wheat protein hydrolysate that was applied before. Differences in insulinotropic potential between various protein hydrolysates are likely associated with the amino acid composition and/or the average amino acid chain length of the protein hydrolysate, which determine the rise in plasma amino acid concentrations following ingestion (26-28). More research is warranted to determine the main factors that are responsible for differences in insulinotropic potential between different protein sources. Addition of free leucine to the ingested carbohydrate-protein hydrolysate mixture further increased the insulin response, resulting in a 190 ± 33% greater insulin response compared with the ingestion of carbohydrate only. The latter shows that the addition of free phenylalanine, as applied in our earlier studies, is not necessary to obtain such high postexercise insulin responses.
In line with the two- to threefold greater insulin responses, we observed a 35 ± 5 and 42 ± 11% lower plasma glucose response in the CHO-PRO and the CHO-PRO-LEU trials, respectively, when compared with the ingestion of carbohydrate only (Fig. 2). This could be regarded as a direct consequence of increased insulin stimulated glucose uptake. However, the plasma glucose response does not necessarily reflect plasma glucose uptake, but merely represents the balance between glucose appearing from the gut/liver and the rate of glucose uptake. Therefore, in the present study, we applied continuous [6,6-2H2] glucose tracer infusions to determine the rate of appearance and disappearance of glucose in the circulation. Plasma glucose appearance (Ra) and disappearance rates (Rd) are presented in Figure 3 and Table 2. In the CHO-PRO and CHO-PRO-LEU trials, mean plasma glucose appearance rates were 12 ± 2% lower compared with those observed in the CHO trial (P < 0.05). The latter implies that the lower plasma glucose response in the CHO-PRO and CHO-PRO-LEU trials can at least partly be attributed to a reduced plasma glucose appearance rate during the early phases of postexercise recovery. Coingestion of the casein hydrolysate apparently results in a significant reduction in plasma glucose appearance. The latter could be due to a lower gastric empyting and/or intestinal uptake rate of the carbohydrate-protein mixtures. However, the latter was not accompanied by any gastrointestinal discomfort, as no significant differences between trials were reported in the questionnaires. In addition, it could also be speculated that hepatic glucose output after cessation of exercise is more rapidly reduced in the CHO-PRO and CHO-PRO-LEU trials due to the faster initial rise in plasma insulin concentration.
The 12 ± 2% lower plasma glucose appearance rates in the CHO-PRO and CHO-PRO-LEU trials were accompanied by matching plasma glucose disappearance rates (Table 2 and Fig. 3). After calculating plasma glucose disposal (the percentage of plasma glucose Ra that is taken up from the circulation), we observed that glucose disposal was identical in all trials. Remarkably, in contrast to recent observations in type 2 diabetes patients and healthy controls (29), plasma glucose appearance was fully matched by its disappearance. Plasma glucose Rd represented 100 ± 0.03% of its Ra within 30 min after ingesting the first bolus of test drink in all trials (Table 2). The latter implies that the initial rise in plasma glucose concentration was attributed to a less than 30-min delay in glucose disposal before matching the appearance rate of the ingested glucose in the circulation. Recently, similar nutritional interventions were performed in type 2 diabetes patients and healthy controls at rest (29). With a carbohydrate intake of 0.7 g of carbohydrate per kilogram per hour, it took those subjects 179 ± 8 and 90 ± 8 min, respectively, before Ra glucose was matched by an equivalent Rd. Coingestion of a mixture of protein hydrolysate, free leucine, and phenylalanine substantially reduced the time for glucose Rd to match Ra and, as such, improved plasma glucose disposal (29). In contrast, in the present study, the effects of the preceding exercise trial and/or the training status of the subjects resulted in a glucose disposal capacity that exceeded the amount of carbohydrate that was provided. Furthermore, glucose disposal did not decline substantially during 3.5 h of postexercise recovery (Fig. 3).
Postexercise muscle glycogen synthesis occurs in two distinct phases. The first phase occurs within the first hour following cessation of exercise, is characterized by a high glycogen synthesis rate, and seems to be independent of circulating insulin levels. Thereafter, in the second phase, muscle glycogen synthesis rates are lower and insulin dependent (13,20). Therefore, it has been suggested that the higher insulin levels following protein and/or free amino acid coingestion could explain the elevated muscle glycogen synthesis rates that have been reported (10,23,24,27,31). The latter has been attributed to the stimulating effects of insulin on either plasma glucose uptake and/or glycogen synthase activity (2,11,16). Here, we show that when endurance-trained athletes ingest 0.8 g of carbohydrate per kilogram per hour during the early stages of postexercise recovery, circulating insulin levels are not limiting whole-body plasma glucose disposal. The latter implies that a carbohydrate intake of 0.8 g of carbohydrate per kilogram per hour does not represent an optimal ingestion rate during acute postexercise recovery. Furthermore, these data also suggest that the reported stimulatory effect of protein and/or amino acid coingestion on postexercise muscle glycogen synthesis is more likely attributed to the insulin stimulated activation of glycogen synthase activity. In addition, it could be speculated that the elevated insulin response leads to a more tissue specific increase in insulin-stimulated glucose uptake. More research is warranted to elucidate the mechanism responsible as well as the optimal time frame for the suggested stimulatory effects of protein/amino acid coingestion on postexercise muscle glycogen synthesis.
From the respiratory measurements during postexercise recovery, we observed that protein and/or free leucine coingestion strongly affects energy metabolism. Oxygen uptake and energy expenditure were substantially greater during postexercise recovery in the CHO-PRO and CHO-PRO-LEU trials when compared with the CHO trial (Table 3). The latter can be explained by the strong thermogenic response of protein ingestion, which increased oxygen uptake by approximately 15% in the CHO-PRO and CHO-PRO-LEU trials. These findings seem to be in line with multiple studies on the effects of high protein diets on thermogenesis as a strategy for weight loss (8). Addition of leucine increased the insulin response, but did not further modulate the thermogenic response (Table 3).
In conclusion, coingestion of carbohydrate with a protein hydrolysate and/or free leucine substantially augments the insulin response during postexercise recovery in trained endurance athletes. When trained athletes ingest up to 0.8 g of carbohydrate per kilogram per hour during acute postexercise recovery, plasma glucose appearance is closely matched by its disposal rate. Even a two- to threefold increase in plasma insulin response does not modulate whole-body plasma glucose disposal during the first few hours of postexercise recovery during conditions when up to 0.8 g of carbohydrate per kilogram per hour is ingested.
We gratefully acknowledge the expert assistance of Gabby Hul and Joan Senden and the enthusiastic support of the subjects who volunteered to participate in these trials. The results of the study do not constitute endorsement of any product or food ingredient by the authors or ACSM. This research was supported by a grant from DSM Food Specialties (Delft, The Netherlands).
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