Glycemic response to carbohydrate and the effects of exercise and protein

A bout of aerobic exercise in the heat increases carbohydrate use but does not enhance the disposal of an oral glucose load, in healthy active individuals

We investigated if a bout of exercise in a hot environment (HEAT) would reduce the postprandial hyperglycemia induced by glucose ingestion. The hypothesis was that HEAT stimulating carbohydrate oxidation and glycogen use would increase the disposal of an ingested glucose load [i.e., oral glucose tolerance test (OGTT); 75 g of glucose]. Separated by at least 1 wk, nine young healthy individuals underwent three trials after an overnight fast in a randomized order. Two trials included 50 min of pedaling at 58 ± 5% V̇o2max either in a thermoneutral (21 ± 1°C; NEUTRAL) or in a hot environment (33 ± 1°C; HEAT) eliciting similar energy expenditure (503 ± 101 kcal). These two trials were compared with a no-exercise trial (NO EXER). Twenty minutes after exercise (or rest), subjects underwent an OGTT, while carbohydrate oxidation (CHOxid, using indirect calorimetry) plasma blood glucose, insulin concentrations (i.e., [glucose], [insulin]), and double tracer glucose kinetics ([U-13C] glucose ingestion and [6,6-2H2] glucose infusion) were monitored for 120 min. At rest, [glucose], [insulin], and rates of appearance/disappearance of glucose in plasma (glucose Ra/Rd) were similar among trials. During exercise, heart rate, tympanic temperature, [glucose], glycogen oxidation, and total CHOxid were higher during HEAT than NEUTRAL (i.e., 149 ± 35 vs. 124 ± 31 µmol·kg-1·min-1, P = 0.010). However, during the following OGTT, glucose Rd was similar in HEAT and NEUTRAL trials (i.e., 25.1 ± 3.6 vs. 25.2 ± 5.3 µmol·kg-1·min-1, P = 0.981). Insulin sensitivity (i.e., ISIndexMATSUDA) only improved in NEUTRAL compared with NO EXER (10.1 ± 4.6 vs. 8.8 ± 3.7 au; P = 0.044). In summary, stimulating carbohydrate use with exercise in a hot environment does not improve postprandial plasma glucose disposal or insulin sensitivity in a subsequent OGTT.NEW & NOTEWORTHY Exercise in the heat increases estimated muscle glycogen use. Reduced muscle glycogen after exercise in the heat could increase insulin-mediated glucose uptake during a subsequent oral glucose tolerance test (OGTT). However, plasma glucose kinetics are not improved during the OGTT in response to a bout of exercise in the heat, and insulin sensitivity worsens. Heat stress activates glucose counterregulatory hormones whose actions may linger during the OGTT, preventing increased glucose uptake.

The role of exercise and hypoxia on glucose transport and regulation

Muscle glucose transport activity increases with an acute bout of exercise, a process that is accomplished by the translocation of glucose transporters to the plasma membrane. This process remains intact in the skeletal muscle of individuals with insulin resistance and type 2 diabetes mellitus (T2DM). Exercise training is, therefore, an important cornerstone in the management of individuals with T2DM. However, the acute systemic glucose responses to carbohydrate ingestion are often augmented during the early recovery period from exercise, despite increased glucose uptake into skeletal muscle. Accordingly, the first aim of this review is to summarize the knowledge associated with insulin action and glucose uptake in skeletal muscle and apply these to explain the disparate responses between systemic and localized glucose responses post-exercise. Herein, the importance of muscle glycogen depletion and the key glucoregulatory hormones will be discussed. Glucose uptake can also be stimulated independently by hypoxia; therefore, hypoxic training presents as an emerging method for enhancing the effects of exercise on glucose regulation. Thus, the second aim of this review is to discuss the potential for systemic hypoxia to enhance the effects of exercise on glucose regulation.

Adding protein to a carbohydrate pre-exercise beverage does not influence running performance and metabolism

BACKGROUND

To analyze whether pre-exercise CHO+PRO vs. CHO intake distinctly influences running performance and metabolic biomarkers along a various of exercise intensities.

METHODS

In a randomized, double blind, counterbalanced, crossover and placebo control design, 10 middle distance runners were tested in 3 occasions. After 10 h of fasting, participants ingested isovolumic beverages (0.75+0.25g·BW^-1 of CHO+PRO, 1.0g·BW^-1 of CHO and placebo control) 30 min before a treadmill running incremental protocol of 4 min steps until exhaustion. Venous blood was collected at fasting, 30 min after beverage ingestion and after the 3^rd and 7^th running steps. Oxygen uptake-related variables, including respiratory exchange ratio, heart rate, plasma glucose, insulin, glucagon, free fatty acids, blood lactate concentrations, gastrointestinal discomfort and rate of perceived exertion were measured.

RESULTS

The addition of PRO to CHO had no influence on the measured variables, which did not differ between conditions along all incremental protocol intensities. The intake of CHO+PRO (compared to CHO) tended to decrease glycemia (106.5±21.3 vs. 113.6±26.5) and to increase insulinemia (14.4±15.1 vs. 12.7±10.8) at intensities close to maximum oxygen uptake.

CONCLUSIONS

The addition of PRO to a pre-exercise CHO beverage had no impact on running performance and related metabolic variables at a wide spectrum of exercise intensities.

Effect of exercise on acute postprandial glucose concentrations and interleukin-6 responses in sedentary and overweight males

This study examined the effect of 2 forms of exercise on glucose tolerance and the concurrent changes in markers associated with the interleukin (IL)-6 pathways. Fifteen sedentary, overweight males (29.0 ± 3.1 kg/m2) completed 2 separate, 3-day trials in randomised and counterbalanced order. An oral glucose tolerance test (OGTT; 75 g) was performed at the same time on each day of the trial. Day 2 of each trial consisted of a single 30-min workload-matched bout of either high-intensity intermittent exercise (HIIE; alternating 100% and 50% of peak oxygen uptake) or continuous moderate-intensity exercise (CME; 60 % of peak oxygen uptake) completed 1 h prior to the OGTT. Venous blood samples were collected before, immediately after, 1 h after, and 25 h after exercise for measurement of insulin, C-peptide, IL-6, and the soluble IL-6 receptors (sIL-6R; soluble glycoprotein 130 (sgp130)). Glucose area under the curve (AUC) was calculated from capillary blood samples collected throughout the OGTT. Exercise resulted in a modest (4.4%; p = 0.003) decrease in the glucose AUC when compared with the pre-exercise AUC; however, no differences were observed between exercise conditions (p = 0.65). IL-6 was elevated immediately after and 1 h after exercise, whilst sgp130 and sIL-6R concentrations were reduced immediately after exercise. In summary, exercise was effective in reducing glucose AUC, which was attributed to improvements that took place between 60 and 120 min into the OGTT, and was in parallel with an increased ratio of IL-6 to sIL-6R, which accords with an increased activation via the "classical" IL-6 signalling pathway. Our findings suggest that acute HIIE did not improve glycaemic response when compared with CME.

Effects of acute ingestion of whey protein with or without prior aerobic exercise on postprandial glycemia in type 2 diabetics

PURPOSE

Acute protein co-ingestion or a single bout of aerobic exercise can attenuate postprandial glycemia, but their combined effect has not been investigated in type 2 diabetics.

METHODS

Using a randomised crossover design, male type 2 diabetics (n = 8) [mean (95% CI); age, 55.0 (45.2, 64.8) year; BMI, 33.7 (25.6, 41.8) kg·m^- 2; 2 h glucose 14.0 (12.5, 15.5) mM] completed (1) 75 g oral glucose tolerance test (OGTT) (CON); (2) OGTT supplemented with 0.33 g·kg BM^- 1 of whey protein concentrate (PRO); or OGTT supplemented with PRO but preceded by a bout of aerobic cycling exercise (PRO + EX). Postprandial venous blood samples were collected for glucose, insulin, C-peptide and glucagon.

RESULTS

Despite a fold-increase of 1.90 (1.26, 2.56; p < 0.05) in postprandial insulin compared to CON, PRO failed to attenuate postprandial glycemia measured by 2 h glucose area under the curve. During PRO + EX, plasma glucose was elevated by 1.51 (0.5, 2.5) mM and 1.3 (0.3, 2.3) mM at 15 and 30 min, respectively, compared to CON, but was lower by 1.60 (0.6, 2.6) mM and 1.5 (0.5, 2.5) mM at 90 and 120 min, respectively (all p < 0.01). The additive effect of exercise and protein ingestion resulted in a fold-increase of 1.67 (1.35, 2.00; p < 0.05) in postprandial glucagon compared to CON.

CONCLUSION

In type 2 diabetics, prior aerobic exercise altered the humoral response to co-ingestion of whey protein with a carbohydrate load, but neither protein ingestion alone nor when preceded by prior exercise attenuated postprandial glycemia.

The Effect of Mode of Transport on Intraindividual Variability in Glycemic and Insulinemic Response Testing

The effect of light- to moderate-intensity exercise, such as that used as a mode of transport, on glycemic response testing is unclear. The aim was to investigate the effect of acute exercise (walking and cycling), simulated to act as a mode of transport, prior to glycemic response testing on the intraindividual variability of blood glucose and insulin. A total of 11 male participants visited the laboratory four times. Initially, they undertook a maximum oxygen uptake and two submaximal exercise tests. For the other three visits, they either rested (25 min), cycled, or walked 5 km followed by a 2-hr glycemic response test after consuming a glucose drink (50 g of available carbohydrate). The mean coefficient of variation of each transport group was below the International Organization for Standardization cutoff of 30%. The highest mean coefficient of variation of glucose area under the curve (AUC) was between the rest and the walking trials (30%) followed by walking and cycling (26%). For insulin AUC, the highest mean coefficient of variation was between walking and cycling (28%) followed by rest and walking (24%). The lowest glucose AUC and insulin AUC were between rest and cycling (25% and 14%, respectively). This study did not find differences (p > .05) between the conditions for glucose AUC (at 120 min, rest: 134.5 ± 104.6 mmol/L; walking: 115.5 ± 71.7 mmol/L; and cycling: 142.5 ± 75 mmol/L) and insulin AUC (at 120 min, rest: 19.45 ± 9.12 μmol/ml; walking: 16.49 ± 8.42 μmol/ml; and cycling: 18.55 ± 9.23 μmol/ml). The results indicate no difference between the tests undertaken; however, further research should ensure the inclusion of two rest conditions.

ACUTE CARDIOVASCULAR RESPONSE TO PRE-PRANDIAL AND POSTPRANDIAL EXERCISE IN ACTIVE MEN

ABSTRACT Introduction: Pre-prandial exercise promotes greater mobilization of fat metabolism due to the increased release of catecholamines, cortisol, and glucagon. However, this response affects how the cardiovascular system responds to exercise. Objective: To evaluate the response of systolic, diastolic, and mean blood pressure, heart rate (HR) and rate-pressure product (RPP) to pre- and postprandial exercise. Methods: Ten physically active male subjects (25.50 ± 2.22 years) underwent two treadmill protocols (pre- and postprandial) performed for 36 minutes at 65% of VO2max on different days. On both days, subjects attended the laboratory on a 10-hour fasting state. For the postprandial session, volunteers ingested a pre-exercise meal of 349.17 kcal containing 59.3 g of carbohydrates (76.73%), 9.97 g of protein (12.90%), and 8.01 g of lipids (10.37%). Blood pressure, HR and RPP were measured before and after exercise. The 2x2 factorial Anova with the multiple comparisons test of Bonferroni was applied to analyze cardiovascular variables in both moments (pre- vs. postprandial). The significance level was set at p<0.05. Results: Systolic (121.70 ± 7.80 vs. 139.78 ± 12.91 mmHg) and diastolic blood pressure (66.40 ± 9.81 vs. 80.22 ± 8.68 mmHg) increased significantly after exercise only in the postprandial session (p<0.05). HR increased significantly (p<0.05) after both protocols (64.20 ± 15.87 vs. 141.20 ± 10.33 bpm pre-prandial and 63.60 ± 8.82 vs. 139.20 ± 10.82 bpm postprandial). RPP had a similar result (8052.10 ± 1790.68 vs. 18382.60 ± 2341.66 mmHg.bpm in the pre-prandial session and 7772.60 ± 1413.76 vs. 19564.60 ± 3128.99 mmHg.bpm in the postprandial session). Conclusion: These data suggest that fasted exercise does not significantly alter the blood pressure. Furthermore, the meal provided before the postprandial exercise may promote a greater blood pressure responsiveness during exercise.

Effect of macronutrients and fiber on postprandial glycemic responses and meal glycemic index and glycemic load value determinations

Background: The potential confounding effect of different amounts and proportions of macronutrients across eating patterns on meal or dietary glycemic index (GI) and glycemic load (GL) value determinations has remained partially unaddressed.Objective: The study aimed to determine the effects of different amounts of macronutrients and fiber on measured meal GI and GL values.Design: Four studies were conducted during which participants [n = 20-22; women: 50%; age: 50-80 y; body mass index (in kg/m²): 25-30)] received food challenges containing different amounts of the variable nutrient in a random order. Added to the standard 50 g available carbohydrate from white bread was 12.5, 25, or 50 g carbohydrate; 12.5, 25, or 50 g protein; and 5.6, 11.1, or 22.2 g fat from rice cereal, tuna, and unsalted butter, respectively, and 4.8 or 9.6 g fiber from oat cereal. Arterialized venous blood was sampled for 2 h, and measured meal GI and GL and insulin index (II) values were calculated by using the incremental area under the curve (AUC_i) method.Results: Adding carbohydrate to the standard white-bread challenge increased glucose AUC_i (P < 0.0001), measured meal GI (P = 0.0066), and mean GL (P < 0.0001). Adding protein (50 g only) decreased glucose AUC_i (P = 0.0026), measured meal GI (P = 0.0139), and meal GL (P = 0.0140). Adding fat or fiber had no significant effect on these variables. Adding carbohydrate (50 g), protein (50 g), and fat (11.1 g) increased the insulin AUC_i or II; fiber had no effect.Conclusions: These data indicate that uncertainty in the determination of meal GI and GL values is introduced when carbohydrate-containing foods are consumed concurrently with protein (equal amount of carbohydrate challenge) but not with carbohydrate-, fat-, or fiber-containing foods. Future studies are needed to evaluate whether this uncertainty also influences the prediction of average dietary GI and GL values for eating patterns. This trial was registered at clinicaltrials.gov as NCT01023646.

Nighttime feeding likely alters morning metabolism but not exercise performance in female athletes

The timing of morning endurance competition may limit proper pre-race fueling and resulting performance. A nighttime, pre-sleep nutritional strategy could be an alternative method to target the metabolic and hydrating needs of the early morning athlete without compromising sleep or gastrointestinal comfort during exercise. Therefore, the purpose of this investigation was to examine the acute effects of pre-sleep chocolate milk (CM) ingestion on next-morning running performance, metabolism, and hydration status. Twelve competitive female runners and triathletes (age, 30 ± 7 years; peak oxygen consumption, 53 ± 4 mL·kg(-1)·min(-1)) randomly ingested either pre-sleep CM or non-nutritive placebo (PL) ∼30 min before sleep and 7-9 h before a morning exercise trial. Resting metabolic rate (RMR) was assessed prior to exercise. The exercise trial included a warm-up, three 5-min incremental workloads at 55%, 65%, and 75% peak oxygen consumption, and a 10-km treadmill time trial (TT). Physiological responses were assessed prior, during (incremental and TT), and postexercise. Paired t tests and magnitude-based inferences were used to determine treatment differences. TT performances were not different ("most likely trivial" improvement with CM) between conditions (PL: 52.8 ± 8.4 min vs CM: 52.8 ± 8.0 min). RMR was "likely" increased (4.8%) and total carbohydrate oxidation (g·min(-1)) during exercise was "possibly" or likely increased (18.8%, 10.1%, 9.1% for stage 1-3, respectively) with CM versus PL. There were no consistent changes to hydration indices. In conclusion, pre-sleep CM may alter next-morning resting and exercise metabolism to favor carbohydrate oxidation, but effects did not translate to 10-km running performance improvements.

The immediate effects of a single bout of aerobic exercise on oral glucose tolerance across the glucose tolerance continuum

We investigated glucose tolerance and postprandial glucose fluxes immediately after a single bout of aerobic exercise in subjects representing the entire glucose tolerance continuum. Twenty‐four men with normal glucose tolerance (NGT), impaired glucose tolerance (IGT), or type 2 diabetes (T2D; age: 56 ± 1 years; body mass index: 27.8 ± 0.7 kg/m², P > 0.05) underwent a 180‐min oral glucose tolerance test (OGTT) combined with constant intravenous infusion of [6,6‐²H₂]glucose and ingestion of [U‐¹³C]glucose, following 1 h of exercise (50% of peak aerobic power) or rest. In both trials, plasma glucose concentrations and kinetics, insulin, C‐peptide, and glucagon were measured. Rates (mg kg⁻¹ min⁻¹) of glucose appearance from endogenous (R_aEndo) and exogenous (oral glucose; R_aOGTT) sources, and glucose disappearance (R_d) were determined. We found that exercise increased R_aEndo, R_aOGTT, and R_d (all P < 0.0001) in all groups with a tendency for a greater (~20%) peak R_aOGTT value in NGT subjects when compared to IGT and T2D subjects. Accordingly, following exercise, the plasma glucose concentration during the OGTT was increased in NGT subjects (P < 0.05), while unchanged in subjects with IGT and T2D. In conclusion, while a single bout of moderate‐intensity exercise increased the postprandial glucose response in NGT subjects, glucose tolerance following exercise was preserved in the two hyperglycemic groups. Thus, postprandial plasma glucose responses immediately following exercise are dependent on the underlying degree of glycemic control.

This study shows that following an exercise bout, plasma glucose concentrations during an oral glucose tolerance test are increased in subjects with normal glucose tolerance, but unchanged in subjects with impaired glucose tolerance or type 2 diabetes. While rates of glucose disappearance and rates of glucose appearance from endogenous sources and from orally ingested glucose were all increased following exercise, there was a 20% greater peak value for the rate of orally ingested glucose appearance in normal glucose tolerant subjects, when compared to IGT and T2D subjects. In summary, postprandial plasma glucose responses immediately following exercise are dependent on the underlying level of glycemic control.

Pre-exercise nutrition: the role of macronutrients, modified starches and supplements on metabolism and endurance performance

Endurance athletes rarely compete in the fasted state, as this may compromise fuel stores. Thus, the timing and composition of the pre-exercise meal is a significant consideration for optimizing metabolism and subsequent endurance performance. Carbohydrate feedings prior to endurance exercise are common and have generally been shown to enhance performance, despite increasing insulin levels and reducing fat oxidation. These metabolic effects may be attenuated by consuming low glycemic index carbohydrates and/or modified starches before exercise. High fat meals seem to have beneficial metabolic effects (e.g., increasing fat oxidation and possibly sparing muscle glycogen). However, these effects do not necessarily translate into enhanced performance. Relatively little research has examined the effects of a pre-exercise high protein meal on subsequent performance, but there is some evidence to suggest enhanced pre-exercise glycogen synthesis and benefits to metabolism during exercise. Finally, various supplements (i.e., caffeine and beetroot juice) also warrant possible inclusion into pre-race nutrition for endurance athletes. Ultimately, further research is needed to optimize pre-exercise nutritional strategies for endurance performance.