Availability of glucose ingested during muscle exercise performed under acipimox-induced lipolysis blockade

Effects of reduced free fatty acid availability on skeletal muscle PDH activation during aerobic exercise. Pyruvate dehydrogenase

This study investigated the effect of reduced free fatty acid (FFA) availability on pyruvate dehydrogenase activation (PDHa) and carbohydrate metabolism during moderate aerobic exercise. Eight active male subjects cycled for 40 min at 55% Vo(2 peak) on two occasions. During one trial, subjects ingested 20 mg/kg body mass of the antilipolytic drug nicotinic acid (NA) during the hour before exercise to reduce FFA. Nothing was ingested in the control trial (CON). Blood and expired gas measurements were obtained throughout the trials, and muscle biopsy samples were obtained immediately before exercise and at 5, 20, and 40 min of exercise. Plasma FFA were lower in the NA trial (0.13 +/- 0.01 vs. 0.48 +/- 0.03 mM, P < 0.05), and the respiratory exchange ratio (RER) was increased with NA (0.93 +/- 0.01 vs. 0.89 +/- 0.01, P < 0.05), resulting in a 14.5 +/- 1.8% increase in carbohydrate oxidation compared with CON. PDHa increased rapidly in both trials at exercise onset but was approximately 15% higher (P < 0.05) throughout exercise in the NA trial (2.44 +/- 0.19 and 2.07 +/- 0.12 mmol x kg wet muscle(-1) x min(-1) for NA and CON at 40 min). Muscle glycogenolysis was 15.3 +/- 9.6% greater in the NA trial vs. the CON trial but did not reach statistical significance. Glucose 6-phosphate contents were elevated (P < 0.05) in the NA trial at 30 and 40 min of exercise, but pyruvate and lactate contents were unaffected. These data demonstrate that the reduction of exogenous FFA availability increased the activation of PDH and carbohydrate oxidation during moderate aerobic exercise in men. The increased activation of PDH was not explained by changes in muscle pyruvate or the ATP/ADP ratio but may be related to a decrease in the NADH/NAD(+) ratio or an epinephrine-induced increase in calcium concentration.

Carbon isotope fractionation between blood and expired CO2 at rest and exercise

Carbon isotope fractionation occurs between bicarbonates and gaseous CO2. Accordingly, expired CO2 could be impoverished in 13C vs. blood CO2 (approximately 90% bicarbonates). The ratio 13C/12C in expired and blood CO2 was measured in six healthy subjects at rest and at the end of exercise (90 min; 65+/-5% VO2max), with ingestion of water (300 ml) without or with glucose (30 g) naturally or artificially enriched in 13C, in order to study a range of 13C/12C in blood (-17.5+/-0.3 to 3.4+/-0.6% delta 13C PDB-1). At rest, 13C/12C in expired CO2 was 4.7+/-0.2% delta 13C PDB-1 lower than in blood CO2. This difference was not modified in response to exercise with ingestion of water or 13C-glucose (average difference 4.6+/-0.4 % delta 13C PDB-1). Carbon isotope fractionation across the lung was approximately 30% lower than predicted from the fractionation factor between bicarbonates and gaseous CO2 (1.00674 at 37 degrees C, or a approximately 6.6% delta 13C PDB-1 difference). This is consistent with the fact that approximately 40% of expired CO2 is released from carbamates and dissolved CO2. From a methodological point of view, these results indicate that 13C/12C in expired CO2 adequately tracks 13C/12C in blood CO2 with a constant approximately 4.6 % delta 13C PDB-1 difference.

The reduction in postprandial lipemia after exercise is independent of the relative contributions of fat and carbohydrate to energy metabolism during exercise

A single session of exercise several hours before a high-fat meal reduces postprandial lipemia. The purpose of the present study was to test the hypothesis that this effect is independent of substrate metabolism during exercise. Twelve men aged 21 to 36 years underwent three oral fat tolerance tests with intervals of at least 1 week. On one occasion, only activities of daily living were allowed the preceding day (control). On the other two occasions, subjects ran on a treadmill for 90 minutes on the afternoon preceding the fat tolerance test; 90 minutes before running, they ingested either acipimox, an inhibitor of lipolysis in adipose tissue, or placebo. Acipimox abolished the increase in the nonesterified fatty acid (NEFA) concentration observed during the run after placebo and reduced lipid oxidation (placebo, 37 +/- 7 g; acipimox, 21 +/- 3 g; P < .05, mean +/- SEM), but had no effect on gross energy expenditure (placebo, 4.86 +/- 0.20 MJ; acipimox, 4.83 +/- 0.18 MJ). Before each of the three fat tolerance tests, subjects reported to the laboratory after an overnight fast. Blood samples were obtained in the fasted state and for 6 hours after consumption of a high-fat meal (per kilogram of body mass: 1.2 g fat, 1.2 g carbohydrate, and 61 kJ energy). Plasma concentrations of NEFA were higher postprandially with acipimox, compared with control and placebo (P < .05), as were glucose concentrations measured over the first 4 hours. The insulin response to the meal was lower in placebo compared with control and acipimox (P < .05). Despite these counterregulatory responses, postprandial lipemia was reduced to the same degree (compared with control, P < .05) by exercise preceded by acipimox and by exercise preceded by placebo (area under the plasma triacylglycerol concentration v time curve: control, 8.77 +/- 1.17 mmol/L x 6 h; placebo, 6.95 +/- 0.97 mmol/L x 6 h; acipimox, 6.81 +/- 0.81 mmol/L x 6 h). These findings suggest that some factor other than the nature of the metabolic substrate used during exercise determines the attenuating effect of prior exercise on postprandial lipemia.

Effects of exercise on neuroendocrine secretions and glucose regulation at different times of day

To study the effects of time of day on neuroendocrine and metabolic responses to exercise, body temperature, plasma glucose, insulin secretion rates (ISR), and plasma cortisol, growth hormone (GH) and thyrotropin (TSH) were measured in young men, both at bed rest and during a 3-h exercise period (40-60% maximal O2 uptake). Exercise was performed at three times of day characterized by marked differences in cortisol levels, i.e., early morning (n = 5), afternoon (n = 8), and around midnight (n = 9). The subjects were kept awake and fasted, but they received a constant glucose infusion to avoid hypoglycemia. Exercise-induced elevations of temperature were higher in the early morning than at other times of day. The exercise-induced glucose decrease was approximately 50% greater around midnight, when cortisol was minimal and not stimulated by exercise, than in the afternoon or early morning (P < 0.05). This effect of time of day appeared unrelated to decreases in ISR or increases in temperature and GH. Robust TSH increases occurred in all exercise periods and were maximal at night. The results demonstrate the existence of circadian variations in neuroendocrine and metabolic responses to exercise.

Exogenous carbohydrate utilisation: effects on metabolism and exercise performance

It is generally recognized that a decrease in carbohydrate availability can lead to the development of fatigue during prolonged exercise in humans. Administration of glucose or other carbohydrates before or during exercise has been shown to postpone fatigue, conserve muscle glycogen and improve performance. Carbohydrates can be categorised according to their ability to increase blood glucose concentration (known as glycaemic index) and by the extent they stimulate the release of insulin. The glycaemic index is reflected in the rate at which consumed carbohydrate is made available in the blood. Glucose is the only type of carbohydrate that can readily be oxidised by skeletal muscle for energy production. Gastric emptying is the primary factor limiting the rate of carbohydrate delivery to the blood and therefore influences the utilisation of exogenous carbohydrate ingested before or during exercise. Various methods have been used to assess the oxidation of exogenous carbohydrates during exercise. Peak rates of CHO oxidation during exercise have been reported between 0.4 and 1.0 g/min, and the rates of oxidation do not appear to be influenced to a major extent by the use of multiple drinking schedule in comparison with a single bolus schedule. Previous studies also suggest that the ingestion of fructose during exercise does not offer any additional benefits over ingestion of glucose or glucose polymer solutions of similar concentration. The hormones insulin, glucagon and adrenaline together with cortisol and growth hormone play key roles in the regulation of carbohydrate metabolism during exercise. Ingestion of moderately concentrated carbohydrate solutions (4-8%) enhances prolonged exercise performance and is appropriate for optimising energy and fluid delivery without causing adverse effects. The ergogenic effects of carbohydrate ingestion on performance during intermittent exercise such as competitive sports are less well established, although the evidence to date suggests diminished performance when carbohydrate are limiting.