Use of endogenous carbohydrate and fat as fuels during exercise

Effects of exogenous lactate on lipid, protein, and glucose metabolism-a randomized crossover trial in healthy males

Lactate may inhibit lipolysis and thus enhance insulin sensitivity, but there is a lack of metabolic human studies. This study aimed to determine how hyperlactatemia affects lipolysis, glucose- and protein metabolism, and insulin sensitivity in healthy men. In a single-blind, randomized, crossover design, eight healthy men were studied after an overnight fast on two occasions: 1) during a sodium-lactate infusion (LAC) and 2) during a sodium-matched NaCl infusion (CTR). Both days consisted of a 3-h postabsorptive period followed by a 3-h hyperinsulinemic-euglycemic clamp (HEC). Lipolysis rate, endogenous glucose production (EGP), and delta glucose rate of disappearance (ΔRdglu) were evaluated using [9,10-3H]palmitate and [3-3H]glucose tracers. In addition, whole body- and forearm protein metabolism was assessed using [15N]phenylalanine, [2H4]tyrosine, [15N]tyrosine, and [13C]urea tracers. In the postabsorptive period, plasma lactate increased to 2.7 ± 0.5 mmol/L during LAC vs. 0.6 ± 0.3 mmol/L during CTR (P < 0.001). In the postabsorptive period, palmitate flux was 30% lower during LAC compared with CTR (84 ± 32 µmol/min vs. 120 ± 35 µmol/min, P = 0.003). During the HEC, palmitate flux was suppressed similarly during both interventions (P = 0.7). EGP, ΔRdglu, and M value were similar during LAC and CTR. During HEC, LAC increased whole body phenylalanine flux (P = 0.02) and protein synthesis (P = 0.03) compared with CTR; LAC did not affect forearm protein metabolism compared with CTR. Lactate infusion inhibited lipolysis by 30% under postabsorptive conditions but did not affect glucose metabolism or improve insulin sensitivity. In addition, whole body phenylalanine flux was increased. Clinical trial registrations: NCT04710875.NEW & NOTEWORTHY Lactate is a decisive intermediary metabolite, serving as an energy substrate and a signaling molecule. The present study examines the effects of lactate on substrate metabolism and insulin sensitivity in healthy males. Hyperlactatemia reduces lipolysis by 30% without affecting insulin sensitivity and glucose metabolism. In addition, hyperlactatemia increases whole body amino acid turnover rate.

Fat Oxidation during Exercise in People with Spinal Cord Injury, and Protocols Used: A Systematic Review

BACKGROUND

The aim of this study was to summarize evidence on energy metabolism through peak fat oxidation (PFO) and maximum fat oxidation (Fat_max), as well as to analyze the protocols used in people with spinal cord injury (SCI) and to examine the main factors related to fat oxidation ability (i.e., age, sex, level of physical activity, and level and degree of injury).

METHODS

Studies to determine PFO and Fat_max using indirect calorimetry with an arm exercise protocol for SCI patients were included after a systematic search. Other endpoints included study design, sample size, control group, demographic data, level of injury, physical condition, protocol, outcomes measured, and statistical findings.

RESULTS

Eight studies (n = 560) were included. The mean value of VO_2peak was 1.86 L∙min^-1 (range 0.75-2.60 L∙min^-1) (lowest value in the tetraplegic subjects). The PFO ranged between 0.06 and 0.30 g∙min^-1 (lowest rates: the non-trained subjects with cervical SCI; highest: the tetraplegic subjects). Two types of exercise protocol were found: arm cycle ergometer, and wheelchair propulsion with a computerized ergometer. Five studies used an incremental protocol (2-3 min/stage, different load increments); the rest performed tests of 20 min/stage at three intensities.

CONCLUSION

There are few existing studies measuring fat oxidation in SCI, many of which used small and heterogeneous samples. PFO was lower in SCI subjects when compared with non-injured people performing lower-limb exercise; however, comparing upper-limb exercise, people with SCI showed higher values.

Effects of a Short-Term "Fat Adaptation with Carbohydrate Restoration" Diet on Metabolic Responses and Exercise Performance in Well-Trained Runners

Periodized carbohydrate availability can enhance exercise capacity, but the effects of short-term fat adaptation carbohydrate restoration (FACR) diets on metabolic responses and exercise performance in endurance athletes have not been conclusively determined. This study aimed to investigate the effect of a FACR diet on measures of resting metabolism, exercise metabolism, and exercise performance. Well-trained male runners (n = 8) completed a FACR dietary intervention (five days' carbohydrate < 20% and fat > 60% energy, plus one-day carbohydrate ≥ 70% energy), and a control high-carbohydrate (HCHO) diet for six days (carbohydrate > 60% energy; fat < 20% energy) in a randomized crossover design. Pre- and post-intervention metabolic measures included resting metabolic rate (RMR), respiratory quotient (RQ), maximum fat oxidation rate during exercise (MFO), and maximum fat oxidation intensity (FATmax). Measures of exercise performance included maximal oxygen uptake (VO₂max), running economy (RE), and 5 km running time trial (5 km-TT). In FACR compared with HCHO, there were significant improvements in FATmax (p = 0.006) and RE (p = 0.048). There were no significant differences (p > 0.05) between FACR and HCHO in RMR, RQ, VO₂max, or 5 km-TT. Findings suggest that a short-term (six days) FACR diet may facilitate increased fat oxidation and submaximal exercise economy but does not improve 5 km-TT performance.

Inhibitory Effects of Intranasal Administration of Insulin on Fat Oxidation during Exercise Are Diminished in Young Overweight Individuals

It remains unknown whether the high insulin (INS) levels in the brain affect fat oxidation during exercise. We examined the effects of the intranasal administration of INS, which increases the INS concentration in the cerebrospinal fluid when peripheral effects are lacking, on the maximum fat oxidation rate (maxFOR) and its intensity (FATmax) during exercise in 15 young normal-weight (N group) and eight young overweight (O group) individuals. On two separate days, either INS or placebo (PL) was randomly administered intranasally before a graded exercise test. Indirect calorimetry was used to assess maxFOR and FATmax during exercise. Blood INS and glucose levels did not change after INS administration. In the N group, maxFOR and FATmax were significantly smaller in the INS trial than in the PL trial. MaxFOR was significantly smaller in the O group than in the N group and was not influenced by INS administration. Exercise-induced elevation in blood epinephrine levels tended to be reduced by INS administration only in the N group. Intranasal INS administration reduces fat oxidation during exercise without any peripheral effects, possibly by suppressing sympathetic nerve activity. This inhibitory effect is diminished in overweight subjects, suggesting that cerebral insulin effects are attenuated in this population.

Carbohydrate dependence during prolonged simulated cycling time trials

PURPOSE

We determined the effect of suppressing lipolysis via administration of Nicotinic acid (NA) and pre-exercise feeding on rates of whole-body substrate utilisation and cycling time trial (TT) performance.

METHODS

In a randomised, single-blind, crossover design, eight trained male cyclists/triathletes completed two series of TTs in which they performed a predetermined amount of work calculated to last ~60, 90 and 120 min. TTs were undertaken after a standardised breakfast (2 g kg(-1) BM of carbohydrate (CHO)) and ingestion of capsules containing either NA or placebo (PL).

RESULTS

Plasma [free fatty acids] were suppressed with NA, but increased in the later stages of TT90 and TT120 with PL (p < 0.05). There was no treatment effect on time to complete TT60 (60.4 ± 4.1 vs. 59.3 ± 3.4 min) or TT90 (90.4 ± 9.1 vs. 89.5 ± 6.6 min) for NA and PL, respectively. However, TT120 was slower with NA (123.1 ± 5.7 vs. 120.1 ± 8.7 min, p < 0.001), which coincided with a decline in plasma [glucose] during the later stages of this ride (p < 0.05). For TTs of the same duration, the rates of whole-body CHO oxidation were unaffected by NA, but decreased with increasing TT time (p < 0.05). CHO was the predominant substrate for all TTs contributing between 83 and 94 % to total energy expenditure, although there was a small use of lipid-based fuels for all rides.

CONCLUSION

(1) NA impaired cycling TT performance lasting 120 min, (2) cycling TTs lasting from 60 to 120 min are CHO dependent, and (3) there is an obligatory use of lipid-based fuels in TTs lasting 1-2 h.

Dietary composition regulates Drosophila mobility and cardiac physiology

The impact of dietary composition on exercise capacity is a subject of intense study in both humans and model organisms. Interactions between diet and genetics are a crucial component of optimized dietary design. However, the genetic factors governing exercise response are still not well understood. The recent development of invertebrate models for endurance exercise is likely to facilitate study designs examining the conserved interactions between diet, exercise and genetics. As a first step, we used the Drosophila model to describe the effects of varying dietary composition on several physiological indices, including fatigue tolerance and climbing speed, cardiac performance, lipid storage and autophagy. We found that flies of two divergent genetic backgrounds optimize endurance and cardiac performance on relatively balanced low calorie diets. When flies are provided with unbalanced diets, diets higher in sugar than in yeast facilitate greater endurance at the expense of cardiac performance. Importantly, we found that dietary composition has a profound effect on various physiological indices, whereas total caloric intake per se has very little predictive value for performance. We also found that the effects of diet on endurance are completely reversible within 48 h if flies are switched to a different diet.

Impaired aerobic exercise adaptation in children and adolescents with craniopharyngioma is associated with hypothalamic involvement

OBJECTIVE

Many patients treated for craniopharyngioma (CP) complain of a relative incapacity for physical activity. Whether this is due to an objective decrease in adaptation to exercise is unclear. We assessed exercise tolerance in children with surgically treated CP and appropriate pituitary hormone replacement therapy compared with healthy controls and we examined the potential relationships with hypothalamic involvement, GH replacement, and the catecholamine deficiency frequently observed in these subjects.

DESIGN AND METHODS

Seventeen subjects (12 males and five females) with CP and 22 healthy controls (14 males and eight females) aged 15.3±2.5 years (7.3-18 years) underwent a standardized cycle ergometer test. Maximum aerobic capacity was expressed as the ratio of VO(2max) to fat-free mass (VO(2max)/FFM), a measure independent of age and fat mass in children.

RESULTS

VO(2max)/FFM was 20% lower in children with CP compared with controls (P<0.05), even after adjustment for gender. Children with hypothalamic involvement (n=10) had a higher percentage of fat mass (P<0.05) than those without hypothalamic involvement (n=7) and lower VO(2max)/FFM (P<0.05), whereas children without hypothalamic involvement had VO(2max)/FFM close to that of controls (P>0.05). GH treatment was associated with a significant positive effect on aerobic capacity (P<0.05) only in the absence of hypothalamic involvement. No relationship was found between exercise capacity parameters and daily urine epinephrine excretion or epinephrine peak response to insulin-induced hypoglycemia.

CONCLUSIONS

Children with CP have a decrease in aerobic capacity mainly related to hypothalamic involvement. The hypothalamic factors altering aerobic capacity remain to be determined.

LXR activation prevents exhaustive exercise-induced hypoglycaemia and spares muscle glycogen but does not enhance running endurance in untrained rats

AIM

Liver X receptors (LXRs) are ligand-activated transcription factors that play an important role in regulation of hepatic lipid and carbohydrate metabolism. However, to date there is very few information on the role of LXRs in skeletal muscle. Moreover, it remains obscure whether LXR activation affects physical endurance. Therefore, we aimed to examine effects of selective LXR activator--T0901317--on running endurance and skeletal muscle exercise metabolism in rats.

METHODS

The animals were assigned to two groups (n=20) receiving either vehicle or T0901317 (10 mg kg(-1) day(-1) ) for 1 week. One day after the final administration, half of the rats in each group were exercised until exhaustion on the electrically driven treadmill. All animals were then anaesthetized and samples of the soleus, red and white sections of the gastrocnemius muscle, epididymal fat pad and liver were excised.

RESULTS

We found that LXR activation prevented exhaustive exercise-induced hypoglycaemia. T0901317 also shifted substrate utilization in working muscles in favour of fatty acids as indicated by its glycogen sparing effect, enhanced consumption of intramuscular triacylglycerol and upregulation of genes promoting fatty acid oxidation and suppressing carbohydrate oxidation. However, running time to exhaustion was not improved.

CONCLUSION

We conclude that LXR activation increases fatty acid utilization during exercise which, however, does not translate into measurable enhancement of exercise endurance.

Determination of the exercise intensity that elicits maximal fat oxidation

PURPOSE

The aim of this study was to develop a test protocol to determine the exercise intensity at which fat oxidation rate is maximal (Fat(max)).

METHOD

Eighteen moderately trained cyclists performed a graded exercise test to exhaustion, with 5-min stages and 35-W increments (GE(35/5)). In addition, four to six continuous prolonged exercise tests (CE) at constant work rates, corresponding to the work rates of the GE test, were performed on separate days. The duration of each test was chosen so that all trials would result in an equal energy expenditure. Seven other subjects performed three different GE tests to exhaustion. The test protocols differed in stage duration and in increment size. Fat oxidation was measured using indirect calorimetry.

RESULTS

No significant differences were found in Fat(max) determined with the GE(35/5), the average fat oxidation of the CE tests, or fat oxidation measured during the first 5 min of the CE tests (56 +/- 3, 64 +/- 3, 58 +/- 3%VO(2max), respectively). Results of the GE(35/5) protocol were used to construct an exercise intensity versus fat oxidation curve for each individual. Fat(max) was equivalent to 64 +/- 4%VO(2max) and 74 +/- 3%HR(max). The Fat(max) zone (range of intensities with fat oxidation rates within 10% of the peak rate) was located between 55 +/- 3 and 72 +/- 4%VO(2max). The contribution of fat oxidation to energy expenditure became negligible above 89 +/- 3%VO(2max) (92 +/- 1%HR(max)). When stage duration was reduced from 5 to 3 min or when increment size was reduced from 35 to 20 W, no significant differences were found in Fat(max), Fat(min), or the Fat(max) zone.

CONCLUSION

It is concluded that a protocol with 3-min stages and 35-W increments in work rate can be used to determine Fat(max). Fat oxidation rates are high over a large range of intensities; however, at exercise intensities above Fat(max), fat oxidation rates drop markedly.