Endogenous CHO Oxidation Research Articles

Glucose and Fructose Hydrogel Enhances Running Performance, Exogenous Carbohydrate Oxidation, and Gastrointestinal Tolerance.

Beneficial effects of carbohydrate (CHO) ingestion on exogenous CHO oxidation and endurance performance require a well-functioning gastrointestinal (GI) tract. However, GI complaints are common during endurance running. This study investigated the effect of a CHO solution-containing sodium alginate and pectin (hydrogel) on endurance running performance, exogenous and endogenous CHO oxidation, and GI symptoms. Eleven trained male runners, using a randomized, double-blind design, completed three 120-min steady-state runs at 68% V˙O2max, followed by a 5-km time-trial. Participants ingested 90 g·h-1 of 2:1 glucose-fructose (13C enriched) as a CHO hydrogel, a standard CHO solution (nonhydrogel), or a CHO-free placebo during the 120 min. Fat oxidation, total and exogenous CHO oxidation, plasma glucose oxidation, and endogenous glucose oxidation from liver and muscle glycogen were calculated using indirect calorimetry and isotope ratio mass spectrometry. GI symptoms were recorded throughout the trial. Time-trial performance was 7.6% and 5.6% faster after hydrogel ([min:s] 19:29 ± 2:24, P < 0.001) and nonhydrogel (19:54 ± 2:23, P = 0.002), respectively, versus placebo (21:05 ± 2:34). Time-trial performance after hydrogel was 2.1% faster (P = 0.033) than nonhydrogel. Absolute and relative exogenous CHO oxidation was greater with hydrogel (68.6 ± 10.8 g, 31.9% ± 2.7%; P = 0.01) versus nonhydrogel (63.4 ± 8.1 g, 29.3% ± 2.0%; P = 0.003). Absolute and relative endogenous CHO oxidation was lower in both CHO conditions compared with placebo (P < 0.001), with no difference between CHO conditions. Absolute and relative liver glucose oxidation and muscle glycogen oxidation were not different between CHO conditions. Total GI symptoms were not different between hydrogel and placebo, but GI symptoms were higher in nonhydrogel compared with placebo and hydrogel (P < 0.001). The ingestion of glucose and fructose in hydrogel form during running benefited endurance performance, exogenous CHO oxidation, and GI symptoms compared with a standard CHO solution.

Impact of slow versus rapid digesting carbohydrates on substrate oxidation in pre-pubertal children: A randomized crossover trial

Consumption of rapid digesting sugars by children are under increased scrutiny because of their contribution to unhealthy weight gain. Previous studies in adults and children have suggested that altering the blend of carbohydrates (CHOs) consumed may cause shifts in substrate utilization. The purpose of this study was to examine the effects of consuming a slow digesting carbohydrate (SDC) and rapid digesting carbohydrate (RDC) on CHO and fat oxidation, glucose, and insulin responses at rest, during exercise, and post-exercise rest in pre-pubescent children. A randomized, double-blind, crossover design was used. Nineteen pre-pubescent children (n=10 boys, n=9 girls, mean±standard error, age=9.84±0.37-yrs) participated. Visits to the laboratory began with a 30-min measurement of resting metabolism followed by consumption of either an RDC or SDC drink. Postprandial resting metabolism was recorded for 60-min, immediately followed by 60-min of submaximal cycling exercise while metabolism was recorded, which was immediately followed by another 60-min recording of post-exercise metabolism. Total CHO and fat oxidation, endogenous and exogenous CHO oxidation, blood glucose, and insulin were assessed. Total CHO oxidation rate (g∙min-1) was greater after the RDC drink at 60min (p=0.032). Endogenous CHO oxidation rate (g∙min-1) was greater after the SDC drink at 15min (p≤0.010). Cumulative endogenous CHO oxidation (g) was greater after the SDC drink at 45min (p=0.009). Endogenous CHO oxidation accounted for a greater proportion of substrate oxidation after the first 60-min rest period (p=0.028), while exogenous CHO oxidation accounted for a greater proportion of substrate oxidation for the RDC at all time points (p≤0.019). The present study provides novel data suggesting that an SDC promotes greater endogenous substrate utilization in pre-pubertal children, which may have beneficial health impacts on energy intake and carbohydrate regulation/metabolism during growth and development. NCT03185884, clinicaltrials.gov.

Endogenous versus exogenous carbohydrate oxidation measured by stable isotopes in pre-pubescent children plus 13C abundances in foods consumed three days prior

PurposeThe purposes of the present study were to (a) examine resting metabolism, substrate utilization, and endogenous versus exogenous carbohydrate (CHO) oxidation before and after 30-g rapidly-digesting carbohydrate (RDC) ingestion using indirect calorimetry and breath test analysis of stable isotope concentrations in pre-pubescent children and (b) report the 13C abundances in foods consumed for three days prior. MethodsNineteen children (n = 10 boys, n = 9 girls) at Tanner stage I or II participated (mean age ± 95% CI = 9.84 ± 0.77 y) in this study. Food was administered to the children for three days preceding their scheduled breath tests. Breath tests and indirect calorimetry were performed after an 8-h fast before and 60 min following consumption of a 30-g simple RDC drink consisting of maltodextrin and sucrose. Open circuit spirometry and indirect calorimetry monitored resting metabolism and CHO oxidation. Separate breath samples were taken every 15 min. Samples of all foods and breath samples were analyzed for 13C and 12C abundances with a stable-isotope mass spectrometer. Results13C in expired breath samples were −23.81 ± 1.64‰ at baseline and increased every 15 min after consumption of the CHO drink (p < 0.001–0.009). Cumulative total, endogenous, and exogenous CHO utilization increased during the post-prandial period (p < 0.001). Endogenous CHO oxidation was consistently greater than exogenous CHO oxidation (p < 0.001–0.002).Blood glucose was elevated from baseline at 30- and 60-min post-prandial (p < 0.001). Insulin did not change over time (p = 0.184). ConclusionsThe foods provided during the 3-day controlled diet effectively minimized 13C variation prior to metabolic testing. The 13C abundances of foods reported herein should serve as practical recommendations to reduce 13C intake before breath tests. While endogenous CHO oxidation remained greater in proportion to exogenous CHO oxidation, these findings suggest that even a relatively small amount of RDC can increase exogenous CHO oxidation and blood glucose in normal-weight children. To further examine shifts in endogenous versus exogenous CHO utilization, we recommend that future studies take steps to minimize 13C variation before breath tests and examine changes in substrate metabolism at rest and during exercise in normal weight and overweight pre-pubescent children. Clinical trial registration numberNCT03185884.

Comparable Exogenous Carbohydrate Oxidation With Lactose Or Sucrose Feeding During Endurance Exercise

PURPOSE: Endogenous carbohydrate (CHO) availability can limit endurance exercise performance. CHO ingested during exercise such as glucose, glucose polymers and sucrose are readily oxidized and can improve endurance performance. The extent to which lactose can be utilized as a fuel source during exercise is unknown. The purpose is to elucidate the metabolic response to lactose ingestion during endurance exercise, compared to sucrose or water. METHODS: 11 participants (mean ± SD: 22 ± 4 years, 50.9 ± 4.7 ml.min-1.kg-1) cycled for 150 min at 50% of Wmax on 3 occasions. During exercise participants ingested, in a randomized order, water (WAT), lactose (LAC) or sucrose (SUC) containing beverages; a CHO dose of 0.8 g.min-1 (48 g.h-1) was used. Indirect calorimetry was used to calculate fat and total CHO oxidation and stable isotope tracer methodology (natural high 13C abundance CHO ingestion, 13C:12C ratio determination in expired breath by isotope-ratio mass spectrometry) was used to quantify exogenous CHO oxidation. Venous blood samples were taken throughout exercise and analyzed for glucose, lactate and non-esterified fatty acids (NEFA). RESULTS: Mean exogenous CHO oxidation rates in LAC (0.55 ± 0.19 g.min-1) and SUC (0.58 ± 0.10 g.min-1) were similar (P=0.68). Endogenous CHO oxidation contributed less to energy expenditure (EE) in LAC (39 ± 14 %) than in WAT (50 ± 11 %, P=0.01) and SUC (50 ± 8 %, P=0.04). Fat oxidation contributed most to EE in WAT (50 ± 11 %), which was significantly greater than in LAC (42 ± 8 %, P=0.04) and SUC (28 ± 6 %, P<0.01), with fat oxidation higher in LAC than SUC (P<0.01). Plasma glucose was significantly higher in LAC (5.2 ± 0.4 mmol.L-1) and SUC (5.3 ± 0.5 mmol.L-1) than WAT (4.6 ± 0.5 mmol.L-1, P≤0.01). Plasma lactate was significantly higher in SUC (1.7 ± 0.5 mmol.L-1) than WAT (1.5 ± 0.4 mmol.L-1, P<0.01). Plasma NEFA were significantly higher in WAT (1.1 ± 0.4 mmol.L-1) than SUC (0.5 ± 0.3 mmol.L-1, P<0.01), and tended to be higher in WAT than LAC (0.7 ± 0.4 mmol.L-1, P=0.08). CONCLUSIONS: Exogenous CHO oxidation during exercise was comparable from ingested lactose and sucrose. However, lactose ingestion promoted higher fat and lower endogenous CHO oxidation during exercise than sucrose. This suggests that lactose may be at least as efficacious as commonly used CHO sources for improving exercise performance.

Comparable Exogenous Carbohydrate Oxidation from Lactose or Sucrose during Exercise.

ABSTRACTPurposeIngesting readily oxidized carbohydrates (CHO) such as sucrose during exercise can improve endurance performance. Whether lactose can be utilized as a fuel source during exercise is unknown. The purpose of this study was to investigate the metabolic response to lactose ingestion during exercise, compared with sucrose or water.MethodsEleven participants (age, 22 ± 4 yr; V̇O2peak, 50.9 ± 4.7 mL·min−1·kg−1) cycled at 50% Wmax for 150 min on five occasions. Participants ingested CHO beverages (lactose or sucrose; 48 g·h−1, 0.8 g·min−1) or water throughout exercise. Total substrate and exogenous CHO oxidation was estimated using indirect calorimetry and stable isotope techniques (naturally high 13C-abundance CHO ingestion). Naturally low 13C-abundance CHO trials were conducted to correct background shifts in breath 13CO2 production. Venous blood samples were taken to determine plasma glucose, lactate, and nonesterified fatty acid concentrations.ResultsMean exogenous CHO oxidation rates were comparable with lactose (0.56 ± 0.19 g·min−1) and sucrose (0.61 ± 0.10 g·min−1; P = 0.49) ingestion. Endogenous CHO oxidation contributed less to energy expenditure in lactose (38% ± 14%) versus water (50% ± 11%, P = 0.01) and sucrose (50% ± 7%, P ≤ 0.05). Fat oxidation was higher in lactose (42% ± 8%) than in sucrose (28% ± 6%; P ≤ 0.01); CHO conditions were lower than water (50% ± 11%; P ≤ 0.05). Plasma glucose was higher in lactose and sucrose than in water (P ≤ 0.01); plasma lactate was higher in sucrose than in water (P ≤ 0.01); plasma nonesterified fatty acids were higher in water than in sucrose (P ≤ 0.01).ConclusionsLactose and sucrose exhibited similar exogenous CHO oxidation rates during exercise at moderate ingestion rates. Compared with sucrose ingestion, lactose resulted in higher fat and lower endogenous CHO oxidation.

Co-ingestion Of Glucose And Fructose During Exercise On Acute Exposure To Altitude

The reduction in partial pressure of oxygen (O2) leading to reduced O2 delivery to tissues at altitude has the potential to alter substrate utilization during exercise. Experiments at sea level suggest performance benefits from consuming multiple transportable carbohydrates, the impact of this at altitude is not known. PURPOSE: To investigate how carbohydrate (CHO) (glucose and fructose) ingestion effects fuel utilization during exercise at 2500m and the impact of this on performance. METHODS: Ingestion of 1.2 g.min-1 glucose plus 0.8 g.min-1 fructose was compared to 1.2 g.min-1 glucose and a placebo during 120 minutes running at 70% V[Combining Dot Above]O2max performed by eight males in a normobaric hypoxic chamber set at 2500m altitude. Exogenous and endogenous oxidation was quantified using 13C stable mass isotope tracing techniques. Performance was measured in a 5 km TT. RESULTS: CHO ingestion shifted fuel use to predominantly CHO (79.51 ± 7.11, 79.45 ± 1.88 and 56.66 ± 20.18% in glucose plus fructose, glucose alone and placebo, respectively) compared with placebo where fat (20.49 ± 7.11, 20.55 ± 11.88 and 43.34 ± 20.18% in glucose plus fructose, glucose alone and placebo, respectively) was the dominant fuel. Co-ingestion of glucose and fructose led to significantly (p< 0.001; ES= 3.82) greater peak exogenous CHO oxidation rates compared to glucose alone (1.51 ± 0.14 g.min-1 and 0.96 ± 0.07 g.min-1, respectively) and this resulted in a significantly (p= 0.007; ES= 1.01) lower contribution from endogenous CHO oxidation (49.15 ± 8.85% and 59.65 ± 1.80%, respectively). Co-ingestion of glucose and fructose did not improve performance in the 5 km TT (25:37 ± 03:01 minutes) compared to glucose alone (25:13 ± 02:53 minutes). However, time to complete the 5 km TT was significantly (p= 0.002; ES= 0.47) faster in the glucose only trial compared to placebo (26:33 ± 2:59 minutes). CONCLUSION: CHO ingestion reduced the reliance on fat oxidation compared to placebo and the co-ingestion of glucose and fructose increased exogenous and spared endogenous CHO oxidation over glucose alone. However, the co-ingestion of glucose and fructose did not provide an ergogenic benefit at 2500m.

Fructose–Maltodextrin Ratio Governs Exogenous and Other CHO Oxidation and Performance

Fructose coingested with glucose in carbohydrate (CHO) drinks increases exogenous-CHO oxidation, gut comfort, and physical performance. This study aimed to determine the effect of different fructose-maltodextrin-glucose ratios on CHO oxidation and fluid absorption while controlling for osmolality and caloricity. In a crossover design, 12 male cyclists rode 2 h at 57% peak power then performed 10 sprints while ingesting artificially sweetened water or three equiosmotic 11.25% CHO-salt drinks at 200 mL·15 min, comprising weighed fructose and maltodextrin-glucose in ratios of 0.5:1 (0.5 ratio), 0.8:1 (0.8 ratio), and 1.25:1 (1.25 ratio). Fluid absorption was traced with D2O, whereas C-fructose and C-maltodextrin-glucose permitted fructose and glucose oxidation rate evaluation. The mean exogenous-fructose and exogenous-glucose oxidation rates were 0.27, 0.39, and 0.46 g·min and 0.65, 0.71, and 0.58 g·min in 0.5, 0.8, and 1.25 ratio drinks, representing mean oxidation efficiencies of 54%, 59%, and 55% and 65%, 85%, and 86% for fructose and glucose, respectively. With the 0.8 ratio drink, total exogenous-CHO oxidation rate was 18% (90% confidence interval, ±5%) and 5.2% (±4.6%) higher relative to 0.5 and 1.25 ratios, respectively, whereas respective differences in total exogenous-CHO oxidation efficiency were 17% (±5%) and 5.3% (±4.8%), associated with 8.6% and 7.8% (±4.2%) higher fructose oxidation efficiency. The effects of CHO ratio on water absorption were inconclusive. Mean sprint power with the 0.8 ratio drink was moderately higher than that with the 0.5 ratio (2.9%; 99% confidence interval, ±2.8%) and 1.25 ratio (3.1%; ±2.7%) drinks, with total- and endogenous-CHO oxidation rate, abdominal cramps, and drink sweetness qualifying as explanatory mechanisms. Enhanced high-intensity endurance performance with a 0.8 ratio fructose-maltodextrin-glucose drink is characterized by higher exogenous-CHO oxidation efficiency and reduced endogenous-CHO oxidation. The gut-hepatic or other physiological site responsible requires further research.

Carbohydrate Supplementation and Sex Differences in Fuel Selection during Exercise

To compare the effects of a high-CHO diet (80% CHO) and glucose ingestion (2 g x kg(-1)) during exercise (120 min, 57% VO2max) on fuel selection in women taking (W+OC) or not (W-OC) oral contraceptives and in men (six in each group). Substrate oxidation was measured using indirect respiratory calorimetry in combination with a tracer technique to compute the oxidation of exogenous (13C-glucose) and endogenous CHO. In the control situation (mixed diet with water ingestion during exercise), the percent contribution to the energy yield (%En) of CHO oxidation was higher in men than in women (62 vs 53 %En). The high-CHO diet and glucose ingestion during exercise separately increased the %En from CHO oxidation in both men (+12%) and women (+24%), and the sex difference observed in the control situation disappeared. However, the increase in the %En from total CHO oxidation observed when glucose was ingested during exercise and when combined with a high-CHO diet was larger in women than in men (+47 vs +17 %En). This was not attributable to a higher %En from exogenous glucose oxidation in women, for which no sex difference was observed (25 and 27 %En in men and women), but was attributable to a smaller decrease in endogenous glucose oxidation. No significant difference in fuel selection was observed between W+OC and W-OC. The increase in total CHO oxidation after the high-CHO diet was not different between sexes. Glucose ingestion during exercise, separately and combined to the high-CHO diet, had a greater effect in women than in men; this was mostly attributable to the smaller reduction in endogenous CHO oxidation.

Fuel selection and cycling endurance performance with ingestion of [13C]glucose: evidence for a carbohydrate dose response

Endurance performance and fuel selection while ingesting glucose (15, 30, and 60 g/h) was studied in 12 cyclists during a 2-h constant-load ride [approximately 77% peak O2 uptake] followed by a 20-km time trial. Total fat and carbohydrate (CHO) oxidation and oxidation of exogenous glucose, plasma glucose, glucose released from the liver, and muscle glycogen were computed using indirect respiratory calorimetry and tracer techniques. Relative to placebo (210+/-36 W), glucose ingestion increased the time trial mean power output (%improvement, 90% confidence limits: 7.4, 1.4 to 13.4 for 15 g/h; 8.3, 1.4 to 15.2 for 30 g/h; and 10.7, 1.8 to 19.6 for 60 g/h glucose ingested; effect size=0.46). With 60 g/h glucose, mean power was 2.3, 0.4 to 4.2% higher, and 3.1, 0.5 to 5.7% higher than with 30 and 15 g/h, respectively, suggesting a relationship between the dose of glucose ingested and improvements in endurance performance. Exogenous glucose oxidation increased with ingestion rate (0.17+/-0.04, 0.33+/-0.04, and 0.52+/-0.09 g/min for 15, 30, and 60 g/h glucose), but endogenous CHO oxidation was reduced only with 30 and 60 g/h due to the progressive inhibition of glucose released from the liver (probably related to higher plasma insulin concentration) with increasing ingestion rate without evidence for muscle glycogen sparing. Thus ingestion of glucose at low rates improved cycling time trial performance in a dose-dependent manner. This was associated with a small increase in CHO oxidation without any reduction in muscle glycogen utilization.

Fuel Selection during Prolonged Arm and Leg Exercise with 13C-Glucose Ingestion

To compare fuel selection during prolonged arm (AE) and leg exercise (LE) with water or glucose ingestion. Ten subjects (VO2max: 4.77 +/- 0.20 and 3.36 +/- 0.15 L x min(-1) for LE and AE, respectively) completed 120 min of LE and AE at 50% of the mode-specific maximal power output (353 +/- 18 and 160 +/- 9 W, respectively) with ingestion of water (20 mL x kg(-1)) or 13C-glucose (2 g x kg(-1)). Substrate oxidation was measured using indirect respiratory calorimetry corrected for urea excretion and 13CO2 production at the mouth. The contribution of protein oxidation to the energy yield (%En) was higher during AE than LE (approximately 8% vs approximately 4%) because of the lower energy expenditure and was not significantly modified with glucose ingestion. With water ingestion, the %En from CHO oxidation was not significantly different during LE and AE (64 +/- 2% and 66 +/- 2%, respectively). Glucose ingestion significantly increased the %En from total CHO oxidation during AE (78 +/- 3%) but not during LE (71 +/- 2%). Exogenous glucose oxidation was not significantly different in AE and LE (56 +/- 4 and 65 +/- 3 g, respectively), but the %En from exogenous glucose was higher during AE than LE (30 +/- 1% and 24 +/- 1%) because of the lower energy expenditure. When glucose was ingested, the %En from endogenous CHO oxidation was significantly reduced during both AE (66 +/- 2% to 48 +/- 3%) and LE (64 +/- 2% to 47 +/- 3%) and was not significantly different in the two modes of exercise. The difference in fuel selection between AE and LE when water was ingested was modest with a slightly higher reliance on CHO oxidation during AE. The amount of exogenous glucose oxidized was lower but its %En was higher during AE because of the lower energy expenditure.

Energy substrate utilization during prolonged exercise with and without carbohydrate intake in preadolescent and adolescent girls

Little information is available on energy metabolism during exercise in girls, particularly the contribution of exogenous carbohydrate (CHO(exo)). The purpose of this study was to determine substrate utilization during exercise with and without CHO(exo) intake in healthy girls. Twelve-yr-old preadolescent (YG; n = 12) and 14-yr-old adolescent (OG; n = 10) girls consumed flavored water (WT) or (13)C-enriched 6% CHO (CT) while cycling for 60 min at approximately 70% maximal aerobic power (Vo(2max)). Substrate utilization was calculated for the final 15 min of exercise. CHO(exo) decreased fat oxidation by approximately 50% in YG but not in OG (P < 0.001) and decreased endogenous CHO oxidation by approximately 15% in OG but not in YG (P = 0.006). Endogenous CHO oxidation was lower in YG than in OG regardless of trial (P < or = 0.01), whereas fat oxidation was higher in YG only during WT (P < 0.001). CHO(exo) oxidation rate was similar between YG and OG (7.1 +/- 0.5 and 6.8 +/- 0.4 mg.kg(-1).min(-1), respectively, P = 0.67), contributing approximately 19% to total energy expenditure. Serum estradiol levels in all girls correlated with fat (r = -0.50 to -0.59, P = 0.03 to 0.005) and endogenous CHO oxidation (r = 0.50 to 0.63, P = 0.03 to 0.005) but not with CHO(exo) oxidation (r = -0.09, P = 0.71). We conclude that CHO(exo) influences endogenous substrate utilization in an age-dependent manner in healthy girls but that total CHO(exo) oxidation during exercise is not different between YG and OG. Our results also point to potential sex-related differences in energy substrate utilization even during childhood.

Exogenous Oxidation of Isomaltulose Is Lower than That of Sucrose during Exercise in Men

Isomaltulose (ISO) is a disaccharide that is slowly digested, resulting in a slow availability for absorption. The aim of this study was to compare the blood substrate responses and exogenous carbohydrate (CHO) oxidation rates from orally ingested sucrose (SUC) and ISO during moderate intensity exercise. We hypothesized that the oxidation of ISO is lower compared with SUC, resulting in lower plasma glucose and insulin concentrations and subsequent lower CHO and higher fat oxidation rates. Ten trained men [maximal oxygen uptake (VO2max), 64 ± 1 mL/(kg body mass·min)] cycled on 3 occasions for 150 min at 59 ± 2% VO2max and consumed either water (WAT) or 1 of 2 CHO solutions providing 1.1 g/min of CHO in the form of either SUC or ISO. Peak exogenous CHO oxidation rates were higher (P < 0.05) during the SUC trial (0.92 ± 0.03 g/min) than during the ISO trial (0.54 ± 0.05 g/min). Total endogenous CHO oxidation over the final 90 min of exercise was lower (P < 0.05) in the SUC trial (107 ± 10 g) than in the WAT (137 ± 7 g) and ISO (127 ± 9 g) trials. Fat oxidation was higher during the WAT trial than during the SUC and ISO trials. ISO resulted in a lower plasma insulin response at 30 min compared with SUC, whereas the glucose response did not differ between the 2 CHO. Oxidation of ingested ISO was significantly less than that of SUC, most likely due to the lower rate of digestion of ISO. A lower CHO delivery and a small difference in plasma insulin may have resulted in higher endogenous CHO use and higher fat oxidation during the ISO trial than during the SUC trial.

Dose-Response Effects of Ingested Carbohydrate on Exercise Metabolism in Women

The effect of different quantities of carbohydrate (CHO) intake on CHO metabolism during prolonged exercise was examined in endurance-trained females. On four occasions, eight females performed 2 h of cycling at approximately 60% .VO2max with ingestion of beverages containing low (LOW, 0.5 g.min(-1)), moderate (MOD, 1.0 g.min(-1)), or high (HIGH, 1.5 g.min(-1)) amounts of CHO, or water only (WAT). Test solutions contained trace amounts of [U-13C] glucose. Indirect calorimetry combined with measurement of expired 13CO2 and plasma 13C enrichment enabled calculation of exogenous CHO, liver-derived glucose, and muscle glycogen oxidation during the last 30 min of exercise. The highest rates of exogenous CHO oxidation were observed in MOD, with no further increases in HIGH (peak rates of 0.33 +/- 0.02, 0.50 +/- 0.03, and 0.48 +/- 0.05 g.min(-1) for LOW, MOD, and HIGH, respectively; P < 0.05 for LOW vs MOD and HIGH). Endogenous CHO oxidation was lowest in MOD (0.99 +/- 0.06, 0.82 +/- 0.08, 0.70 +/- 0.07, and 0.89 +/- 0.09 g.min(-1); P < 0.05 for MOD vs all other trials). Compared with WAT, CHO ingestion reduced liver glucose oxidation during exercise by approximately 30% (P < 0.05 for WAT vs all CHO). Differential rates of muscle glycogen oxidation were observed with different CHO doses (0.57 +/- 0.07, 0.53 +/- 0.08, 0.41 +/- 0.07, and 0.60 +/- 0.09 g.min(-1) for WAT, LOW, MOD, and HIGH respectively; P < 0.05 for MOD vs HIGH). In endurance-trained women, the highest rates of exogenous CHO oxidation and greatest endogenous CHO sparing was observed when CHO was ingested at moderate rates (1.0 g.min(-1), 60 g.h(-1)) during exercise.

Exogenous carbohydrate oxidation rates are elevated after combined ingestion of glucose and fructose during exercise in the heat

The first purpose of this study was to investigate whether a glucose (GLU)+fructose (FRUC) beverage would result in a higher exogenous carbohydrate (CHO) oxidation rate and a higher fluid availability during exercise in the heat compared with an isoenergetic GLU beverage. A second aim of the study was to examine whether ingestion of GLU at a rate of 1.5 g/min during exercise in the heat would lead to a reduced muscle glycogen oxidation rate compared with ingestion of water (WAT). Eight trained male cyclists (maximal oxygen uptake: 64+/-1 ml.kg-1.min-1) cycled on three different occasions for 120 min at 50% maximum power output at an ambient temperature of 31.9+/-0.1 degrees C. Subjects received, in random order, a solution providing either 1.5 g/min of GLU, 1.0 g/min of GLU+0.5 g/min of FRUC, or WAT. Exogenous CHO oxidation during the last hour of exercise was approximately 36% higher (P<0.05) in GLU+FRUC compared with GLU, and peak oxidation rates were 1.14+/-0.05 and 0.77+/-0.08 g/min, respectively. Endogenous CHO oxidation was significantly lower (P<0.05) in GLU+FRUC compared with WAT. Muscle glycogen oxidation was not different after ingestion of GLU or WAT. Plasma deuterium enrichments were significantly higher (P<0.05) in WAT and GLU+FRUC compared with GLU. Furthermore, at 60 and 75 min of exercise, plasma deuterium enrichments were higher (P<0.05) in WAT compared with GLU+FRUC. Ingestion of GLU+FRUC during exercise in the heat resulted in higher exogenous CHO oxidation rates and fluid availability compared with ingestion of GLU and reduced endogenous CHO oxidation compared with ingestion of WAT.

High Exogenous Carbohydrate Oxidation Rates Following Glucose And Fructose Ingestion During Exercise In The Heat

Exercise in the heat results in greater reliance on carbohydrate (CHO) metabolism. However, the oxidation rate of ingested CHO is reduced in the heat compared with a cool environment possibly due to reduced CHO absorption in the heat. Interestingly, combined ingestion of glucose and fructose (GLU+FRUC) during exercise in a cool environment results in 20–55% higher exogenous CHO oxidation rates compared with ingestion of an isoenergetic amount of glucose (GLU) only. Exercise in the heat is also accompanied by significant losses of fluid and when not replaced, this may impair endurance performance. A CHO beverage that leads to high CHO and fluid delivery rates may have important implications for exercise performed in hot environments. PURPOSE The first purpose of this study was to investigate whether a GLU+FRUC beverage would result in a higher exogenous CHO oxidation rate and a higher fluid availability during exercise in the heat compared with a beverage containing an isoenergetic amount of GLU only. A second aim of the study was to examine whether ingestion of GLU at a rate of 1.5 g/min during exercise in the heat would lead to a reduced rate of muscle glycogen oxidation when compared with ingestion of water (WAT). METHODS Eight trained male cyclists (VO2max: 64±1 ml/kg/min) cycled on three different occasions for 120 min at 50% maximum power output (64±1 VO2max) at an ambient temperature of 31.9±0.1°C. Subjects received, in random order, a solution providing either 1.5 g/min of GLU, 1.0 g/min of GLU + 0.5 g/min of FRUC, or WAT. 2H2O was added to all drinks administered at t=60 min. A continuous infusion of [6,6–2H2]-glucose was performed in GLU and WAT. RESULTS Exogenous CHO oxidation rates during the last hour of exercise were ∼36% higher (P < 0.05) in GLU+FRUC compared with GLU and peak oxidation rates were 1.14±0.05 g/min and 0.77±0.08 g/min, respectively. Endogenous CHO oxidation was significantly lower (P < 0.05) in GLU+FRUC compared with WAT. No other differences were found in endogenous CHO oxidation rates between trials. Muscle glycogen oxidation was not different following ingestion of GLU or WAT. Plasma deuterium enrichments were significantly higher (P < 0.05) in WAT and GLU+FRUC compared with GLU. Furthermore, at 60 and 75 min of exercise, plasma deuterium enrichments were higher (P < 0.05) in WAT compared with GLU+FRUC. CONCLUSION Ingestion of GLU+FRUC during exercise in the heat resulted in higher exogenous CHO oxidation rates and fluid availability when compared with ingestion of GLU and reduced endogenous CHO oxidation when compared with ingestion of WAT. Supported by a grant of GlaxoSmithKline, Consumer Healthcare (UK)

High Exogenous Carbohydrate Oxidation Rates Following Glucose And Fructose Ingestion During Exercise In The Heat

Exercise in the heat results in greater reliance on carbohydrate (CHO) metabolism. However, the oxidation rate of ingested CHO is reduced in the heat compared with a cool environment possibly due to reduced CHO absorption in the heat. Interestingly, combined ingestion of glucose and fructose (GLU+FRUC) during exercise in a cool environment results in 20–55% higher exogenous CHO oxidation rates compared with ingestion of an isoenergetic amount of glucose (GLU) only. Exercise in the heat is also accompanied by significant losses of fluid and when not replaced, this may impair endurance performance. A CHO beverage that leads to high CHO and fluid delivery rates may have important implications for exercise performed in hot environments. PURPOSE The first purpose of this study was to investigate whether a GLU+FRUC beverage would result in a higher exogenous CHO oxidation rate and a higher fluid availability during exercise in the heat compared with a beverage containing an isoenergetic amount of GLU only. A second aim of the study was to examine whether ingestion of GLU at a rate of 1.5 g/min during exercise in the heat would lead to a reduced rate of muscle glycogen oxidation when compared with ingestion of water (WAT). METHODS Eight trained male cyclists (VO2max: 64±1 ml/kg/min) cycled on three different occasions for 120 min at 50% maximum power output (64±1 VO2max) at an ambient temperature of 31.9±0.1°C. Subjects received, in random order, a solution providing either 1.5 g/min of GLU, 1.0 g/min of GLU + 0.5 g/min of FRUC, or WAT. 2H2O was added to all drinks administered at t=60 min. A continuous infusion of [6,6–2H2]-glucose was performed in GLU and WAT. RESULTS Exogenous CHO oxidation rates during the last hour of exercise were ∼36% higher (P < 0.05) in GLU+FRUC compared with GLU and peak oxidation rates were 1.14±0.05 g/min and 0.77±0.08 g/min, respectively. Endogenous CHO oxidation was significantly lower (P < 0.05) in GLU+FRUC compared with WAT. No other differences were found in endogenous CHO oxidation rates between trials. Muscle glycogen oxidation was not different following ingestion of GLU or WAT. Plasma deuterium enrichments were significantly higher (P < 0.05) in WAT and GLU+FRUC compared with GLU. Furthermore, at 60 and 75 min of exercise, plasma deuterium enrichments were higher (P < 0.05) in WAT compared with GLU+FRUC. CONCLUSION Ingestion of GLU+FRUC during exercise in the heat resulted in higher exogenous CHO oxidation rates and fluid availability when compared with ingestion of GLU and reduced endogenous CHO oxidation when compared with ingestion of WAT. Supported by a grant of GlaxoSmithKline, Consumer Healthcare (UK)

Caffeine increases exogenous carbohydrate oxidation during exercise

Both carbohydrate (CHO) and caffeine have been used as ergogenic aids during exercise. It has been suggested that caffeine increases intestinal glucose absorption, but there are also suggestions that it may decrease muscle glucose uptake. The purpose of the study was to investigate the effect of caffeine on exogenous CHO oxidation. In a randomized crossover design, eight male cyclists (age 27 +/- 2 yr, body mass 71.2 +/- 2.3 kg, maximal oxygen uptake 65.7 +/- 2.2 ml x kg(-1) x min(-1)) exercised at 64 +/- 3% of maximal oxygen uptake for 120 min on three occasions. During exercise subjects ingested either a 5.8% glucose solution (Glu; 48 g/h), glucose with caffeine (Glu+Caf, 48 g/h + 5 mg x kg(-1) x h(-1)), or plain water (Wat). The glucose solution contained trace amounts of [U-13C]glucose so that exogenous CHO oxidation could be calculated. CHO and fat oxidation were measured by indirect calorimetry, and 13C appearance in the expired gases was measured by continuous-flow IRMS. Average exogenous CHO oxidation over the 90- to 120-min period was 26% higher (P < 0.05) in Glu+Caf (0.72 +/- 0.04 g/min) compared with Glu (0.57 +/- 0.04 g/min). Total CHO oxidation rates were higher (P < 0.05) in the CHO ingestion trials compared with Wat, but they were highest when Glu+Caf was ingested (1.21 +/- 0.37, 1.84 +/- 0.14, and 2.47 +/- 0.23 g/min for Wat, Glu, and Glu+Caf, respectively; P < 0.05). There was also a trend (P = 0.082) toward an increased endogenous CHO oxidation with Glu+Caf (1.81 +/- 0.22 g/min vs. 1.27 +/- 0.13 g/min for Glu and 1.12 +/- 0.37 g/min for Wat). In conclusion, compared with glucose alone, 5 mg x kg(-1) x h(-1) of caffeine coingested with glucose increases exogenous CHO oxidation, possibly as a result of an enhanced intestinal absorption.

High oxidation rates from combined carbohydrates ingested during exercise.

Studies that have investigated oxidation of a single carbohydrate (CHO) during exercise have reported oxidation rates of up to 1 g x min(-1). Recent studies from our laboratory have shown that a mixture of glucose and sucrose or glucose and fructose ingested at a high rate (1.8 g x min(-1)) leads to peak oxidation rates of approximately 1.3 g x min(-1) and results in approximately 20 to 55% higher exogenous CHO oxidation rates compared with the ingestion of an isocaloric amount of glucose. The purpose of the present study was to examine whether a mixture of glucose, sucrose and fructose ingested at a high rate would result in even higher exogenous CHO oxidation rates (>1.3 g x min(-1)). Eight trained male cyclists (VO2max: 64 +/- 1 mL x kg(-1) BM x min(-1)) cycled on three different occasions for 150 min at 62 +/- 1% VO2max and consumed either water (WAT) or a CHO solution providing 2.4 g x min(-1) of glucose (GLU) or 1.2 g x min(-1) of glucose + 0.6 g x min(-1) of fructose + 0.6 g x min(-1) of sucrose (MIX). High peak exogenous CHO oxidation rates were found in the MIX trial (1.70 +/- 0.07 g x min(-1)), which were approximately 44% higher (P < 0.01) compared with the GLU trial (1.18 +/- 0.04 g x min(-1)). Endogenous CHO oxidation was lower (P < 0.05) in MIX compared with GLU (0.76 +/- 0.12 and 1.05 +/- 0.06 g x min(-1), respectively). When glucose, fructose and sucrose are ingested simultaneously at high rates (2.4 g x min(-1)) during cycling exercise, exogenous CHO oxidation rates can reach peak values of approximately 1.7 g x min(-1) and estimated endogenous CHO oxidation is reduced compared with the ingestion of an isocaloric amount of glucose.

High Oxidation Rates from a Mixture of Glucose, Sucrose and Fructose Ingested During Prolonged Exercise

0151 It has been demonstrated that oxidation of carbohydrate (CHO) ingested during cycling exercise does not exceed ∼1 g/min. Suggestions have been made that intestinal CHO absorption could be a potential factor limiting the rate of exogenous CHO oxidation. Sucrose and fructose are absorbed by intestinal transport mechanisms that are, at least in part, different from intestinal glucose transport which may become saturated at an ingestion rate of 1.2 g/min. Recent studies from our laboratory have shown that a mixture of glucose and sucrose or glucose and fructose when ingested at a high rate (1.8 g/min) leads to peak oxidation rates of ∼1.25 g/min and results in ∼20 to 50% higher exogenous CHO oxidation rates compared with the ingestion of an isoenergetic amount of glucose. Combined ingestion of glucose, sucrose and fructose at high rates might provide CHO for these different transport mechanisms and this might further increase the rate of exogenous CHO oxidation. PURPOSE: The purpose of the present study was to examine whether a mixture of glucose, sucrose and fructose when ingested at a high rate would lead to exogenous CHO oxidation rates higher than 1.25 g/min. METHODS: Eight trained male cyclists (VO2max: 64±1 ml/kg/min) cycled on three different occasions in random order and separated by at least one week. Each trial consisted of 150-min of cycling at 50% of maximal power output (62±1% VO2max) while subjects received a solution providing either 2.4 g/min of glucose (GLU) or 1.2 g/min of glucose + 0.6 g/min of fructose + 0.6 g/min of sucrose (MIX) or plain water (WAT). RESULTS: Peak exogenous CHO oxidation rates were ∼44% higher (P <0.01) in the MIX (1.70±0.07 g/min) compared with the GLU trial (1.18±0.04 g/min). Endogenous CHO oxidation was lower (P <0.05) in MIX compared with GLU (0.76±0.12 and 1.05 ±0.06 g/min, respectively). CONCLUSION: In conclusion, when glucose, fructose and sucrose are ingested simultaneously at high rates (2.4 g/min) during cycling exercise, exogenous CHO oxidation rates can reach peak values of ∼1.7 g/min and endogenous CHO oxidation is reduced compared with the ingestion of an isoenergetic amount of glucose. Supported by a grant of GlaxoSmithKline, Consumer Healthcare (UK)

Effects of carbohydrate (CHO) and fat supplementation on CHO metabolism during prolonged exercise

The aim of the study was to examine carbohydrate (CHO) utilization in subjects receiving CHO or CHO + medium-chain triglycerides (MCT) supplements during 180 minutes of exercise at 50% maximal aerobic work rate ([Wmax] 57% maximal oxygen consumption [Vo 2max]). In a double-blind crossover design, nine trained athletes cycled four times. Subjects received a bolus of 4 mL · kg −1 at the start and 2 mL · kg −1 every 20 minutes during exercise of either a 150-g · L −1 CHO solution (CHO trial), an equicaloric 70 energy % (en%) CHO—30 en% MCT suspension containing 29 g MCT (CHO + MCT trial), or a 150-g · L −1 CHO (high-CHO [HCHO]) solution plus 29 g MCT (HCHO + MCT trial). A fourth trial consisted of a 13C-background control trial (CON). The four trials were randomized. Before and after the exercise bout, muscle biopsies were taken from the quadriceps muscle and muscle glycogen levels were determined. During exercise, breath samples were collected for estimation of exogenous and endogenous CHO oxidation. No significant differences were detected in glycogen breakdown among the trials (277 ± 14 14 mmol · kg dry weight −1 CHO, 249 ± 20 CHO + MCT, and 240 ± 18 HCHO + MCT) or in the respiratory exchange ratio during exercise. Mean exogenous CHO oxidation rates during the final hour of exercise were 0.79, 0.63, and 0.73 g · min −1, respectively. No differences were observed between the trials regarding exogenous or endogenous CHO oxidation. Plasma free fatty acid (FFA) concentrations were elevated during exercise to a level of approximately 500 μmol · L −1 and were comparable in all trials, whereas plasma ketone concentrations significantly increased after MCT ingestion as compared with the CHO trial. It is concluded that 29 g MCT co-ingested with CHO during 180 minutes of exercise does not influence CHO utilization or glycogen breakdown.