Autoregulation of glucose production in men with a glycerol load during rest and exercise

Quantification of nutrient fluxes during acute exercise in mice

Despite the known metabolic benefits of exercise, an integrated metabolic understanding of exercise is lacking. Here, we use in vivo steady-state isotope-labeled infusions to quantify fuel flux and oxidation during exercise in fasted, fed, and exhausted female mice, revealing several novel findings. Exercise strongly promoted glucose fluxes from liver glycogen, lactate, and glycerol, distinct from humans. Several organs spared glucose, a process that broke down in exhausted mice despite concomitant hypoglycemia. Proteolysis increased markedly, also divergent from humans. Fatty acid oxidation dominated during fasted exercise. Ketone production and oxidation rose rapidly, seemingly driven by a hepatic bottleneck caused by gluconeogenesis-induced cataplerotic stress. Altered fuel consumption was observed in organs not directly involved in muscle contraction, including the pancreas and brown fat. Several futile cycles surprisingly persisted during exercise, despite their energy cost. In sum, we provide a comprehensive, integrated, holistic, and quantitative accounting of metabolism during exercise in an intact organism.

Effects of pre-exercise glycerol supplementation on dehydration, metabolic, kinematic, and thermographic variables in international race walkers

BACKGROUND

Due to the increase in global temperature, it is necessary to investigate solutions so that athletes competing in hot conditions can perform in optimal conditions avoiding loss of performance and health problems. Therefore, this study aims to evaluate the effect of pre-exercise glycerol supplementation during a rectangular test at ambient temperature mid (28.2ºC) on dehydration variables in international race walkers.

METHODS

Eight international male race walkers (age: 28.0 years (4.4); weight: 65.6 kg (6.6); height: 180.0 cm (5.0); fat mass: 6.72% (0.66); muscle mass: 33.3 kg (3.3); VO2MAX: 66.5 ml · kg-1·min-1 (1.9)) completed this randomized crossover design clinical trial. Subjects underwent two interventions: they consumed placebo (n = 8) and glycerol (n = 8) acutely, before a rectangular test where dehydration, RPE, metabolic, kinematic, and thermographic variables were analyzed before, during and after the test.

RESULTS

After the intervention, significant differences were found between groups in body mass in favor of the placebo (Placebo: -2.23 kg vs Glycerol: -2.48 kg; p = 0.033). For other variables, no significant differences were found.

CONCLUSION

Therefore, pre-exercise glycerol supplementation was not able to improve any dehydration, metabolic, kinematic, or thermographic variables during a rectangular test at temperature mid in international race walkers. Possibly, a higher environmental temperature could have generated a higher metabolic and thermoregulatory stress, generating differences between groups like other previous scientific evidence.

Time course of fractional gluconeogenesis after meat ingestion in healthy adults: a D2O study

In the postprandial state, glucose homeostasis is challenged by macronutrient intake, including proteins that trigger insulin secretion and provide glucose precursors. However, little is known about the postprandial response of gluconeogenesis to a protein meal. We aimed to quantify the evolution of fractional gluconeogenesis after a meat meal. Thirteen healthy subjects received oral doses of D₂O. After fasting overnight, they ingested a steak (120 g). Glycemia, insulinemia, and ²H enrichments in glucose and plasma water were measured for 8 h after the meal. Fractional gluconeogenesis was assessed using the average method. Glucose was stable for 5 h and then decreased. There was a slight increase of insulin 1 h after the meal. ²H enrichment in the carbon 5 position of glucose (C5) increased after 2 h, whereas it decreased in plasma water. Consequently, fractional gluconeogenesis increased from 68.2 ± 7.2% before the meal to 75.5 ± 5.8% 8 h after the meal, the latter corresponding to 22 h without a glucose supply. These values are consistent with the exhaustion of glycogen stores after 24 h but represent the highest among values in the literature. The impact of methodological conditions is discussed.

Liver glycogen metabolism during and after prolonged endurance-type exercise

Carbohydrate and fat are the main substrates utilized during prolonged endurance-type exercise. The relative contribution of each is determined primarily by the intensity and duration of exercise, along with individual training and nutritional status. During moderate- to high-intensity exercise, carbohydrate represents the main substrate source. Because endogenous carbohydrate stores (primarily in liver and muscle) are relatively small, endurance-type exercise performance/capacity is often limited by endogenous carbohydrate availability. Much exercise metabolism research to date has focused on muscle glycogen utilization, with little attention paid to the contribution of liver glycogen. (13)C magnetic resonance spectroscopy permits direct, noninvasive measurements of liver glycogen content and has increased understanding of the relevance of liver glycogen during exercise. In contrast to muscle, endurance-trained athletes do not exhibit elevated basal liver glycogen concentrations. However, there is evidence that liver glycogenolysis may be lower in endurance-trained athletes compared with untrained controls during moderate- to high-intensity exercise. Therefore, liver glycogen sparing in an endurance-trained state may account partly for training-induced performance/capacity adaptations during prolonged (>90 min) exercise. Ingestion of carbohydrate at a relatively high rate (>1.5 g/min) can prevent liver glycogen depletion during moderate-intensity exercise independent of the type of carbohydrate (e.g., glucose vs. sucrose) ingested. To minimize gastrointestinal discomfort, it is recommended to ingest specific combinations or types of carbohydrates (glucose plus fructose and/or sucrose). By coingesting glucose with either galactose or fructose, postexercise liver glycogen repletion rates can be doubled. There are currently no guidelines for carbohydrate ingestion to maximize liver glycogen repletion.

Gluconeogenesis and hepatic glycogenolysis during exercise at the lactate threshold

Because the maintenance of glycemia is essential during prolonged exercise, we examined the effects of endurance training, exercise intensity, and plasma lactate concentration ([lactate]) on gluconeogenesis (GNG) and hepatic glycogenolysis (GLY) in fasted men exercising at, and just below, the lactate threshold (LT), where GNG precursor lactate availability is high. Twelve healthy men (6 untrained, 6 trained) completed 60 min of constant-load exercise at power outputs corresponding to their individual LT. Trained subjects completed two additional 60-min sessions of constant-load exercise: one at 10% below the LT workload (LT-10%), and the other with a lactate clamp (LT-10%+LC) to match the [lactate] of the LT trial. Flux rates were determined by primed continuous infusion of [6,6-(2)H(2)]glucose, [3-(13)C]lactate, and [(13)C]bicarbonate tracers during 90 min of rest and 60 min of cycling. Exercise at LT corresponded to 67.6 ± 1.3 and 74.8 ± 1.7% peak O(2) consumption in the untrained and trained subjects, respectively (P < 0.05). Relative exercise intensity was matched between the untrained group at LT and the trained group at LT-10%, and [lactate] during exercise was matched in the LT and LT-10%+LC trials via exogenous lactate infusion. Glucose kinetics (rate of appearance, rate of disposal, and metabolic clearance rate) were augmented with the lactate clamp. GNG was decreased in the trained subjects exercising at LT and LT-10% compared with the untrained subjects, but increasing [lactate] in the LT-10%+LC trial significantly increased GNG (4.4 ± 0.9 mg·kg(-1)·min(-1)) compared with its corresponding control (1.7 ± 0.4 mg·kg(-1)·min(-1), P < 0.05). Hepatic GLY was higher in the trained than untrained subjects, but not significantly different across conditions. We conclude that GNG plays an essential role in maintaining total glucose production during exercise in fasted men, regardless of training state. However, endurance training increases the ability to achieve a higher relative exercise intensity and absolute power output at the LT without a significant decrease in GNG. Furthermore, raising systemic precursor substrate availability increases GNG during exercise, but not at rest.

Carbohydrate supplementation and sex differences in fuel selection during exercise

PURPOSE

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).

METHODS

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.

RESULTS

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.

CONCLUSIONS

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.

Predicting metabolic adaptation, body weight change, and energy intake in humans

Complex interactions between carbohydrate, fat, and protein metabolism underlie the body's remarkable ability to adapt to a variety of diets. But any imbalances between the intake and utilization rates of these macronutrients will result in changes in body weight and composition. Here, I present the first computational model that simulates how diet perturbations result in adaptations of fuel selection and energy expenditure that predict body weight and composition changes in both obese and nonobese men and women. No model parameters were adjusted to fit these data other than the initial conditions for each subject group (e.g., initial body weight and body fat mass). The model provides the first realistic simulations of how diet perturbations result in adaptations of whole body energy expenditure, fuel selection, and various metabolic fluxes that ultimately give rise to body weight change. The validated model was used to estimate free-living energy intake during a long-term weight loss intervention, a variable that has never previously been measured accurately.

Fuel selection during prolonged arm and leg exercise with 13C-glucose ingestion

PURPOSE

To compare fuel selection during prolonged arm (AE) and leg exercise (LE) with water or glucose ingestion.

METHODS

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.

RESULTS

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.

CONCLUSIONS

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.

Regulation of hepatic glucose production and the role of gluconeogenesis in humans: is the rate of gluconeogenesis constant?

We have been interested in the metabolic effects of ingested fuels, both in normal subjects and in people with type 2 diabetes. Recently, we have become interested in the regulation of glucose production and the regulation of gluconeogenesis in particular. We are not aware of a recent comprehensive review of these topics. Therefore, we have reviewed the currently available literature. The pertinent papers obtained from a Medline search of the words gluconeogenesis, glycogenolysis, hepatic glucose output, as well as papers from our personal files, form the basis of this review. In order to analyse the data, it also was necessary to review the relevant methodology used in determining gluconeogenesis. Pathway diagrams have been included with this review in order to illustrate and highlight key aspects of the methodologies. Current data support the hypothesis that the rate of glucose appearance changes but the rate of gluconeogenesis remains remarkably stable in widely varying metabolic conditions in people without diabetes. In people with diabetes, whether gluconeogenesis remains unchanged is at present uncertain. Available data are very limited. The mechanism by which gluconeogenesis remains relatively constant, even in the setting of excess substrates, is not known. One interesting speculation is that gluconeogenic substrates substitute for each other depending on availability. Thus, the overall rate is either unaffected or only modestly changed. This requires further confirmation.

Retention of intravenously infused [13C]bicarbonate is transiently increased during recovery from hard exercise

The effects of exercise on energy substrate metabolism persist into the postexercise recovery period. We sought to derive bicarbonate retention factors (k) to correct for carbon tracer oxidized, but retained from pulmonary excretion before, during, and after exercise. Ten men and nine women received a primed-continuous infusion of [(13)C]bicarbonate (sodium salt) under three different conditions: 1) before, during, and 3 h after 90 min of exercise at 45% peak oxygen consumption (Vo(2peak)); 2) before, during, and 3 h after 60 min of exercise at 65% Vo(2peak); and 3) during a time-matched resting control trial, with breath samples collected for determination of (13)CO(2) excretion rates. Throughout the resting control trial, k was stable and averaged 0.83 in men and women. During exercise, average k in men was 0.93 at 45% Vo(2peak) and 0.94 at 65% Vo(2peak), and in women k was 0.91 at 45% Vo(2peak) and 0.92 at 65% Vo(2peak), with no significant differences between intensities or sexes. After exercise at 45% Vo(2peak), k returned rapidly to control values in men and women, but following exercise at 65% Vo(2peak), k was significantly less than control at 30 and 60 min postexercise in men (0.74 and 0.72, respectively, P < 0.05) and women (0.75 and 0.76, respectively, P < 0.05) with no significant postexercise differences between men and women. We conclude that bicarbonate/CO(2) retention is transiently increased in men and women for the first hour of postexercise recovery following endurance exercise bouts of hard but not moderate intensity.

Hepatic autoregulation: response of glucose production and gluconeogenesis to increased glycogenolysis

The effect of increased glycogenolysis, simulated by galactose's conversion to glucose, on the contribution of gluconeogenesis (GNG) to hepatic glucose production (GP) was determined. The conversion of galactose to glucose is by the same pathway as glycogen's conversion to glucose, i.e., glucose 1-phosphate --> glucose 6-phosphate --> glucose. Healthy men (n = 7) were fasted for 44 h. At 40 h, hepatic glycogen stores were depleted. GNG then contributed approximately 90% to a GP of approximately 8 micromol.kg(-1).min(-1). Galactose, 9 g/h, was infused over the next 4 h. The contribution of GNG to GP declined from approximately 90% to 65%, i.e., by approximately 2 micromol.kg(-1).min(-1). The rate of galactose conversion to blood glucose, measured by labeling the infused galactose with [1-(2)H]galactose (n = 4), was also approximately 2 micromol.kg(-1).min(-1). The 41st h GP rose by approximately 1.5 micromol.kg(-1).min(-1) and then returned to approximately 9 micromol.kg(-1).min(-1), while plasma glucose concentration increased from approximately 4.5 to 5.3 mM, accompanied by a rise in plasma insulin concentration. Over 50% of the galactose infused was accounted for in blood glucose and hepatic glycogen formation. Thus an increase in the rate of GP via the glycogenolytic pathway resulted in a concomitant decrease in the rate of GP via GNG. While the compensatory response to the galactose administration was not complete, since GP increased, hepatic autoregulation is operative in healthy humans during prolonged fasting.

Contributions of working muscle to whole body lipid metabolism are altered by exercise intensity and training

To evaluate the contribution of working muscle to whole body lipid oxidation, we examined the effects of exercise intensity and endurance training (9 wk, 5 days/wk, 1 h, 75% Vo(2 peak)) on whole body and leg free fatty acid (FFA) kinetics in eight male subjects (26 +/- 1 yr, means +/- SE). Two pretraining trials [45 and 65% Vo(2 max) (45UT, 65UT)] and two posttraining trials [65% of pretraining Vo(2 peak) (ABT), and 65% of posttraining Vo(2 peak) (RLT)] were performed using [1-(13)C]palmitate infusion and femoral arteriovenous sampling. Training increased Vo(2 peak) by 15% (45.2 +/- 1.2 to 52.0 +/- 1.8 ml.kg(-1).min(-1), P < 0.05). Muscle FFA fractional extraction was lower during exercise (EX) compared with rest regardless of workload or training status ( approximately 20 vs. 48%, P < 0.05). Two-leg net FFA balance increased from net release at rest ( approximately -36 micromol/min) to net uptake during EX for 45UT (179 +/- 75), ABT (236 +/- 63), and RLT (136 +/- 110) (P < 0.05), but not 65UT (51 +/- 127). Leg FFA tracer measured uptake was higher during EX than rest for all trials and greater during posttraining in RLT (716 +/- 173 micromol/min) compared with pretraining (45UT 450 +/- 80, 65UT 461 +/- 72, P < 0.05). Leg muscle lipid oxidation increased with training in ABT (730 +/- 163 micromol/min) vs. 65UT (187 +/- 94, P < 0.05). Leg muscle lipid oxidation represented approximately 62 and 30% of whole body lipid oxidation at lower and higher relative intensities, respectively. In summary, training can increase working muscle tracer measured FFA uptake and lipid oxidation for a given power output, but both before and after training the association between whole body and leg lipid metabolism is reduced as exercise intensity increases.

Computational model of in vivo human energy metabolism during semistarvation and refeeding

Changes in body weight and composition are the result of complex interactions among metabolic fluxes contributing to macronutrient balances. To better understand these interactions, a mathematical model was constructed that used the measured dietary macronutrient intake during semistarvation and refeeding as model inputs and computed whole body energy expenditure, de novo lipogenesis, and gluconeogenesis as well as turnover and oxidation of carbohydrate, fat, and protein. Published in vivo human data provided the basis for the model components that were integrated by fitting a few unknown parameters to the classic Minnesota human starvation experiment. The model simulated the measured body weight and fat mass changes during semistarvation and refeeding and predicted the unmeasured metabolic fluxes underlying the body composition changes. The resting metabolic rate matched the experimental measurements and required a model of adaptive thermogenesis. Refeeding caused an elevation of de novo lipogenesis that, along with increased fat intake, resulted in a rapid repletion and overshoot of body fat. By continuing the computer simulation with the prestarvation diet and physical activity, the original body weight and composition were eventually restored, but body fat mass was predicted to take more than one additional year to return to within 5% of its original value. The model was validated by simulating a recently published short-term caloric restriction experiment without changing the model parameters. The predicted changes in body weight, fat mass, resting metabolic rate, and nitrogen balance matched the experimental measurements, thereby providing support for the validity of the model.

Lactate delivery (not oxygen) limits hepatic gluconeogenesis when blood flow is reduced

The purpose of this study was to determine, using the isolated liver perfusion technique, whether the limiting factor for hepatic gluconeogenesis (GNG) from lactate was precursor delivery or oxygen availability during reduced flow rates of 0.85 or 0.60 ml.min(-1).g liver(-1). After a 24-h fast, three different experimental protocols were employed. Protocol 1 examined the impact on GNG when reservoir lactate concentration was maintained but oxygen delivery was elevated via increases in hematocrit (Hct). Elevating the Hct from 22.5+/- 0.8% to 30.9+/- 0.4% at a blood flow of 0.89+/- 0.01 ml.min(-1).g liver(-1) increased the oxygen consumption (Vo(2)) but did not augment GNG. Similarly, when the Hct was elevated from 22.5+/- 0.8% to 41.5+/- 0.7% at 0.59+/- 0.04 ml.min(-1).g liver(-1), Vo(2) was increased, but GNG was unaffected. Protocol 2 examined the impact on GNG when Hct was maintained but precursor delivery was elevated via increases in reservoir lactate concentration ([LA]). Specifically, elevating the [LA] from 2.31+/- 0.07 to 3.61+/- 0.33 mM at a flow rate of 0.82+/- 0.04 ml.min(-1).g liver(-1) significantly increased GNG. Similarly, elevating the [LA] from 2.31+/- 0.07 to 4.24+/- 0.37 mM at a flow rate of 0.58+/- 0.02 ml.min(-1).g liver(-1) increased GNG. Finally, we examined the impact of increasing both the oxygen and lactate delivery (Protocol 3). Again, Vo(2) was elevated with increased oxygen delivery, but GNG was not augmented beyond that observed with elevations in lactate delivery alone, i.e., Protocol 2. The results indicate that, during decrements in blood flow, GNG is limited primarily by precursor delivery, not oxygen availability.

Impact of flow rate on lactate uptake and gluconeogenesis in glucagon-stimulated perfused livers

The impact of reduced hepatic flow on lactate uptake and gluconeogenesis was examined in isolated glucagon-stimulated perfused livers from 24-h-fasted rats. After surgical isolation, livers were perfused (single pass) for 30 min with Krebs-Henseleit (KH) bicarbonate buffer, fresh bovine erythrocytes (hematocrit approximately 20%), and no added substrate. After this "washout" period, steady-state perfusions were initiated with a second reservoir containing the KH buffer, bovine erythrocytes, [U-(14)C]lactate (10,000 dpm/ml), lactate (2.5 mM), and glucagon (250 microg/ml). Perfusion flow rate was adjusted to one of five rates (i.e., 1.8, 2.7, 3.9, 7.4, and 11.0 ml.min(-1).100 g body wt(-1)). After the perfusion, the liver was dissected out and weighed so as to establish the actual flow rate per gram of liver. The resulting flow rates ranged from 0.52 to 4.03 ml.min(-1).g liver(-1). As a function of flow rate, lactate uptake rose in a hyperbolic fashion to an apparent plateau of 2.34 micromol.min(-1).g liver(-1). Fractional extraction (FX) of lactate from the perfusate demonstrated an exponential decline with increased flow rates (r=0.97). At flow rates above 1.0 ml.min(-1).g liver(-1), adjustments in FX compensated for changes in lactate delivery, resulting in steady rates of lactate uptake and gluconeogenesis. Below 1.0.min(-1).g liver(-1) the increased FX was unable to compensate for the decline in lactate delivery and lactate uptake declined rapidly. Gluconeogenesis demonstrated similar kinetics to lactate uptake, reflecting its dominant role among pathways for lactate removal under the current conditions.

Relative importance of liver, kidney, and substrates in epinephrine-induced increased gluconeogenesis in humans

Splanchnic and renal net balance measurements indicate that lactate and glycerol may be important precursors for epinephrine-stimulated gluconeogenesis (GNG) in liver and kidney, but the effects of epinephrine on their renal and hepatic conversion to glucose in humans have not yet been reported. We therefore used a combination of renal balance and isotopic techniques in nine postabsorptive volunteers to measure systemic and renal GNG from these precursors before and during a 3-h infusion of epinephrine (270 pmol. kg-1. min-1) and calculated hepatic GNG as the difference between systemic and renal rates. During infusion of epinephrine, renal and hepatic GNG from lactate increased 4- to 6-fold and accounted for approximately 85 and 70% of renal and hepatic glucose release, respectively, at the end of study; renal and hepatic GNG from glycerol increased approximately 1.5- to 2-fold and accounted for approximately 7-9% of renal and hepatic glucose release at the end of study. The increased renal GNG from lactate and glycerol was due not only to their increased renal uptake (approximately 3.3- and 1.4-fold, respectively) but also increased renal gluconeogenic efficiency (approximately 1.8- and 1.5-fold). The increased renal uptake of lactate and glycerol was wholly due to their increased arterial concentrations, since their renal fractional extraction remained unchanged and renal blood flow decreased. We conclude that 1) lactate is the predominant precursor for epinephrine-stimulated GNG in both liver and kidney, 2) hepatic and renal GNG from lactate and glycerol are similarly sensitive to stimulation by epinephrine, and 3) epinephrine increases renal GNG from lactate and glycerol by increasing substrate availability and the gluconeogenic efficiency of the kidney.

Gluconeogenesis in humans with induced hyperlactatemia during low-intensity exercise

We studied the role of lactate in gluconeogenesis (GNG) during exercise in untrained fasting humans. During the final hour of a 4-h cycle exercise at 33-34% maximal O(2) uptake, seven subjects received, in random order, either a sodium lactate infusion (60 micromol x kg(-1) x min(-1)) or an isomolar sodium bicarbonate infusion. The contribution of lactate to gluconeogenic glucose was quantified by measuring (2)H incorporation into glucose after body water was labeled with deuterium oxide, and glucose rate of appearance (R(a)) was measured by [6,6-(2)H(2)]glucose dilution. Infusion of lactate increased lactate concentration to 4.4 +/- 0.6 mM (mean +/- SE). Exercise induced a decrease in blood glucose concentration from 5.0 +/- 0.2 to 4.2 +/- 0.3 mM (P < 0.05); lactate infusion abolished this decrease (5.0 +/- 0.3 mM; P < 0.001) and increased glucose R(a) compared with bicarbonate infusion (P < 0.05). Lactate infusion increased both GNG from lactate (29 +/- 4 to 46 +/- 4% of glucose R(a), P < 0.001) and total GNG. We conclude that lactate infusion during low-intensity exercise in fasting humans 1). increased GNG from lactate and 2). increased glucose production, thus increasing the blood glucose concentration. These results indicate that GNG capacity is available in humans after an overnight fast and can be used to sustain blood glucose levels during low-intensity exercise when lactate, a known precursor of GNG, is available at elevated plasma levels.

Lactate and glucose interactions during rest and exercise in men: effect of exogenous lactate infusion

To test the hypothesis that lactate plays a central role in the distribution of carbohydrate (CHO) potential energy for oxidation and glucose production (GP), we performed a lactate clamp (LC) procedure during rest and moderate intensity exercise. Blood [lactate] was clamped at approximately 4 mM by exogenous lactate infusion. Subjects performed 90 min exercise trials at 65 % of the peak rate of oxygen consumption (V(O(2))(,peak); 65 %), 55 % V(O(2))(,peak) (55 %) and 55 % V(O(2))(,peak) with lactate clamped to the blood [lactate] that was measured at 65 % V(O(2))(,peak) (55 %-LC). Lactate and glucose rates of appearance (R(a)), disappearance (R(d)) and oxidation (R(ox)) were measured with a combination of [3-(13)C]lactate, H(13)CO(3)(-), and [6,6-(2)H(2)]glucose tracers. During rest and exercise, lactate R(a) and R(d) were increased at 55 %-LC compared to 55 %. Glucose R(a) and R(d) were decreased during 55 %-LC compared to 55 %. Lactate R(ox) was increased by LC during exercise (55 %: 6.52 +/- 0.65 and 55 %-LC: 10.01 +/- 0.68 mg kg(-1) min(-1)) which was concurrent with a decrease in glucose oxidation (55 %: 7.64 +/- 0.4 and 55 %-LC: 4.35 +/- 0.31 mg kg(-1) min(-1)). With LC, incorporation of (13)C from tracer lactate into blood glucose (L GNG) increased while both GP and calculated hepatic glycogenolysis (GLY) decreased. Therefore, increased blood [lactate] during moderate intensity exercise increased lactate oxidation, spared blood glucose and decreased glucose production. Further, exogenous lactate infusion did not affect rating of perceived exertion (RPE) during exercise. These results demonstrate that lactate is a useful carbohydrate in times of increased energy demand.

Metabolic and cardiorespiratory responses to "the lactate clamp"

To evaluate the hypothesis that precursor supply limits gluconeogenesis (GNG) during exercise, we examined training-induced changes in glucose kinetics [rates of appearance (R(a)) and disappearance (R(d))], oxidation (R(ox)), and recycling (R(r)) with an exogenous lactate infusion to 3.5-4.0 mM during rest and to pretraining 65% peak O(2) consumption (VO(2 peak)) levels during exercise. Control and clamped trials (LC) were performed at rest pre- (P(R)R, P(R)R-LC) and posttraining (P(O)R, P(O)R-LC) and during exercise pre- (P(R)E(X)) and posttraining at absolute (P(O)A(B), P(O)A(B)-LC) and relative (P(O)R(L), P(O)R(L)-LC) intensities. Glucose R(r) was not different in any rest or exercise condition. Glucose R(a) did not differ as a result of LC. Glucose R(ox) was significantly decreased with LC at P(O)R (0.38 +/- 0.03 vs. 0.56 +/- 0.04 mg. kg(-1). min(-1)) and P(O)A(B) (3.82 +/- 0.51 vs. 5.0 +/- 0.62 mg. kg(-1). min(-1)). Percent glucose R(d) oxidized decreased with all LC except P(O)R(L)-LC (P(R)R, 32%; P(R)R-LC, 22%; P(O)R, 27%; P(O)R-LC, 20%; P(O)A(B), 95%; P(O)A(B)-LC, 77%), which resulted in a significant increase in oxidation from alternative carbohydrate (CHO) sources at rest and P(O)A(B). We conclude that 1) increased arterial [lactate] did not increase glucose R(r) measured during rest or exercise after training, 2) glucose disposal or production did not change with increased precursor supply, and 3) infusion of exogenous CHO in the form of lactate resulted in the decrease of glucose R(ox).

Evidence against glycogen cycling of gluconeogenic substrates in various liver preparations

The effect of inhibition of glycogen phosphorylase by 1,4-dideoxy-1,4-imino-d-arabinitol on rates of gluconeogenesis, gluconeogenic deposition into glycogen, and glycogen recycling was investigated in primary cultured hepatocytes, in perfused rat liver, and in fed or fasted rats in vivo clamped at high physiological levels of plasma lactate. 1,4-Dideoxy-1,4-imino-d-arabinitol did not alter the synthesis of glycerol-derived glucose in hepatocytes or lactate-derived glucose in perfused liver or fed or fasted rats in vivo. Thus, 1,4-dideoxy-1,4-imino-d-arabinitol inhibited hepatic glucose output in the perfused rat liver (0.77 +/- 0.19 versus 0.33 +/- 0.09, p < 0.05), whereas the rate of lactate-derived gluconeogenesis was unaltered (0.22 +/- 0.09 versus 0.18 +/- 0.08, p = not significant) (1,4-dideoxy-1,4-imino-d-arabinitol versus vehicle, micromol/min * g). Overall, the data suggest that 1,4-dideoxy-1,4-imino-d-arabinitol inhibited glycogen breakdown with no direct or indirect effects on the rates of gluconeogenesis. Total end point glycogen content (micromol of glycosyl units/g of wet liver) were similar in fed (235 +/- 19 versus 217 +/- 22, p = not significant) or fasted rats (10 +/- 2 versus 7 +/- 2, p = not significant) with or without 1,4-dideoxy-1,4-imino-d-arabinitol, respectively. The data demonstrate no glycogen cycling under the investigated conditions and no effect of 1,4-dideoxy-1,4-imino-d-arabinitol on gluconeogenic deposition into glycogen. Taken together, these data also suggest that inhibition of glycogen phosphorylase may prove beneficial in the treatment of type 2 diabetes.

Luteal and follicular glucose fluxes during rest and exercise in 3-h postabsorptive women

We examined the effects of exercise intensity and menstrual cycle phase on glucose flux rates during rest and exercise in rested and fed (3-h postabsorptive) women. Eight moderately active, eumenorrheic women were studied under conditions of rest (90 min) and exercise (60 min, leg ergometer cycling at 45 and 65% peak oxygen consumption) during follicular and luteal phases. In both menstrual phases, an effect of exercise intensity was evident with glucose rates of appearance and disappearance and metabolic clearance rates: rest < 45% intensity < 65% intensity (P < 0.05). In addition, we observed no significant effect of menstrual phase on glucose rates of appearance and disappearance and metabolic clearance rate during rest or exercise at either intensity. These results are interpreted to mean that in women fed several hours before study 1) glucose flux is directly related to exercise intensity, 2) menstrual cycle phase does not alter glucose flux during rest and exercise, and 3) the subtle effects of endogenous ovarian hormones on glucose kinetics are subordinate to the much larger effects of exercise and recent carbohydrate nutrition.

Measurement of gluconeogenesis in exercising men by mass isotopomer distribution analysis

We evaluated the hypothesis that coordinated adjustments in absolute rates of gluconeogenesis (GNG(ab)) and hepatic glycogenolysis (Gly) would maintain euglycemia and match glucose production (GP) to peripheral utilization during rest and exercise. Specifically, we evaluated the extent to which gradations in exercise power output would affect the contribution of GNG(ab) to GP. For these purposes, we employed mass isotopomer distribution analysis (MIDA) and isotope-dilution techniques on eight postabsorptive (PA) endurance-trained men during 90 min of leg cycle ergometry at 45 and 65% peak O(2) consumption (VO(2 peak); moderate and hard intensities, respectively) and the preceding rest period. GP was constant in resting subjects, whereas the fraction from GNG (f(GNG)) increased over time during rest (22.3 +/- 0.9% at 11.25 h PA vs. 25.6 +/- 0.9% at 12.0 h PA, P < 0.05). In the transition from rest to exercise, GP increased in an intensity-dependent manner (rest, 2.0 +/- 0.1; 45%, 4.0 +/- 0.4; 65%, 5.84 +/- 0.64 mg. kg(-1). min(-1), P < 0.05), although glucose rate of disappearance exceeded rate of appearance during the last 30 min of exercise at 65% VO(2 peak). Compared with rest, increases in GP were sustained by 92 and 135% increments in GNG(ab) during moderate- and hard-intensity exercises, respectively. Correspondingly, Gly (calculated as the difference between GP and MIDA-measured GNG(ab)) increased 100 and 203% over rest during the two exercise intensities. During moderate-intensity exercise, f(GNG) was the same as at rest; however, during the harder exercise f(GNG) decreased significantly to account for only 21% of GP. The highest sustained GNG(ab) observed in these trials on PA men was 1.24 +/- 0.3 mg. kg(-1). min(-1). We conclude that, after an overnight fast, 1) absolute GNG rates increased with intensity of effort despite a reduced f(GNG) at 65% VO(2 peak), 2) during exercise Gly is more responsible than GNG(ab) for maintaining GP, and 3) in 12-h fasted men, neither increased Gly or GNG(ab) nor was their combination able to maintain euglycemia during prolonged hard (65% VO(2 peak)) exercise.

Effects of free fatty acids and glycerol on splanchnic glucose metabolism and insulin extraction in nondiabetic humans

The present study sought to determine whether elevated plasma free fatty acids (FFAs) alter the ability of insulin and glucose to regulate splanchnic as well as muscle glucose metabolism. To do so, FFAs were increased in 10 subjects to approximately 1 mmol/l by an 8-h Intralipid/heparin (IL/Hep) infusion, whereas they fell to levels near the detection limit of the assay (<0.05 mmol/l) in 13 other subjects who were infused with glycerol alone at rates sufficient to either match (n = 5, low glycerol) or double (n = 8, high glycerol) the plasma glycerol concentrations observed during the IL/Hep infusion. Glucose was clamped at approximately 8.3 mmol/l, and insulin was increased to approximately 300 pmol/l to stimulate both muscle and hepatic glucose uptake. Insulin secretion was inhibited with somatostatin. Leg and splanchnic glucose metabolism were assessed using a combined catheter and tracer dilution approach. Leg glucose uptake (21.7 +/- 3.5 vs. 48.3 +/- 9.3 and 57.8 +/- 11.7 micromol x kg(-1) leg x min(-1)) was lower (P < 0.001) during IL/Hep than the low- or high-glycerol infusions, confirming that elevated FFAs caused insulin resistance in muscle. IL/Hep did not alter splanchnic glucose uptake or the contribution of the extracellular direct pathway to UDP-glucose flux. On the other hand, total UDP-glucose flux (13.2 +/- 1.7 and 12.5 +/- 1.0 vs. 8.1 +/- 0.5 micromol x kg(-1) x min(-1)) and flux via the indirect intracellular pathway (8.4 +/- 1.2 and 8.1 +/- 0.6 vs. 4.8 +/- 0.05 micromol x kg(-1) x min(-1)) were greater (P < 0.05) during both the IL/Hep and high-glycerol infusions than the low-glycerol infusion. In contrast, only IL/Hep increased (P < 0.05) splanchnic glucose production, indicating that elevated FFAs impaired the ability of the liver to autoregulate. Splanchnic insulin extraction, directly measured using the arterial and hepatic vein catheters, did not differ (67 +/- 3 vs. 71 +/- 5 vs. 69 +/- 1%) during IL/Hep and high- and low-glycerol infusions. We conclude that elevated FFAs exert multiple effects on glucose metabolism. They inhibit insulin- and glucose-induced stimulation of muscle glucose uptake and suppression of splanchnic glucose production. They increase the contribution of the indirect pathway to glycogen synthesis and impair hepatic autoregulation. On the other hand, they do not alter either splanchnic glucose uptake or splanchnic insulin extraction in nondiabetic humans.

Recovery of (13)CO2 during rest and exercise after [1-(13)C]acetate, [2-(13)C]acetate, and NaH(13)CO3 infusions

For estimating the oxidation rates (Rox) of glucose and other substrates by use of (13)C-labeled tracers, we obtained correction factors to account for label dilution in endogenous bicarbonate pools and TCA cycle exchange reactions. Fractional recoveries of (13)C label in respiratory gases were determined during 225 min of rest and 90 min of leg cycle ergometry at 45 and 65% peak oxygen uptake (VO(2 peak)) after continuous infusions of [1-(13)C]acetate, [2-(13)C]acetate, or NaH(13)CO(3). In parallel trials, [6,6-(2)H]glucose and [1-(13)C]glucose were given. Experiments were conducted after an overnight fast with exercise commencing 12 h after the last meal. During the transition from rest to exercise, CO(2) production increased (P < 0.05) in an intensity-dependent manner. Significant differences were observed in the fractional recoveries of (13)C label as (13)CO(2) at rest (NaH(13)CO(3), 77.5 +/- 2.8%; [1-(13)C]acetate, 49.8 +/- 2.4%; [2-(13)C]acetate, 26.1 +/- 1.4%). During exercise, fractional recoveries of (13)C label from [1-(13)C]acetate, [2-(13)C]acetate, and NaH(13)CO(3) were increased compared with rest. Magnitudes of label recoveries during both exercise intensities were tracer specific (NaH(13)CO(3), 93%; [1-(13)C]acetate, 80%; [2-(13)C]acetate, 65%). Use of an acetate-derived correction factor for estimating glucose oxidation resulted in Rox values in excess (P < 0.05) of glucose rate of disappearance during hard exercise. We conclude that, after an overnight fast: 1) recovery of (13)C label as (13)CO(2) from [(13)C]acetate is decreased compared with bicarbonate; 2) the position of (13)C acetate label affects carbon dilution estimations; 3) recovery of (13)C label increases in the transition from rest to exercise in an isotope-dependent manner; and 4) application of an acetate correction factor in glucose oxidation measurements results in oxidation rates in excess of glucose disappearance during exercise at 65% of VO(2 peak). Therefore, bicarbonate, not acetate, correction factors are advocated for estimating glucose oxidation from carbon tracers in exercising men.