Glucose infusion attenuates endogenous glucose production and enhances glucose use of horses during exercise

Effect of high-starch or high-fibre diets on the energy metabolism and physical performance of horses during an 8-week training period

Large amounts of high-starch concentrates are traditionally fed to horses in training. However, this has been associated with digestive or muscle diseases and behavioural modifications. In parallel, it has been demonstrated that horses fed high-fibre, low-starch diets achieve the same performance over an exercise test as the ones fed high-starch diets. However, whether the same performance level can be maintained over a longer training cycle is still being determined. This study aimed to compare the evolution in physical performance and cardiorespiratory responses of two groups of French Trotters fed either a control high-starch (15.0 g dry matter hay/kg body mass/day + 6.6 g dry matter oats/kg body mass/day) or a high-fibre diet (75% of oats replaced by dehydrated alfalfa) over an 8-week training period. The horses that entered the trial were untrained for ≥4 months and previously fed hay only. Track training with speed monitoring included interval training sessions and 2400 m performance tests from week 1 to week 8 (W8). Before (week 0, W0) and after (week 9, W9) the training period, horses performed an incremental continuous exercise test during which cardiorespiratory parameters were measured. Both groups progressed to the same extent regarding physical performance measured during interval training sessions (acceleration: 0.16 m.s-2 at W0 and 0.40 m.s-2 at W8; p < 0.0001), the 2400 m performance test (average speed: 8.88 m.s-1 at W0 and 10.55 m.s-1 at W8; p < 0.0001), and the incremental continuous exercise test (speed during the fastest stage: 9.57 m.s-1 at W0 and 10.53 m.s-1 at W9; p = 0.030). Although oxygen consumption increased with training (p = 0.071), it was not influenced by the diet. On the contrary, carbon dioxide production increased in the high-starch group only (high-starch group: 84.0 vs. high-fibre group: 77.7 mL.kg-1.min-1 at W9; p = 0.031). The results illustrate that horses in both groups progressed similarly but did not use the same metabolic pathways during exercise. This hypothesis is supported by carbohydrate oxidation, which tended to increase in the high-starch group at W9 but decreased in the high-fibre group (p = 0.061). In conclusion, the substitution of high-starch by high-fibre diets enabled similar performance over an 8-week training period and altered energy metabolism in a way that could be beneficial during high-intensity exercise.

Nutritional Influences on Skeletal Muscle and Muscular Disease

Skeletal muscle comprises 40% to 55% of mature body weight in horses, and its mass is determined largely by rates of muscle protein synthesis. In order to support exercise, appropriate energy sources are essential: glucose can support both anaerobic and aerobic exercise, whereas fat can only be metabolized aerobically. Following exercise, ingestion of nonfiber carbohydrates and protein can aid muscle growth and recovery. Muscle glycogen replenishment is slow in horses, regardless of dietary interventions. Several heritable muscle disorders, including type 1 and 2 polysaccharide storage myopathy and recurrent exertional rhabdomyolysis, can be managed in part by restricting dietary nonstructural carbohydrate intake.

The effects of different beverage intake on blood components during exercise under high-temperature environment

High temperature environment causes detrimental effects on health. In the present study, the effects of intake of several kinds of beverage on blood components during exercise under the high temperature environment were evaluated. The 10 subjects were student of the H University. Exercise intensity was 50–60% O_2maxx and treadmill exercise was continued for 1 h. The kinds of beverage were water, ion beverage, cucumber drink. Blood sampling was performed before the exercise, immediately finishing exercise, and 30 min after rest. In the present results, glucose concentration was increased by intake of water, ion beverage, and cucumber drink immediately after exercise. In the water intake group, glucose concentration was decrease 30 min after rest. Free fatty acid concentration was increased by intake of water, ion beverage, and cucumber drink 60 min after exercise. In the ion beverage and cucumber drink intake groups, free fatty acid concentration was decreased 30 min after rest. Insulin concentration was increased by intake of water, ion beverage, and cucumber drink 60 min after exercise. In the water intake group, insulin concentration was decrease 30 min after rest. There was no difference in the efficacy among water, ion beverage, and cucumber drink, but ion beverage and cucumber drink showed more potent effect on metabolic parameters.

Assessment of glucose disposal with the hyperglycaemic clamp technique during low intensity exercise in Warmblood horses

REASONS FOR PERFORMING STUDY

The quantity of glucose disposal during exercise (walk and trot) compared to rest by use of the hyperglycaemic clamp technique has not been reported previously and has relevance to nutritional requirements.

HYPOTHESIS

Exercise (walk and trot) significantly increases glucose disposal compared to rest.

METHODS

Seven healthy Dutch Warmblood mares, all in dioestrus, mean ± s.d. age 11.6 ± 2.4 years and weighing 569 ± 40 kg were fasted for 12 h prior to a hyperglycaemic clamp at rest (maintaining a steady state of the blood glucose concentration during 30 min), walk (10 min, 1.5 m/s), trot (20 min, 4.4 m/s), walk (10 min, 1.5 m/s) and rest again (maintaining a steady state during 30 min). Plasma glucose concentrations were measured every 5 min. The mean rate of glucose disposal was calculated by corrections for glucose loss via the glucose space and urine. A one-way ANOVA with post hoc Bonferroni was performed.

RESULTS

The mean ± s.d. rate of glucose disposal was 15.0 ± 2.1 at first rest, 25.1 ± 6.2 at first walk, 37.4 ± 9.1 at trot, 33.0 ± 13.1 at second walk and 18.7 ± 4.6 µmol/kg bwt/min at second rest. Values at trot and at second walk differed significantly from values at first rest, whereas values at both rests were similar as well as at first rest and at first walk.

CONCLUSIONS

Mean rate of glucose disposal of Warmblood horses increased 2.5 times during trot compared to basal.

POTENTIAL RELEVANCE

The hyperglycaemic clamp technique is an attractive nonisotope method to assess the rate of glucose disposal in exercising horses.

Beneficial metabolic adaptations due to endurance exercise training in the fasted state

Training with limited carbohydrate availability can stimulate adaptations in muscle cells to facilitate energy production via fat oxidation. Here we investigated the effect of consistent training in the fasted state, vs. training in the fed state, on muscle metabolism and substrate selection during fasted exercise. Twenty young male volunteers participated in a 6-wk endurance training program (1-1.5 h cycling at ∼70% Vo(₂max), 4 days/wk) while receiving isocaloric carbohydrate-rich diets. Half of the subjects trained in the fasted state (F; n = 10), while the others ingested ample carbohydrates before (∼160 g) and during (1 g·kg body wt⁻¹·h⁻¹) the training sessions (CHO; n = 10). The training similarly increased Vo(₂max) (+9%) and performance in a 60-min simulated time trial (+8%) in both groups (P < 0.01). Metabolic measurements were made during a 2-h constant-load exercise bout in the fasted state at ∼65% pretraining Vo(₂max). In F, exercise-induced intramyocellular lipid (IMCL) breakdown was enhanced in type I fibers (P < 0.05) and tended to be increased in type IIa fibers (P = 0.07). Training did not affect IMCL breakdown in CHO. In addition, F (+21%) increased the exercise intensity corresponding to the maximal rate of fat oxidation more than did CHO (+6%) (P < 0.05). Furthermore, maximal citrate synthase (+47%) and β-hydroxyacyl coenzyme A dehydrogenase (+34%) activity was significantly upregulated in F (P < 0.05) but not in CHO. Also, only F prevented the development exercise-induced drop in blood glucose concentration (P < 0.05). In conclusion, F is more effective than CHO to increase muscular oxidative capacity and at the same time enhances exercise-induced net IMCL degradation. In addition, F but not CHO prevented drop of blood glucose concentration during fasting exercise.

Effect of triheptanoin on muscle metabolism during submaximal exercise in horses

OBJECTIVE

To compare effects of corn oil or a 7-carbon fat (triheptanoin) on acylcarnitine, lipid, and carbohydrate metabolism in plasma or muscle of exercising horses.

ANIMALS

8 Thoroughbred geldings.

PROCEDURES

Horses received isocaloric diets containing 650 mL of oil (triheptanoin or corn oil)/d for 18 or 25 days in a crossover design with a 26-day washout period. On day 17 or 24 of each feeding period, the respective oil (217 mL) was nasogastrically administered; 120 minutes later, horses performed a 90-minute submaximal exercise test (SET). Blood and muscle samples were obtained before oil administration and immediately before (blood only), during (blood only), immediately after, and 24 hours after SETs.

RESULTS

Compared with values before oil administration, triheptanoin administration increased plasma insulin and C7:0-, C5:0- and C3:0-acylcarnitine concentrations, whereas corn oil administration increased plasma NEFA concentrations. During SETs, plasma C7:0-, C5:0-, and C3:0-acylcarnitine concentrations were higher when triheptanoin, rather than corn oil, was administered to horses. Plasma glucose, NEFA, and C2:0-, C18:1-, and C18:2-acylcarnitine concentrations increased during SETs similarly for both oils. Respiratory quotient and muscle lactate, citrate, malate, glycogen, and ATP concentrations changed similarly from before to after SETs for both oils. Compared with muscle concentrations immediately after SETs, those for glucose-6-phosphate and citrate 24 hours after SETs were lower and for glycogen were similar to values before SETs.

CONCLUSIONS AND CLINICAL RELEVANCE

Fatigue was not associated with depletion of citric acid cycle intermediates for either oil. Triheptanoin induced a significantly higher insulin secretion and did not appear to enhance muscle glycogen repletion.

Dietary energy source affects glucose kinetics in trained Arabian geldings at rest and during endurance exercise

Advances in modeling and tracer techniques provide new perspective into glucose utilization and potential consequences to health or exercise performance. This study used stable isotope and compartmental modeling to evaluate how adaptation to a feed high in sugar and starch (SS) compared with a feed high in fat and fiber (FF) affects glucose kinetics at rest and during exercise in horses. Six trained Arabians adapted to each feed underwent similar tests at rest and while running approximately 4 m/s on a treadmill. For both tests, horses received 100 micromol/kg body weight [6,6-(2)H]glucose through a venous catheter. Circulating tracer glucose was described for 150 min by exponential decay curves and compartmental analysis. All parameters of glucose transfer increased with exercise (P < or = 0.004). Compared with FF horses, SS horses had higher circulating glucose (P = 0.022) and fractional glucose transfer rates (min(-1)) at rest (P = 0.055). Exercise increased glucose irreversible loss (mmol/min) more in SS horses (P = 0.037). Total glucose transfer during exercise tended to be greater in SS horses (0.027 +/- 0.002 mmol/min) compared with FF horses (0.023 +/- 0.002 mmol/min) (P = 0.109). This study characterized the effect of diet on glucose kinetics in resting and exercising horses using new modeling methods. Horses adapted to a fat-supplemented feed utilized less glucose during low-intensity exercise. Fat supplementation in horses may therefore promote greater flexibility in the selection of substrate to meet energy demands for optimal health and performance.

Carbohydrate metabolism in exercising horses

Carbohydrate and fat are the predominant sources of energy during exercise in mammals. Carbohydrates, such as muscle glycogen and plasma glucose, and fats from adipose tissue and intramuscular triglycerides are oxidized during exercise in amounts and proportions that vary depending on the exercise intensity, level of fitness and nutritional status. In horses, muscle glycogen, and to a lesser extent plasma glucose, are oxidized in substantial amounts during low-, moderate- and high-intensity exercise. Carbohydrate availability to skeletal muscle affects exercise performance in humans, however this relationship is not well outlined in horses. Glucose supplementation by intravenous administration during exercise in horses increases duration of moderate-intensity exercise. However, the effect of glucose supplementation by ingestion of a soluble carbohydrate-rich meal prior to exercise on athletic performance has not been established in horses. Low muscle glycogen concentrations prior to exercise in horses are associated with decreased time to exhaustion at moderate- and high-intensity exercise. Nutritional interventions intended to enhance muscle glycogen resynthesis have proved less successful in horses than in other species. Replenishment of muscle glycogen after strenuous exercise in horses is not complete until 48–72 h after exercise, whereas in humans and laboratory animals it is complete by 24 h. The slower rate of muscle glycogen replenishment after exercise in horses may be related to an inherent lower ability to digest starch and other sources of glucose, a lower ability to synthesize muscle glycogen, or both. The aim of this review is to describe the present understanding of carbohydrate metabolism in the exercising horse, its implications on nutrition and athletic performance, and to contrast it with that in other species.

Ingestion of starch-rich meals after exercise increases glucose kinetics but fails to enhance muscle glycogen replenishment in horses

Fatiguing exercise substantially decreases muscle glycogen concentration in horses, impairing athletic performance in subsequent exercise bouts. Our objective was to determine the effect of ingestion of starch-rich meals after exercise on whole body glucose kinetics and muscle glycogen replenishment. In a randomized, cross-over study seven horses with exercise-induced muscle glycogen depletion were either not fed for 8 h, fed half of the daily energy requirements ( approximately 15 Mcal DE) as hay, or fed an isocaloric amount of corn 15 min and 4 h after exercise. Starch-rich meals fed after exercise, when compared to feed withholding, resulted in mild to moderate hyperglycemia (5.7+/-0.3 vs. 4.7+/-0.3 mM, P<0.01) and hyperinsulinemia (79.9+/-9.3 vs. 39.0+/-1.9 pM, P<0.001), 3-fold greater whole body glucose kinetics (15.5+/-1.4 vs. 5.3+/-0.4 micromol kg(-1)min(-1), P<0.05), but these only minimally enhanced muscle glycogen replenishment (171+/-19 vs. 170+/-56 and 260+/-45 vs. 294+/-29 mmol/kg dry weight immediately and 24 h after exercise, P>0.05). It is concluded that after substantial exercise-induced muscle glycogen depletion, feeding status only minimally affects net muscle glycogen concentrations after exercise, despite marked differences in soluble carbohydrate ingestion and availability of glucose to skeletal muscle.

Effects of fat adaptation on glucose kinetics and substrate oxidation during low-intensity exercise

This study was designed to determine the effects of fat adaptation on carbohydrate and fat oxidation in conditioned horses during low-intensity exercise. Five mature Arabians were studied. The study was conducted as a crossover design with 2 dietary periods, each of 10 week's duration: a) a control (CON) diet, and b) a fat-supplemented (FAT) diet. The total amount of digestible energy (DE) supplied by the fat in the CON and FAT diets was 7% and 29%, respectively. During each period, the horses completed exercise tests at the beginning of the period (Week 0) and after 5 and 10 weeks on the diet. Tests consisted of 90 min of exercise at a speed calculated to elicit 35% VO2max on a treadmill inclined to 3 degrees. Oxygen consumption (VO2), carbon dioxide production (VCO2), and respiratory exchange ratio (RER) were measured at 15-min intervals. For determination of glucose kinetics, a stable isotope ([6-6-d2] glucose) technique was used. Compared to the CON diet, FAT diet consumption for 5-10 weeks was associated with an altered metabolic response to low-intensity exercise, as evidenced by a more than 30% reduction in the production and utilisation of glucose; a decrease in RER; a decrease in the estimated rate of whole-body carbohydrate utilisation; and an increase in the whole-body rate of lipid oxidation during exercise.

Training-induced alterations in glucose metabolism during moderate-intensity exercise

In several species, physical conditioning (training) provokes a large shift in substrate utilisation during submaximal exercise. Few studies in horses have quantitatively examined these effects. Therefore, the effects of exercise training on plasma glucose kinetics during submaximal exercise were examined in 7 horses (5 Thoroughbred, 2 Standardbred; age 3-9 years) that had been paddock-rested for at least 6 months. Two days after determination of maximum aerobic capacity (VO2max), horses ran on a treadmill (4 degree incline) at 55% of VO2max (UT) for 60 min or until fatigue and then completed 6 weeks of moderate-intensity training on a treadmill (5 days/week). Following training and a second VO2max test, the horses completed exercise trials at the same absolute (ABS) and relative (REL) workload in random order, with at least 3 days between tests. After training, VO2max had increased (P<0.05) by 14.9% (mean +/- s.e. pretraining 118.4 +/- 7.4 ml/kg bwt/min; post-training 136.1 +/- 7.8 ml/kg bwt/min). Mean exercise duration was longer (P<0.05) in the ABS trial (57 +/- 1.9 min) than in the UT (46 +/- 3.9 min) and REL (49 +/- 4.6 min) trials. Plasma glucose concentration increased during exercise, and was lower (P<0.05) in ABS than in UT and REL at the end of exercise. Mean glucose rate of appearance (Ra) and disappearance (Rd) were 22 and 21% lower (P<0.05), respectively, in ABS than in UT, but mean glucose Ra and Rd did not differ between the UT and REL trials. Exercise-induced changes in glucagon, epinephrine and norepinephrine were blunted (P<0.05) in ABS, but not REL, when compared to UT. It is concluded that 6 weeks of moderate-intensity training results in a decrease in glucose flux during submaximal exercise at the same absolute, but not relative, workload. The training-induced decrease in glucose flux may, in part, be due to altered plasma concentrations of the major glucoregulatory hormones.

Glycemic index of a meal fed before exercise alters substrate use and glucose flux in exercising horses

In a randomized, balanced, crossover study each of six fit, adult horses ran on a treadmill at 50% of maximal rate of oxygen consumption for 60 min after being denied access to food for 18 h and then 1) fed corn (51.4 kJ/kg digestible energy), or 2) fed an isocaloric amount of alfalfa 2-3 h before exercise, or 3) not fed before exercise. Feeding corn, compared with fasting, resulted in higher plasma glucose and serum insulin and lower serum nonesterified fatty acid concentrations before exercise (P < 0.05) and in lower plasma glucose, serum glycerol, and serum nonesterified fatty acid concentrations and higher skeletal muscle utilization of blood-borne glucose during exercise (P < 0.05). Feeding corn, compared with feeding alfalfa, resulted in higher carbohydrate oxidation and lower lipid oxidation during exercise (P < 0.05). Feeding a soluble carbohydrate-rich meal (corn) to horses before exercise results in increased muscle utilization of blood-borne glucose and carbohydrate oxidation and in decreased lipid oxidation compared with a meal of insoluble carbohydrate (alfalfa) or not feeding. Carbohydrate feedings did not produce a sparing of muscle glycogen compared with fasting.

beta-adrenergic blockade augments glucose utilization in horses during graded exercise

To examine the role of beta-adrenergic mechanisms in the regulation of endogenous glucose (Glu) production [rate of appearance (R(a))] and utilization [rate of disappearance (R(d))] and carbohydrate (CHO) metabolism, six horses completed consecutive 30-min bouts of exercise at approximately 30% (Lo) and approximately 60% (Hi) of estimated maximum O(2) uptake with (P) and without (C) prior administration of the beta-blocker propranolol (0.22 mg/kg iv). All horses completed exercise in C; exercise duration in P was 49.9 +/- 1.2 (SE) min. Plasma Glu was unchanged in C during Lo but increased progressively in Hi. In P, plasma Glu rose steadily during Lo and Hi and was higher (P < 0.05) than in C throughout exercise. Plasma insulin declined during exercise in P but not in C; beta-blockade attenuated (P < 0.05) the rise in plasma glucagon and free fatty acids and exaggerated the increases in epinephrine and norepinephrine. Glu R(a) was 8.1 +/- 0.8 and 8.4 +/- 1.0 micromol. kg(-1). min(-1) at rest and 30.5 +/- 3.6 and 42.8 +/- 4.1 micromol. kg(-1). min(-1) at the end of Lo in C and P, respectively. During Hi, Glu R(a) increased to 54.4 +/- 4.4 and 73.8 +/- 4.7 micromol. kg(-1). min(-1) in C and P, respectively. Similarly, Glu R(d) was approximately 40% higher in P than in C during Lo (27.3 +/- 2.0 and 39.5 +/- 3.3 micromol. kg(-1). min(-1) in C and P, respectively) and Hi (37.4 +/- 2.6 and 61.5 +/- 5.3 micromol. kg(-1). min(-1) in C and P, respectively). beta-Blockade augmented CHO oxidation (CHO(ox)) with a concomitant reduction in fat oxidation. Inasmuch as estimated muscle glycogen utilization was similar between trials, the increase in CHO(ox) in P was due to increased use of plasma Glu. We conclude that beta-blockade increases Glu R(a) and R(d) and CHO(ox) in horses during exercise. The increase in Glu R(d) under beta-blockade suggests that beta-adrenergic mechanisms restrain Glu R(d) during exercise.

Epinephrine inhibits exogenous glucose utilization in exercising horses

This study examined the effects of preexercise glucose administration, with and without epinephrine infusion, on carbohydrate metabolism in horses during exercise. Six horses completed 60 min of treadmill exercise at 55 +/- 1% maximum O(2) uptake 1) 1 h after oral administration of glucose (2 g/kg; G trial); 2) 1 h after oral glucose and with an intravenous infusion of epinephrine (0.2 micromol. kg(-1). min(-1); GE trial) during exercise, and 3) 1 h after water only (F trial). Glucose administration (G and GE) caused hyperinsulinemia and hyperglycemia ( approximately 8 mM). In GE, plasma epinephrine concentrations were three- to fourfold higher than in the other trials. Compared with F, the glucose rate of appearance was approximately 50% and approximately 33% higher in G and GE, respectively, during exercise. The glucose rate of disappearance was approximately 100% higher in G than in F, but epinephrine infusion completely inhibited the increase in glucose uptake associated with glucose administration. Muscle glycogen utilization was higher in GE [349 +/- 44 mmol/kg dry muscle (dm)] than in F (218 +/- 28 mmol/kg dm) and G (201 +/- 35 mmol/kg dm). We conclude that 1) preexercise glucose augments utilization of plasma glucose in horses during moderate-intensity exercise but does not alter muscle glycogen usage and 2) increased circulating epinephrine inhibits the increase in glucose rate of disappearance associated with preexercise glucose administration and increases reliance on muscle glycogen for energy transduction.