Splanchnic glucose metabolism during leg exercise in 60-hour-fasted human subjects

The effect of a short-term ketogenic diet on exercise efficiency during graded exercise in healthy adults

OBJECTIVE

We examined the effects of short-term KD on exercise efficiency and hormonal response during and after the graded exercise testing.

METHODS

Fourteen untrained healthy adults (8 males, 6 females, age 26.4 ± 3.1 [SD] years; BMI 24.8 ± 4.6 kg/m2; peak VO2max 54.0 ± 5.8 ml/kg FFM/min) completed 3-days of a mixed diet (MD) followed by another 3-days of KD after 3-days of washout period. Upon completion of each diet arm, participants underwent graded exercise testing with low- (LIE; 40% of VO2max), moderate- (MIE; 55%), and high-intensity exercise (HIE; 70%). Exercise efficiency was calculated as work done (kcal/min)/energy expenditure (kcal/min).

RESULTS

Fat oxidation during the recovery period was higher in KD vs. MD. Despite identical workload during HIE, participants after having KD vs. MD showed higher energy expenditure and lower exercise efficiency (10.1 ± 0.7 vs. 12.5 ± 0.3%, p < .01). After KD, free fatty acid (FFA) concentrations were higher during MIE and recovery vs. resting, and beta-hydroxybutylate (BOHB) was lower at HIE vs. resting. Cortisol concentrations after KD was higher during recovery vs. resting, with no significant changes during graded exercise testing after MD.

CONCLUSIONS

Our data suggest that short-term KD is favorable to fat metabolism leading increased circulating FFA and BOHB during LIE to MIE. However, it is notable that KD may cause 1) exercise inefficiency manifested by increased energy expenditure and 2) elevated exercise stress during HIE and recovery. Trial registration: KCT0005172, International Clinical Trials Registry Platform.

Acute effects of a single moderate-intensity exercise bout performed in fast or fed states on cell metabolism and signaling: Comparison between lean and obese rats

AIMS

Although the benefits of exercise can be potentiated by fasting in healthy subjects, few studies evaluated the effects of this intervention on the metabolism of obese subjects. This study investigated the immediate effects of a single moderate-intensity exercise bout performed in fast or fed states on the metabolism of gastrocnemius and soleus of lean and obese rats.

MAIN METHODS

Male rats received a high-fat diet (HFD) for twelve weeks to induce obesity or were fed standard diet (SD). After this period, the animals were subdivided in groups: fed and rest (FER), fed and exercise (30 min treadmill, FEE), 8 h fasted and rest (FAR) and fasted and exercise (FAE). Muscle samples were used to investigate the oxidative capacity and gene expression of AMPK, PGC1α, SIRT1, HSF1 and HSP70.

KEY FINDINGS

In relation to lean animals, obese animals' gastrocnemius glycogen decreased 60 %, triglycerides increased 31 %; glucose and alanine oxidation decreased 26 % and 38 %, respectively; in soleus, triglycerides reduced 46 % and glucose oxidation decreased 37 %. Exercise and fasting induced different effects in glycolytic and oxidative muscles of obese rats. In soleus, fasting exercise spared glycogen and increased palmitate oxidation, while in gastrocnemius, glucose oxidation increased. In obese animals' gastrocnemius, AMPK expression decreased 29 % and SIRT1 increased 28 % in relation to lean. The AMPK response was more sensitive to exercise and fasting in lean than obese rats.

SIGNIFICANCE

Exercise and fasting induced different effects on the metabolism of glycolytic and oxidative muscles of obese rats that can promote health benefits in these animals.

Effects of Low/Medium-Intensity Exercise on Fat Metabolism after a 6-h Fast

The effects of fasting and different exercise intensities on lipid metabolism were investigated in 12 male students aged 19.9 ± 1.4 years, with maximal oxygen consumption (VO_2max) of 50.33 ± 4.0 mL/kg/min, using a counterbalanced design. Each participant ran on a treadmill at 45% and 65% VO_2max continuously for 20 min, followed by running at 85% VO_2max for 20 min (or until exhaustion) under a fed or fasted state (6 h). The respiratory exchange ratio (RER), blood glucose (BGLU), blood lactate (BLA), and blood triglyceride (TG) were analyzed during exercise. The results showed that the intensity of exercise did not significantly affect the BGLU and TG in the fed state. The levels of both RER and BLA increased as the intensity of exercise increased from low to high (45, 65, and 85% VO_2max), and more energy was converted from fat into glucose at a high intensity of exercise. In the fasted state of 6 h, the BGLU level increased parallel to the intensity of exercise. The RER was close to 1.0 at a high intensity of exercise, indicating that more energy was converted from glycogen. At the intensities of 45 and 65% VO_2max, the RER and concentration of TG were both lower in the fasted than in the fed state, showing that a higher percentage of energy comes from fat than in the fed state at 45 and 65% VO_2max. When running at 85% VO_2max, the BGLU concentration was higher in the fasted than in the fed state, indicating that the liver tissues release more BGLU for energy in the fasted state. Therefore, in the fasted state, running at 45% and 65% of VO_2max significantly affects lipid metabolism. On the contrary, the higher RER and BGLU concentrations when running at 85% VO_2max revealed no significant difference between the two probes. This study suggests that medium- and low-intensity exercise (45 and 65% VO_2max) in the fasted state enhances lipid metabolism.

Effect of combining pre-exercise carbohydrate intake and repeated short sprints on the blood glucose response to moderate-intensity exercise in young individuals with Type 1 diabetes

AIMS

To determine whether pre-exercise ingestion of carbohydrates to maintain stable glycaemia during moderate-intensity exercise results in excessive hyperglycaemia if combined with repeated sprints in individuals with Type 1 diabetes.

METHODS

Eight overnight-fasted people with Type 1 diabetes completed the following four 40-min exercise sessions on separate days in a randomized counterbalanced order under basal insulinaemic conditions: continuous moderate-intensity exercise at 50% V ˙ O 2 peak; intermittent high-intensity exercise (moderate-intensity exercise interspersed with 4-s sprints every 2 min and a final 10-s sprint); continuous moderate-intensity exercise with prior carbohydrate intake (~10 g per person); and intermittent high-intensity exercise with prior carbohydrate intake. Venous blood was sampled during and 2 h after exercise to measure glucose and lactate levels.

RESULTS

The difference in marginal mean time-averaged area under the blood glucose curve between continuous moderate-intensity exercise + prior carbohydrate and intermittent high-intensity exercise + prior carbohydrate during exercise and recovery was not significant [0.2 mmol/l (95% CI -0.7, 1.1); P = 0.635], nor was the difference in peak blood glucose level after adjusting for baseline level [0.2 mmol/l (95% CI -0.7, 1.1); P = 0.695]. The difference in marginal mean time-averaged area under the blood glucose curve between continuous moderate-intensity and intermittent high-intensity exercise during exercise and recovery was also not significant [-0.2 mmol/l (95% CI -1.2, 0.8); P = 0.651].

CONCLUSIONS

When carbohydrates are ingested prior to moderate-intensity exercise, adding repeated sprints is not significantly detrimental to glycaemic management in overnight fasted people with Type 1 diabetes under basal insulin conditions.

Splanchnic regulation of glucose production

The liver plays a key role for the maintenance of blood glucose homeostasis under widely changing physiological conditions. In the overnight fasted state, breakdown of hepatic glycogen and synthesis of glucose from lactate, amino acids, glycerol, and pyruvate contribute about equally to hepatic glucose production. Postprandial glucose uptake by the liver is determined by the size of the glucose load reaching the liver, the rise in insulin concentration, and the route of glucose delivery. Hepatic glycogen stores are depleted within 36 to 48 hours of fasting, but gluconeogenesis continues to provide glucose for tissues with an obligatory glucose requirement. Glucose output from the liver increases during exercise; during short-term intensive exertion, hepatic glycogenolysis is the primary source of extra glucose for skeletal muscle, and during prolonged exercise, hepatic gluconeogenesis becomes gradually more important in keeping with falling insulin and rising glucagon levels. Type 1 diabetes is accompanied by diminished hepatic glycogen stores, augmented gluconeogenesis, and increased basal hepatic glucose production in proportion to the severity of the diabetic state. The hyperglycemia of type 2 diabetes is in part caused by an overproduction of glucose from the liver that is secondary to accelerated gluconeogenesis.

Hyperinsulinaemia during exercise does not suppress hepatic glycogen concentrations in patients with type 1 diabetes: a magnetic resonance spectroscopy study

AIMS/HYPOTHESIS

We compared in vivo changes in liver glycogen concentration during exercise between patients with type 1 diabetes and healthy volunteers.

METHODS

We studied seven men with type 1 diabetes (mean +/- SEM diabetes duration 10 +/- 2 years, age 33 +/- 3 years, BMI 24 +/- 1 kg/m(2), HbA(1c) 8.1 +/- 0.2% and VO(2) peak 43 +/- 2 ml [kg lean body mass](-1) min(-1)) and five non-diabetic controls (mean +/- SEM age 30 +/- 3 years, BMI 22 +/- 1 kg/m(2), HbA(1c) 5.4 +/- 0.1% and VO(2) peak 52 +/- 4 ml [kg lean body mass](-1) min(-1), before and after a standardised breakfast and after three bouts (EX1, EX2, EX3) of 40 min of cycling at 60% VO(2) peak. (13)C Magnetic resonance spectroscopy of liver glycogen was acquired in a 3.0 T magnet using a surface coil. Whole-body substrate oxidation was determined using indirect calorimetry.

RESULTS

Blood glucose and serum insulin concentrations were significantly higher (p < 0.05) in the fasting state, during the postprandial period and during EX1 and EX2 in subjects with type 1 diabetes compared with controls. Serum insulin concentration was still different between groups during EX3 (p < 0.05), but blood glucose concentration was similar. There was no difference between groups in liver glycogen concentration before or after the three bouts of exercise, despite the relative hyperinsulinaemia in type 1 diabetes. There were also no differences in substrate oxidation rates between groups.

CONCLUSIONS/INTERPRETATION

In patients with type 1 diabetes, hyperinsulinaemic and hyperglycaemic conditions during moderate exercise did not suppress hepatic glycogen concentrations. These findings do not support the hypothesis that exercise-induced hypoglycaemia in patients with type 1 diabetes is due to suppression of hepatic glycogen mobilisation.

Effects of fasting on endurance exercise

Early studies agree that fasting is detrimental to overall physical performance and to endurance performance in humans; however, a study in rats reported an ergogenic effect where time to exhaustion was increased by a glycogen-sparing effect of elevated free fatty acids in blood resulting from a 24-hour fast. Later studies on humans have also found a detrimental effect of fasting on exercise endurance, with the exception of 1 study which found no difference. The discrepancy between humans and rats could not be explained by level of glycogen sparing, mode of exercise, duration of the fast, physiological differences or level of training. The intensity of exercise, and a potential placebo effect of fasting, are possible reasons for the conflicting results. Despite reduced endurance performance, fasted humans are able to exercise and maintain blood glucose homeostasis; the specific cause of an earlier onset of fatigue during a single bout of exercise in the fasted state remains unclear.

Plasma glucose metabolism during exercise in humans

Plasma glucose is an important energy source in exercising humans, supplying between 20 and 50% of the total oxidative energy production and between 25 and 100% of the total carbohydrate oxidised during submaximal exercise. Plasma glucose utilisation increases with the intensity of exercise, due to an increase in glucose utilisation by each active muscle fibre, an increase in the number of active muscle fibres, or both. Plasma glucose utilisation also increases with the duration of exercise, thereby partially compensating for the progressive decrease in muscle glycogen concentration. When compared at the same absolute exercise intensity (i.e. the same VO2), reliance on plasma glucose is also greater during exercise performed with a small muscle mass, i.e. with the arms or just 1 leg. This may be due to differences in the relative exercise intensity (i.e. the %VO2peak), or due to differences between the arms and legs in their fitness for aerobic activity. The rate of plasma glucose utilisation is decreased when plasma free fatty acid or muscle glycogen concentrations are very high, effects which are probably mediated by increases in muscle glucose-6-phosphate concentration. However, glucose utilisation is also reduced during exercise following a low carbohydrate diet, despite the fact that muscle glycogen is also often lower. When exercise is performed at the same absolute intensity before and after endurance training, plasma glucose utilisation is lower in the trained state. During exercise performed at the same relative intensity, however, glucose utilisation may be lower, the same, or actually higher in trained than in untrained subjects, because of the greater absolute VO2 and demand for substrate in trained subjects during exercise at a given relative exercise intensity. Although both hyperglycaemia and hypoglycaemia may occur during exercise, plasma glucose concentration usually remains relatively constant. Factors which increase or decrease the reliance of peripheral tissues on plasma glucose during exercise are therefore generally accompanied by quantitatively similar increases or decreases in glucose production. These changes in total glucose production are mediated by changes in both hepatic glycogenolysis and hepatic gluconeogenesis. Glycogenolysis dominates under most conditions, and is greatest early in exercise, during high intensity exercise, or when dietary carbohydrate intake is high. The rate of gluconeogenesis is increased when exercise is prolonged, preceded by a restricted carbohydrate intake, or performed with the arms. Both glycogenolysis and gluconeogenesis appear to be decreased by endurance exercise training. These effects are due to changes in both the hormonal milieu and in the availability of hepatic glycogen and gluconeogenic precursors.(ABSTRACT TRUNCATED AT 400 WORDS)

Influence of a 24 h fast on high intensity cycle exercise performance in man

The influence of a 24 h fast on endurance performance and the metabolic response to maximal cycle exercise was investigated in 6 healthy men (mean +/- SD: age = 27 +/- 7 years; weight = 73 +/- 10 kg; VO2max = 46 +/- 10 ml.kg-1.min-1). Subjects performed in randomised order two exercise bouts to exhaustion separated by one week. Test rides were performed in fasted (F) and post-absorptive (normal-diet, ND) conditions on an electrically braked cycle ergometer at a workload equivalent to 100% of VO2max. Acid-base status and selected metabolites were measured on arterialised venous blood at rest prior to exercise and at intervals for 15 mins following exercise. Exercise time to exhaustion was shorter after F compared with ND (p less than 0.01). Pre-exercise blood bicarbonate (HCO3-) concentration, PCO2 and base excess (BE) were lower after F compared with ND (p less than 0.05). Prior to exercise, circulating concentrations of free fatty acids (FFA), beta-hydroxybutyrate (B-HB) and glycerol were higher after F compared with ND (p less than 0.01) but blood glucose and lactate concentration were not different. On the F treatment, after exercise, blood pH, HCO3-, and BE were all significantly higher (p less than 0.01) than on ND; blood lactate concentration was significantly lower for the whole of the post-exercise period after F compared with ND (p less than 0.01). Circulating levels of FFA and B-HB after exercise on the F treatment fell but levels of these substrates were not altered by exercise after ND.(ABSTRACT TRUNCATED AT 250 WORDS)

Splanchnic and leg exchange of free fatty acids in patients with liver cirrhosis

Splanchnic and leg exchange of free fatty acids (FFA), glycerol and ketone bodies, as well as FFA turnover, were determined in the post-absorptive state in 8 patients with liver cirrhosis and in 6 healthy control subjects. The catheter technique was used together with tracer ([14C]oleate) infusion. The arterial concentrations of FFA, glycerol and ketone bodies were 2-6-fold higher in the patients than in the controls. The FFA turnover was 230% greater in the patients, while the fractional turnover was similar in the two groups. In the splanchnic region as well as in the leg, both FFA uptake and release were increased 2-4-fold in the patients. The fractional uptake of FFA was reduced in both areas, indicating that the augmented uptake was due to the high circulating FFA levels alone. The splanchnic production of ketone bodies was four times higher in the patients than in the controls (295 +/- 30 vs 87 +/- 11 mumol/min). The fraction of FFA converted to ketone bodies was greater (42 +/- 6 vs 20 +/- 5%, P less than 0.05), indicating that the accelerated ketone body production was a combined effect of raised FFA uptake and altered intrahepatic metabolism of FFA. The splanchnic production of glucose was reduced by approximately 50% in the patients, while the uptake of glycerol was augmented. The leg uptake of 3-hydroxybutyrate was increased 300% and the release of glycerol was 200% greater in the patients.(ABSTRACT TRUNCATED AT 250 WORDS)