Plasma Glucose Oxidation Research Articles

Ketone Monoester Plus Carbohydrate Supplementation Does Not Alter Exogenous and Plasma Glucose Oxidation or Metabolic Clearance Rate During Exercise in Men Compared with Carbohydrate Alone

BackgroundIncreasing β-hydroxybutyrate (βHB) availability through ketone monoester (KE) plus carbohydrate supplementation is suggested to enhance physical performance by sparing glucose use during exercise. However, no studies have examined the effect of ketone supplementation on glucose kinetics during exercise. ObjectivesThis exploratory study primarily aimed to determine the effect of KE plus carbohydrate supplementation on glucose oxidation during steady-state exercise and physical performance compared with carbohydrate alone. MethodsUsing a randomly assigned, crossover design, 12 men consumed 573 mg KE/kg body mass plus 110 g glucose (KE+CHO) or 110 g glucose (CHO) before and during 90 min of steady-state treadmill exercise [54 ± 3% peak oxygen uptake (V˙O2peak)] wearing a weighted vest (30% body mass; 25 ± 3 kg). Glucose oxidation and turnover were determined using indirect calorimetry and stable isotopes. Participants performed an unweighted time to exhaustion (TTE; 85% V˙O2peak) after steady-state exercise and a weighted (25 ± 3 kg) 6.4 km time trial (TT) the next day after consuming a bolus of KE+CHO or CHO. Data were analyzed by paired t-tests and mixed model ANOVA. ResultsβHB concentrations were higher (P < 0.05) after exercise [2.1 mM (95% CI: 1.6, .6)] and the TT [2.6 mM (2.1, 3.1)] in KE+CHO compared with CHO. TTE was lower [−104 s (−201, −8)], and TT performance was slower [141 s (19,262)] in KE+CHO than in CHO (P < 0.05). Exogenous [−0.01 g/min (−0.07, 0.04)] and plasma [−0.02 g/min (−0.08, 0.04)] glucose oxidation and metabolic clearance rate {MCR [0.38 mg·kg−1·min−1 (−0.79, 1.54)]} were not different, and glucose rate of appearance [−0.51 mg·kg−1·min−1 (−0.97, −0.04)], and disappearance [−0.50 mg·kg−1·min−1 (−0.96, −0.04)] were lower (P < 0.05) in KE+CHO compared with CHO during steady-state exercise. ConclusionsIn the current study, rates of exogenous and plasma glucose oxidation and MCR were not different between treatments during steady-state exercise, suggesting blood glucose utilization is similar between KE+CHO and CHO. KE+CHO supplementation also results in lower physical performance compared with CHO alone.This trial was registered at www.clinicaltrials.gov as NCT04737694.

Ketone Ester Plus Carbohydrate Supplementation Impairs Physical Performance And Decreases Glucose Turnover Versus Carbohydrate Alone

BACKGROUND: Ketone esters combined with carbohydrate have been reported to enhance physical performance compared to carbohydrate alone. Improvements in physical performance are suggested to result from sparing of glucose for fuel use during exercise; however, no studies have examined the effect of ketone supplementation on glucose kinetics. PURPOSE: Determine the effect of ketone ester plus carbohydrate (KE + CHO) supplementation on physical performance and glucose kinetics during steady-state aerobic exercise. METHODS: Twelve men (mean ± SD; 29 ± 5 y, 26 ± 3 kg/m2) completed this randomized, double-blind, crossover study. After controlling exercise and diet for 48 h, participants consumed a KE + CHO (KE: 573 mg KE/kg body mass, CHO: 110 g glucose) or CHO (110 g glucose) drink before and during 90 min of steady-state treadmill exercise (54 ± 3 % VO2peak) with a weighted vest (30% body mass; 25 ± 3 kg). Glucose kinetics during exercise were determined using indirect calorimetry and tracer techniques ([6,6-2H2] glucose and 13C glucose). Participants performed an unweighted time to exhaustion test (TTE; 85 % VO2peak) following steady-state exercise. After 24 h of controlled feeding, participants consumed a bolus of KE + CHO or CHO and completed a weighted 4-mile time trial (TT; 25 ± 3 kg). RESULTS: β-hydroxybutyrate concentrations were higher (P < 0.05) after exercise (2.3 ± 0.8 vs. 0.1 ± 0.1 mM) and the TT (2.7 ± 0.8 vs. 0.1 ± 0.1 mM) in KE + CHO than CHO. TTE was lower (761 ± 429 vs. 865 ± 458 sec) and TT performance was slower (55.0 ± 7.4 vs. 52.9 ± 6.6 min) in KE + CHO than CHO (P < 0.05). Exogenous (0.5 ± 0.1 vs. 0.5 ± 0.1 g/min) and plasma (0.6 ± 0.1 vs. 0.6 ± 0.1 g/min) glucose oxidation rates in the last 40 min of steady-state exercise were the same between KE + CHO and CHO (P > 0.05). Glucose rate of appearance (6.5 ± 1.4 vs. 7.0 ± 1.6 mg/kg/min) and disappearance (6.7 ± 1.3 vs. 7.2 ± 1.6 mg/kg/min) during steady-state exercise was lower (P < 0.05) in KE + CHO than CHO, and metabolic clearance rate (MCR) did not differ between treatments (10.5 ± 3.0 vs. 10.1 ± 3.4 mL/kg/min; P > 0.05). CONCLUSION: KE + CHO impaired TT and TTE performance compared to CHO alone. Though glucose turnover was lower, similar rates of exogenous glucose oxidation and MCR during steady-state exercise in KE + CHO and CHO suggests performance decrements occur independent of glucose kinetics.

Spectrophotometric Assay for the Quantification of Plasma Ethanol Levels in Mice through Chromium-Ethanol Oxidation-Reduction Reaction

The quantification of blood/plasma ethanol concentration (BEC/PEC) is of great importance in experiments involving basic research, clinical studies, and bioethanol production. Traditional methods commonly used to measure BEC can be expensive and require high-cost equipment and qualified labor. The aim of this study was to develop a low-cost method that can be performed with simple infrastructure commonly available in research laboratories. For this, we developed a protocol to quantify PEC in mice, using the method of reduction of potassium dichromate by ethanol. However, this oxidation-reduction (redox) reaction is not specific to ethanol. Thus, the PEC was measured following a sequence of chemical reactions to eliminate the reductive interfering substances presented in the samples. Firstly, we evaluated the sensitivity of the dichromate reactive to ethanol and to different reducing substances found in the plasma, in order to determine which the main interfering substances are. Next, once the main interfering substances were determined in the dichromate reduction, plasma was assayed for PEC. First, mice received intraperitoneally (i.p.) saline (basal reading, 0% ethanol) or ethanol injections (0.5, 1, 2, 3, and 4 g/kg) and had their plasma collected. After plasma deproteinization and plasma glucose oxidation, it was mixed with the dichromate/acetic acid reactive, and then the products of the redox reaction were determined by the spectrophotometric method. Then, we determined the PEC with the same plasma samples using a commercial ethanol assay kit as a positive control. We found an excellent correlation between the administered ethanol doses and PECs in both the methods analyzed. The values of PEC found in the dichromate reaction method were similar to those obtained in the literature with the same ethanol doses, and to the commercial enzyme activity assay. Therefore, despite the need for a background reading, this method can be successfully applied to determine PEC using low-cost chemical reagents.

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.

Fuel Use during Exercise at Altitude in Women with Glucose-Fructose Ingestion.

This study compared the coingestion of glucose and fructose on exogenous and endogenous substrate oxidation during prolonged exercise at terrestrial high altitude (HA) versus sea level, in women. Five women completed two bouts of cycling at the same relative workload (55% Wmax) for 120 min on acute exposure to HA (3375 m) and at sea level (~113 m). In each trial, participants ingested 1.2 g·min of glucose (enriched with C glucose) and 0.6 g·min of fructose (enriched with C fructose) before and every 15 min during exercise. Indirect calorimetry and isotope ratio mass spectrometry were used to calculate fat oxidation, total and exogenous carbohydrate oxidation, plasma glucose oxidation, and endogenous glucose oxidation derived from liver and muscle glycogen. The rates and absolute contribution of exogenous carbohydrate oxidation was significantly lower at HA compared with sea level (effect size [ES] > 0.99, P < 0.024), with the relative exogenous carbohydrate contribution approaching significance (32.6% ± 6.1% vs 36.0% ± 6.1%, ES = 0.56, P = 0.059) during the second hour of exercise. In comparison, no significant differences were observed between HA and sea level for the relative and absolute contributions of liver glucose (3.2% ± 1.2% vs 3.1% ± 0.8%, ES = 0.09, P = 0.635 and 5.1 ± 1.8 vs 5.4 ± 1.7 g, ES = 0.19, P = 0.217), and muscle glycogen (14.4% ± 12.2% vs 15.8% ± 9.3%, ES = 0.11, P = 0.934 and 23.1 ± 19.0 vs 28.7 ± 17.8 g, ES = 0.30, P = 0.367). Furthermore, there was no significant difference in total fat oxidation between HA and sea level (66.3 ± 21.4 vs 59.6 ± 7.7 g, ES = 0.32, P = 0.557). In women, acute exposure to HA reduces the reliance on exogenous carbohydrate oxidation during cycling at the same relative exercise intensity.

Effect of a cereal and milk meal with or without fruits and nuts on the postprandial glycemic response in Indian men.

Some cereals, consumed at breakfast, have shown lower glycemic responses. Limited data exist in the Indian context, where the effect could be modified due to genetic or racial differences. This study aimed to investigate the effect of cereal and milk, with or without fruits/nuts, on the glycemic response in healthy Indian men. A randomized cross-over study was carried out on 16 men (18 - 45 years), with 3 interventions providing equal amounts of glycemic carbohydrate: a glucose drink (Reference), cereal and milk (CM), and cereal, milk, fruits and nuts (CMO), on separate days. Plasma glucose, serum insulin, C-peptide, ghrelin, energy expenditure (EE), substrate oxidation and appetite/satiety were measured repeatedly over 3 hours post meal. A significant time effect and time x meal interaction between the meals, higher for the Reference meal, was observed for plasma glucose (p<0.001), insulin (p<0.001), C-peptide (p<0.001), and carbohydrate oxidation (p<0.001); while lower for satiety (p<0.001). The plasma glucose concentrations of CM and CMO meals returned to baseline 60 min postprandially, then remained there, unlike the Reference meal, where the plasma glucose values returned to baseline at 120 min and dipped significantly below baseline at 150 and 180 min. A significant effect of time (p<0.001) was observed for EE between meals. Ghrelin levels did not differ significantly between the test meals. Cereal with milk, along with fruits and nuts at breakfast, has a lower and stable glycemic response, along with increased satiety among healthy male subjects.

Effect of a Carbohydrate Mouth Rinse on Plasma Glucose and Plasma Insulin during a 40‐km Cycling Time Trial

Rinsing the mouth with a carbohydrate solution, without ingesting, improves exercise performance of up to an hour in duration. Performance gains are attributed to the activation of brain regions associated with motivation and motor control. In addition to this neural response, a hormonal cascade may also be present, and include a cephalic‐phase insulin response from oral carbohydrate recognition.PURPOSEThe aim of this study was to examine whether exercise performance gains achieved through a carbohydrate mouth rinse are the partial result of a cephalic‐phase insulin response from oral carbohydrate recognition.METHODSEight trained, competitive cyclists [age (mean ± SEM) = 24 ± 2 y; VO2max = 64.5 ± 2.8 ml/kg/min] completed three simulated, 40‐km time trials comprised of a familiarization trial, carbohydrate mouth rinse trial (CHO) and placebo trial (PLA). Mouth rinse was administered prior to onset of exercise, and every 5‐km throughout exercise. During CHO and PLA trials, plasma glucose and plasma insulin samples were obtained immediately prior to mouth rinsing procedures. Metabolic data and cycling time were collected throughout the exercise session. CHO oxidation rates were calculated based on the equation: g/min = 4.210 VCO2 – 2.962 VO2, while fat oxidation rates were calculated based on: (g/min) = 1.695 VO2 – 1.701 VCO2 (Jeukendrup 2005).RESULTSTime to completion was faster for CHO (67.1 ± 1.1 sec) compared to PLA (67.9 ± 1.0 sec). Plasma glucose did not differ between conditions (p = 0.801) and held steady throughout exercise; additionally, respiratory exchange ratio was not statistically different between conditions (p = 0.081). There was no change in plasma insulin either within trials or between trials (p = 0.638). CHO oxidation rates were not statistically different (p = 0.06) between the CHO (3.43 ± 0.80 g/min) and PLA (2.98 ± 0.18 g/min) groups; fat oxidation rates were not statistically different (p = 0.10) between the CHO (0.59 ± 0.09) and PLA (0.76 ± 0.06) groups.CONCLUSIONSRinsing the mouth with a carbohydrate solution prior to exercise and after each 5‐km interval improved 40‐km time trial performance. This finding shows that improved exercise performance occurs independent of changes in plasma glucose, plasma insulin, RER, or substrate oxidation rates. These findings build on previous research to support the notion of a CHO mouth rinse having primarily a neurological response (Chambers 2009), and not acting via a cephalic‐phase insulin response, in order to improve exercise performance.Support or Funding InformationIndiana University – School of Public Health Student Research Grant Award ‐ March 2016 ($1000)BK Betty Stevens Award (UF funding for trip to Experimental Biology 2017)

Sardine protein diet increases plasma glucagon-like peptide-1 levels and prevents tissue oxidative stress in rats fed a high-fructose diet.

The current study investigated whether sardine protein mitigates the adverse effects of fructose on plasma glucagon‑like peptide-1 (GLP-1) and oxidative stress in rats. Rats were fed casein (C) or sardine protein (S) with or without high‑fructose (HF) for 2 months. Plasma glucose, insulin, GLP‑1, lipid and protein oxidation and antioxidant enzymes were assayed. HF rats developed obesity, hyperglycemia, hyperinsulinemia, insulin resistance and oxidative stress despite reduced energy and food intakes. High plasma creatinine and uric acid levels, in addition to albuminuria were observed in the HF groups. The S‑HF diet reduced plasma glucose, insulin, creatinine, uric acid and homeostasis model assessment‑insulin resistance index levels, however increased GLP‑1 levels compared with the C‑HF diet. Hydroperoxides were reduced in the liver, kidney, heart and muscle of S‑HF fed rats compared with C‑HF fed rats. A reduction in liver, kidney and heart carbonyls was observed in S‑HF fed rats compared with C‑HF fed rats. Reduced levels of nitric oxide (NO) were detected in the liver, kidney and heart of the S‑HF fed rats compared with C‑HF fed rats. The S diet compared with the C diet reduced levels of liver hydroperoxides, heart carbonyls and kidney NO. The S‑HF diet compared with the C‑HF diet increased the levels of liver and kidney superoxide dismutase, liver and muscle catalase, liver, heart and muscle glutathione peroxidase and liver ascorbic acid. The S diet prevented and reversed insulin resistance and oxidative stress, and may have benefits in patients with metabolic syndrome.

Postprandial metabolic events in mini-pigs: new insights from a combined approach using plasma metabolomics, tissue gene expression, and enzyme activity

To unravel metabolic adaptations preceding the occurrence of metabolic dysregulations, a nutritional challenge appears as the best tool capable to reveal metabolic disturbances compared to single-point measurements at the static fasting (PA) state. The aim of the present work was to study the metabolic trajectories at the postprandial (PP) state in a relevant human nutrition animal model combining plasma metabolome analysis with classical metabolism exploration tools. In a first trial, three mini pigs were fed a test meal and arterial blood samples withdrawn before and over 4 h following the meal for plasma metabolites analysis (LC–MS and classical biochemistry). In a second trial, three mini-pigs were euthanized after an overnight fasting and three others 1:15 h after the test meal. The metabolism was explored at the molecular (mRNA levels), biochemical (enzyme activities) and signalling (phosphorylation status) levels in the liver and muscle. As expected, and in accordance with alterations in plasma glucose, urea levels, gluconeogenesis/glycolysis/lipid and amino acid (AA) oxidation genes expression and enzymes activities, the metabolomic approach discriminated the PA from the PP state (R2Ycum = 0.991; Q2Ycum = 0.921). More interestingly hierarchical cluster analysis revealed that the PP metabolome included actually four types of kinetic profiles. Further, PLS-DA analysis revealed a two-step pattern: 1–2 and 3–4 h (R2Ycum = 0.837; Q2cum = 0.635). Among the molecules explaining this discrimination, several AAs and phospholipid species were highlighted and their significance in PP metabolism discussed. Our data showed that the combination of these approaches in mini-pigs could be used to investigate PP metabolic adaptations in various physiological and patho-physiological states.

Brown adipose tissue improves whole-body glucose homeostasis and insulin sensitivity in humans.

Brown adipose tissue (BAT) has attracted scientific interest as an antidiabetic tissue owing to its ability to dissipate energy as heat. Despite a plethora of data concerning the role of BAT in glucose metabolism in rodents, the role of BAT (if any) in glucose metabolism in humans remains unclear. To investigate whether BAT activation alters whole-body glucose homeostasis and insulin sensitivity in humans, we studied seven BAT-positive (BAT+) men and five BAT-negative (BAT−) men under thermoneutral conditions and after prolonged (5–8 h) cold exposure (CE). The two groups were similar in age, BMI, and adiposity. CE significantly increased resting energy expenditure, whole-body glucose disposal, plasma glucose oxidation, and insulin sensitivity in the BAT+ group only. These results demonstrate a physiologically significant role of BAT in whole-body energy expenditure, glucose homeostasis, and insulin sensitivity in humans, and support the notion that BAT may function as an antidiabetic tissue in humans.

The Mediterranean Diet Adoption Improves Metabolic, Oxidative, and Inflammatory Abnormalities in Algerian Metabolic Syndrome Patients

This study was aimed to explore the effects of Mediterranean diet (MD) adoption on insulin resistance, oxidative, and inflammatory status in metabolic syndrome (MS) patients. Eighty four patients with MS were randomly recruited in the medical centers of Oran, Algeria. Eighteen healthy participants were selected as a control group. Among these 84 patients, only 36 patients completed the nutritional advices for 3 months. Patients were instructed to follow a Mediterranean-style diet and received some other selected nutritional and physical activity instructions. Anthropometric measurements were performed and a questionnaire was used to assess dietary intake. Blood samples were drawn at baseline and after 3 months of nutritional intervention from all subjects. At baseline, the MS patients were obese and had altered anthropometric parameters, higher systolic and diastolic blood pressure, plasma lipids, glucose, insulin, HOMA-IR, HbA1c, urea, creatinine, uric acid, and lower albumin compared to healthy subjects. A decrease in plasma, erythrocyte, and platelet antioxidant enzymes, and a rise in lipid and protein oxidation, plasma CRP, and fibrinogen were noted in the MS patients. Moreover, they had an unbalanced dietary pattern when compared to Mediterranean recommendations. Patients following the Mediterranean-style diet had significantly reduced weight, BMI, waist circumference, waist/hip circumference ratio, decreased systolic and diastolic blood pressure, plasma glucose, insulin, HOMA-IR, HbA1c, cholesterol, triacylglycerols, CRP, urea, creatinine, creatinine clearance, lipid and protein oxidation, and higher plasma, erythrocyte, and platelet antioxidant enzymes. In conclusion, a lifestyle intervention based mainly on nutritional advices improves metabolic, oxidative, and inflammatory abnormalities of metabolic syndrome.

The effect of acute dark chocolate consumption on carbohydrate metabolism and performance during rest and exercise

Consumption of cocoa-enriched dark chocolate (DC) has been shown to alter glucose and insulin concentration during rest and exercise compared with cocoa-depleted control (CON). However, the impact of DC consumption on exercise metabolism and performance is uncertain. Therefore, we investigated carbohydrate metabolism via stable isotope tracer techniques during exercise after subjects ingested either DC or CON. Sixteen overnight-fasted male cyclists performed a single-blinded, randomized, crossover design trial, after consuming either DC or CON at 2 h prior to 2.5 h of steady-state (SS) exercise (∼45% peak oxygen uptake). This was followed by an ∼15-min time-trial (TT) and 60 min of recovery. [6,6-(2)H2]Glucose and [U-(13)C]glucose were infused during SS to assess glucose rate of appearance (Ra) and disappearance (Rd). After DC consumption, plasma (-)-glucose and insulin concentrations were significantly (p < 0.001) elevated throughout vs. CON. During SS, there was no difference in [6,6-(2)H2]glucose Ra between treatments, but towards the end of SS (last 60 min) there was a ∼16% decrease in Rd in DC vs. CON (p < 0.05). Accordingly, after DC there was an ∼18% significant decrease in plasma glucose oxidation (trial effect; p = 0.032), and an ∼15% increase in tracer-derived muscle glycogen utilization (p = 0.045) late during SS exercise. The higher blood glucose concentrations during exercise and recovery after DC consumption coincided with high concentrations of epicatechin and (or) theobromine. In summary, DC consumption altered muscle carbohydrate partitioning, between muscle glucose uptake and glycogen oxidation, but did not effect cycling TT performance.

Preexercise Galactose and Glucose Ingestion on Fuel Use during Exercise

This study determined the effect of ingesting galactose and glucose 30 min before exercise on exogenous and endogenous fuel use during exercise. Nine trained male cyclists completed three bouts of cycling at 60% W(max) for 120 min after an overnight fast. Thirty minutes before exercise, the cyclists ingested a fluid formulation containing placebo, 75 g of galactose (Gal), or 75 g of glucose (Glu) to which (13)C tracers had been added, in a double-blind randomized manner. Indirect calorimetry and isotope ratio mass spectrometry were used to calculate fat oxidation, total carbohydrate (CHO) oxidation, exogenous CHO oxidation, plasma glucose oxidation, and endogenous liver and muscle CHO oxidation rates. Peak exogenous CHO oxidation was significantly higher after Glu (0.68 ± 0.08 g.min(-1), P < 0.05) compared with Gal (0.44 ± 0.02 g.min(-1)); however, mean rates were not significantly different (0.40 ± 0.03 vs. 0.36 ± 0.02 g.min(-1), respectively). Glu produced significantly higher exogenous CHO oxidation rates during the initial hour of exercise (P < 0.01), whereas glucose rates derived from Gal were significantly higher during the last hour (P < 0.01). Plasma glucose and liver glucose oxidation at 60 min of exercise were significantly higher for Glu (1.07 ± 0.1 g.min(-1), P < 0.05, and 0.57 ± 0.08 g.min(-1), P < 0.01) compared with Gal (0.64 ± 0.05 and 0.29 ± 0.03 g.min(-1), respectively). There were no significant differences in total CHO, whole body endogenous CHO, muscle glycogen, or fat oxidation between conditions. The preexercise consumption of Glu provides a higher exogenous source of CHO during the initial stages of exercise, but Gal provides the predominant exogenous source of fuel during the latter stages of exercise and reduces the reliance on liver glucose.

The influence of sex, menstrual cycle phase, estrogen, obesity, and endurance training on carbohydrate and lipid metabolism at rest and during moderate-intensity endurance exercise

Carbohydrate (CHO) and fat constitute the main fuels used during endurance exercise. The purpose of this thesis was to determine how sex, menstrual cycle phase, estrogen, obesity, and endurance training (ET) influence substrate metabolism. In study 1, men (n = 11) and women (n = 13) underwent primed constant infusion of [6,6-2H] glucose with muscle biopsies taken before and after a 90 min bike ride at 65% VO2 peak. Luteal-phase women had lower muscle glycogen utilization compared with follicular-phase women. Women had lower respiratory exchange ratio (RER) and plasma glucose utilization than men. In study 2, men (n = 17) and women (n = 19) had muscle biopsies taken before and after a 90 min bike ride at 65% VO2 peak. Women had greater intramyocellular lipid (IMCL) content and lower RER during exercise, with no effect of sex on net IMCL utilization. In study 3, Men (n = 11) were treated with 17-β-estradiol (E2, 2 mg·d–1, 8 d) and placebo and underwent primed constant infusion of [6,6–2H] glucose with muscle biopsies taken before and after a 90 min bike ride at 65% VO2 peak. E2 lowered RER and plasma glucose oxidation during exercise, with no effect on muscle glycogen utilization. In study 4, lean (n = 12) and obese (n = 11) women underwent 12 weeks of ET with muscle biopsies taken before and after training. Obese women were insulin resistant compared with lean women. ET increased oxidative capacity and β-oxidation capacity, as well as mitochondrial content. ET redistributed IMCL from the subsarcolemmal region to the intermyofibrillar region of the muscle fibre. These studies demonstrate that substrate metabolism is affected by many factors in health and disease and the ability to manipulate these factors is of importance to athletes wishing to improve performance, as well as to obese individuals attempting to normalize an altered metabolic state.

Substrate Source Use in Older, Trained Males after Decades of Endurance Training

The purpose of this study was to compare substrate source use in older, long-term exercising, endurance-trained males with sedentary controls. [U-C]palmitate and [6,6-H2]glucose tracers were applied to assess plasma free fatty acid (FFA) and glucose oxidation rates, and to estimate muscle- and/or lipoprotein-derived triacylglycerol (TG) and muscle glycogen use. Subjects were 10 long-term exercising, endurance-trained males and 10 sedentary controls (age 57 +/- 1 and 60 +/- 2 yr, respectively). Muscle biopsy samples were collected before and after exercise to assess muscle fiber type-specific intramyocellular lipid and glycogen content. During exercise, plasma palmitate Ra, Rd, and Rox were significantly greater in the trained subjects compared with the controls (Ra: 0.36 +/- 0.02 and 0.25 +/- 0.02; Rd: 0.36 +/- 0.03 and 0.24 +/- 0.02; Rox: 0.31 +/- 0.02 and 0.20 +/- 0.02 mmol.min, respectively, P < 0.01). This resulted in greater plasma FFA and total fat oxidation rates in the trained versus sedentary subjects (P < 0.001). Muscle- and/or lipoprotein-derived TG use contributed 10 +/- 2 and 11 +/- 3% in the trained and control groups, respectively (NS). No significant net changes in muscle fiber lipid content were observed. Older, endurance-trained males oxidize more fat during moderate-intensity exercise than do sedentary controls. This greater total fat oxidation rate is attributed to a higher plasma FFA release, uptake, and oxidation rate. In contrast, intramyocellular triacylglycerol does not seem to represent a major substrate source during 1 h of moderate-intensity exercise in older trained or sedentary men.

Maintenance of hyperglycaemia does not improve perfomence in a 100 km cycling time trial

Objectives. The aim of this study was to determine whether the elevated plasma glucose oxidation rate (~ 1.8 g.min-1) in the latter stages of prolonged exercise in subjects in which hyperglycaemia (± 10 mmol.l-1) is maintained via a glucose clamp, improves 100 km cycling time-trial (TT) performance. Design. Seven endurance-trained male cyclists (22±4 yrs) participated in this randomised crossover trial. On two occasions, separated by 7 - 10 days, subjects performed a self-paced TT in the laboratory. During one TT blood glucose was maintained at a euglycaemic concentration of ± 5 mmol.l-1 (ETT) and during the other, at ±10 mmol.l-1 (HTT). Each TT was interspersed with 5 X 1 km high-intensity periods (HIP) and 4 X 4 km HIP, in an attempt to mimic the variable intensity of competitive road races. Subjects were instructed to complete the TT in the ‘fastest time possible', taking the 9 HIP (21 km) into consideration. Results. There were no significant differences between ETT and HTT in overall time (143:09±7:14 v. 142:23±7:16 min:s), mean power (275±39 v. 279±39 W) and heart rate (160±9 v. 158±11 beats.min-1). Conclusion. Time trial performance over 100 km is not improved by maintaining a hyperglycaemic (10 mmol.l-1) blood glucose concentration. South African Journal of Sports Medicine Vol. 19 (3) 2007: pp. 94-98

Substrate source utilization during moderate intensity exercise with glucose ingestion in Type 1 diabetic patients

Substrate oxidation and the respective contributions of exogenous glucose, glucose released from the liver, and muscle glycogen oxidation were measured by indirect respiratory calorimetry combined with tracer technique in eight control subjects and eight diabetic patients (5 men and 3 women in both groups) of similar age, height, body mass, and maximal oxygen uptake, over a 60-min exercise period on cycle ergometer at 50.8% (SD 4.0) maximal oxygen uptake [131.0 W (SD 38.2)]. The subjects and patients ingested a breakfast (containing approximately 80 g of carbohydrates) 3 h before and 30 g of glucose (labeled with 13C) 15 min before the beginning of exercise. The diabetic patients also received their usual insulin dose [Humalog = 9.1 U (SD 0.9); Humulin N = 13.9 U (SD 4.4)] immediately before the breakfast. Over the last 30 min of exercise, the oxidation of carbohydrate [1.32 g/min (SD 0.48) and 1.42 g/min (SD 0.63)] and fat [0.33 g/min (SD 0.10) and 0.30 g/min (SD 0.10)] and their contribution to the energy yield were not significantly different in the control subjects and diabetic patients. Exogenous glucose oxidation was also not significantly different in the control subjects and diabetic patients [6.3 g/30 min (SD 1.3) and 5.2 g/30 min (SD 1.6), respectively]. In contrast, the oxidation of plasma glucose and oxidation of glucose released from the liver were significantly lower in the diabetic patients than in control subjects [14.5 g/30 min (SD 4.3) and 9.3 g/30 min (SD 2.8) vs. 27.9 g/30 min (SD 13.3) and 21.6 g/30 min (SD 12.8), respectively], whereas that of muscle glycogen was significantly higher [28.1 g/30 min (SD 15.5) vs. 11.6 g/30 min (SD 8.1)]. These data indicate that, compared with control subjects, in diabetic patients fed glucose before exercise, substrate oxidation and exogenous glucose oxidation overall are similar but plasma glucose oxidation is lower; this is associated with a compensatory higher utilization of muscle glycogen.

Carbohydrate supplementation during prolonged cycling exercise spares muscle glycogen but does not affect intramyocellular lipid use

Using contemporary stable-isotope methodology and fluorescence microscopy, we assessed the impact of carbohydrate supplementation on whole-body and fiber-type-specific intramyocellular triacylglycerol (IMTG) and glycogen use during prolonged endurance exercise. Ten endurance-trained male subjects were studied twice during 3 h of cycling at 63 ± 4% of maximal O2 uptake with either glucose ingestion (CHO trial; 0.7 g CHO kg−1 h−1) or without (CON placebo trial; water only). Continuous infusions with [U-13C] palmitate and [6,6-2H2] glucose were applied to quantify plasma free fatty acids (FFA) and glucose oxidation rates and to estimate intramyocellular lipid and glycogen use. Before and after exercise, muscle biopsy samples were taken to quantify fiber-type-specific IMTG and glycogen content. Plasma glucose rate of appearance (Ra) and carbohydrate oxidation rates were substantially greater in the CHO vs CON trial. Carbohydrate supplementation resulted in a lower muscle glycogen use during the first hour of exercise in the CHO vs CON trial, resulting in a 38 ± 19 and 57 ± 22% decreased utilization in type I and II muscle-fiber glycogen content, respectively. In the CHO trial, both plasma FFA Ra and subsequent plasma FFA concentrations were lower, resulting in a 34 ± 12% reduction in plasma FFA oxidation rates during exercise (P < 0.05). Carbohydrate intake did not augment IMTG utilization, as fluorescence microscopy revealed a 76 ± 21 and 78 ± 22% reduction in type I muscle-fiber lipid content in the CHO and CON trial, respectively. We conclude that carbohydrate supplementation during prolonged cycling exercise does not modulate IMTG use but spares muscle glycogen use during the initial stages of exercise in endurance-trained men.

Significant intramyocellular lipid use during prolonged cycling in endurance-trained males as assessed by three different methodologies

Intramyocellular triacylglycerol (IMTG) has been suggested to represent an important substrate source during exercise. In the present study, IMTG utilization during exercise is assessed through the use of various methodologies. In addition, we identified differences in the use of intramyocellular lipids deposited in the immediate subsarcolemmal (SS) area and those stored in the more central region of the fiber. Contemporary stable isotope technology was applied in combination with muscle tissue sampling before and immediately after 3 h of moderate-intensity cycling exercise (62 +/- 2% Vo(2 max)) in eight well-trained male cyclists. Continuous infusions with [U-13C]palmitate and [6,6-(2)H2]glucose were applied to quantify plasma free fatty acid (FFA) and glucose oxidation rates and to estimate whole body IMTG and glycogen use. Both immunohistochemical analyses of oil red O (ORO)-stained muscle cross sections and biochemical triacylglycerol (TG) extraction were performed to assess muscle lipid content. During exercise, plasma FFA, muscle (and/or lipoprotein)-derived TG, plasma glucose, and muscle glycogen oxidation contributed 24 +/- 2, 22 +/- 3, 11 +/- 1, and 43 +/- 3% to total energy expenditure, respectively. In accordance, a significant net decline in muscle lipid content was observed following exercise as assessed by ORO staining (67 +/- 8%) and biochemical TG extraction (49 +/- 8%), and a positive correlation was observed between methods (r = 0.56; P < 0.05). Lipid depots located in the SS area were utilized to a greater extent than the more centrally located depots. This is the first study to show significant use of IMTG as a substrate source during exercise in healthy males via the concurrent implementation of three major methodologies. In addition, this study shows differences in resting subcellular intramyocellular lipid deposit distribution and in the subsequent net use of these deposits during exercise.

Muscle glycogen oxidation during prolonged exercise measured with oral [13C]glucose: comparison with changes in muscle glycogen content

Plasma glucose and muscle glycogen oxidation during prolonged exercise [75-min at 48 and 76% maximal O(2) uptake (Vo(2 max))] were measured in eight well-trained male subjects [Vo(2 max) = 4.50 l/min (SD 0.63)] using a simplified tracer technique in which a small amount of glucose highly enriched in (13)C was ingested: plasma glucose oxidation was computed from (13)C/(12)C in plasma glucose (which was stable beginning at minute 30 and minute 15 during exercise at 48 and 76% Vo(2 max), respectively) and (13)CO(2) production, and muscle glycogen oxidation was estimated by subtracting plasma glucose oxidation from total carbohydrate oxidation. Consistent data from the literature suggest that this small dose of exogenous glucose does not modify muscle glycogen oxidation and has little effect, if any, on plasma glucose oxidation. The percent contributions of plasma glucose and muscle glycogen oxidation to the energy yield at 48% Vo(2 max) [15.1% (SD 3.8) and 45.9% (SD 5.8)] and at 76% Vo(2 max) [15.4% (SD 3.6) and 59.8% (SD 9.2)] were well in line with data previously reported for similar work loads and exercise durations using conventional tracer techniques. The significant reduction in glycogen concentration measured from pre- and postexercise vastus lateralis muscle biopsies paralleled muscle glycogen oxidation calculated using the tracer technique and was larger at 76% than at 48% Vo(2 max). However, the correlation coefficients between these two estimates of muscle glycogen utilization were not different from zero at each of the two work loads. The simplified tracer technique used in the present experiment appears to be a valid alternative approach to the traditional tracer techniques for computing plasma glucose and muscle glycogen oxidation during prolonged exercise.