Effects of L-carnitine on the pyruvate dehydrogenase complex and carnitine palmitoyl transferase activities in muscle of endurance athletes

Influence of incorporating L-carnitine or Moringa oleifera leaves extract into semen diluent on cryosurvival and in vitro fertilization competence of buck sperm

This study aimed at scrutinizing efficiency of incorporating L-carnitine or M. oleifera leaves extract into semen diluent on improving cryopreservation capacity and in vitro fertilization ability of buck spermatozoa. Ejaculates (n=48) were collected by an artificial vagina from six adult Damascus bucks twice weekly during the breeding season (September-October). Following initial evaluation, ejaculates of each collection session from the same bucks were pooled, diluted (1:10) with glycerolized (3 % glycerol, v/v) tris-citric acid egg yolk diluent and were split into three aliquots. The first aliquot served as control, whereas the second and third aliquots were supplemented with 4 μL/mL L-carnitine and 400 μL/mL moringa leaves extract (v/v), respectively. Thereafter, all specimens were processed for cryopreservation and were stored in liquid nitrogen (-196 °C) for 12 months before post-thaw sperm criteria were analyzed by a computer-assisted sperm analysis (CASA) system. Integrity of sperm DNA post thawing was visualized in all semen groups by fluorescence imaging, and in vitro fertilization ability of spermatozoa was also determined. Inclusion of L-carnitine or moringa leaves extract into the diluent improved (P<0.05) post-thaw sperm physical, morphofunctional and kinematic attributes, whilst maintaining (P<0.05) integrity of sperm DNA throughout the freezing and thawing cycle. Consequently, both supplemented groups yielded higher (P<0.05) in vitro fertilization rates compared to control. These results accentuate the protective roles of these antioxidants on buck sperm against consequences of cryopreservation-induced oxidative stress, hence ameliorating post-thaw sperm quality and fertilization competence. This is crucial for successful application of AI and IVF in goat selective breeding programs.

Effects of L-Carnitine Intake on Exercise-Induced Muscle Damage and Oxidative Stress: A Narrative Scoping Review

Exercise-induced muscle damage results in decreased physical performance that is accompanied by an inflammatory response in muscle tissue. The inflammation process occurs with the infiltration of phagocytes (neutrophils and macrophages) that play a key role in the repair and regeneration of muscle tissue. In this context, high intensity or long-lasting exercise results in the breakdown of cell structures. The removal of cellular debris is performed by infiltrated phagocytes, but with the release of free radicals as collateral products. L-carnitine is a key metabolite in cellular energy metabolism, but at the same time, it exerts antioxidant actions in the neuromuscular system. L-carnitine eliminates reactive oxygen and nitrogen species that, in excess, alter DNA, lipids and proteins, disturbing cell function. Supplementation using L-carnitine results in an increase in serum L-carnitine levels that correlates positively with the decrease in cell alterations induced by oxidative stress situations, such as hypoxia. The present narrative scoping review focuses on the critical evaluation of the efficacy of L-carnitine supplementation on exercise-induced muscle damage, particularly in postexercise inflammatory and oxidative damage. Although both concepts appear associated, only in two studies were evaluated together. In addition, other studies explored the effect of L-carnitine in perception of fatigue and delayed onset of muscle soreness. In view of the studies analyzed and considering the role of L-carnitine in muscle bioenergetics and its antioxidant potential, this supplement could help in postexercise recovery. However, further studies are needed to conclusively clarify the mechanisms underlying these protective effects.

Stable isotope-labeled carnitine reveals its rapid transport into muscle cells and acetylation during contraction

Carnitine plays multiple roles in skeletal muscle metabolism, including fatty acid transport and buffering of excess acetyl-CoA in the mitochondria. The skeletal muscle cannot synthesize carnitine; therefore, carnitine must be taken up from the blood into the cytoplasm. Carnitine metabolism, its uptake into cells, and the subsequent reactions of carnitine are accelerated by muscle contraction. Isotope tracing enables the marking of target molecules and monitoring of tissue distribution. In this study, stable isotope-labeled carnitine tracing was combined with matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) imaging to determine carnitine distribution in mouse skeletal muscle tissues. Deuterium-labeled carnitine (d3-carnitine) was intravenously injected into the mice and diffused to the skeletal muscles for 30 and 60 min. To examine whether muscle contraction changes the distribution of carnitine and its derivatives, unilateral in situ muscle contraction was performed; 60 min muscle contraction showed increased d3-carnitine and its derivative d3-acetylcarnitine in the muscle, indicating that carnitine uptake in cells is promptly converted to acetylcarnitine, consequently, buffering accumulated acetyl-CoA. While the endogenous carnitine was localized in the slow type fibers rather than fast type, the contraction-induced distributions of d3-carnitine and acetylcarnitine were not necessarily associated with muscle fiber type. In conclusion, the combination of isotope tracing and MALDI-MS imaging can reveal carnitine flux during muscle contraction and show the significance of carnitine in skeletal muscles.

Effect of Acute and Chronic Oral l-Carnitine Supplementation on Exercise Performance Based on the Exercise Intensity: A Systematic Review

l-Carnitine (l-C) and any of its forms (glycine-propionyl l-Carnitine (GPL-C) or l-Carnitine l-tartrate (l-CLT)) has been frequently recommended as a supplement to improve sports performance due to, among others, its role in fat metabolism and in maintaining the mitochondrial acetyl-CoA/CoA ratio. The main aim of the present systematic review was to determine the effects of oral l-C supplementation on moderate- (50-79% V˙O2 max) and high-intensity (≥80% V˙O2 max) exercise performance and to show the effective doses and ideal timing of its intake. A structured search was performed according to the PRISMA® statement and the PICOS guidelines in the Web of Science (WOS) and Scopus databases, including selected data obtained up to 24 October 2021. The search included studies where l-C or glycine-propionyl l-Carnitine (GPL-C) supplementation was compared with a placebo in an identical situation and tested its effects on high and/or low-moderate performance. The trials that used the supplementation of l-C together with additional supplements were eliminated. There were no applied filters on physical fitness level, race, or age of the participants. The methodological quality of studies was evaluated by the McMaster Critical Review Form. Of the 220 articles obtained, 11 were finally included in this systematic review. Six studies used l-C, while three studies used l-CLT, and two others combined the molecule propionyl l-Carnitine (PL-C) with GPL-C. Five studies analyzed chronic supplementation (4-24 weeks) and six studies used an acute administration (<7 days). The administration doses in this chronic supplementation varied from 1 to 3 g/day; in acute supplementation, oral l-C supplementation doses ranged from 3 to 4 g. On the one hand, the effects of oral l-C supplementation on high-intensity exercise performance variables were analyzed in nine studies. Four of them measured the effects of chronic supplementation (lower rating of perceived exertion (RPE) after 30 min at 80% V˙O2 max on cycle ergometer and higher work capacity in "all-out" tests, peak power in a Wingate test, and the number of repetitions and volume lifted in leg press exercises), and five studies analyzed the effects of acute supplementation (lower RPE after graded exercise test on the treadmill until exhaustion and higher peak and average power in the Wingate cycle ergometer test). On the other hand, the effects of l-C supplementation on moderate exercise performance variables were observed in six studies. Out of those, three measured the effect of an acute supplementation, and three described the effect of a chronic supplementation, but no significant improvements on performance were found. In summary, l-C supplementation with 3 to 4 g ingested between 60 and 90 min before testing or 2 to 2.72 g/day for 9 to 24 weeks improved high-intensity exercise performance. However, chronic or acute l-C or GPL-C supplementation did not present improvements on moderate exercise performance.

Clinical Effects of L-Carnitine Supplementation on Physical Performance in Healthy Subjects, the Key to Success in Rehabilitation: A Systematic Review and Meta-Analysis from the Rehabilitation Point of View

L-carnitine supplementation improves body strength, sports endurance and exercise capacity, as well as delaying the onset of fatigue. The aim of this study was to identify the correct dosage of supplementation to obtain improvements in physical performance and evaluate the changes related to L-carnitine supplementation in specific metabolic parameters, such as serum lactate, VO₂, serum total and free carnitine at rest and after physical activities, in healthy subjects. The search was conducted on PubMed, EMBASE, Cochrane Library, Scopus and Web of Science and identified 6404 articles with the keywords: "carnitine" AND "exercises" OR "rehabilitation" OR "physical functional performance" OR "physical activity" OR "sports" OR "health" OR "healthy". A total of 30 publications met the inclusion criteria and were included in the systematic review. The meta-analysis did not show any significant differences in serum lactate values at rest and after exercise in healthy subjects who took L-carnitine supplementation (p > 0.05). On the contrary, L-carnitine administration significantly changed maximal oxygen consumption (VO₂) at rest (p < 0.005), serum free and total carnitine at rest and after exercise (p < 0.001). The dosage of supplementation that obtained a significant change in serum total carnitine was 2 g/dL for 4 weeks at rest, 1 g/dL for 3 weeks after exercise, and in serum free carnitine was 2 g/dL for 3 weeks and 2 g/dL for 4 weeks at rest. Based on our study, serum total and free carnitine at rest and after exercise, and VO₂ at rest could be used to clinically follow individuals during physical activity and rehabilitation programs. Moreover, the supplementation should have a correct dosage to have maximum effect. Other robust trials are needed to find the best dosage to obtain positive results in metabolic parameters and in physical performance.

Biological roles of l-carnitine in oocyte and early embryo development

Poor oocyte quality is responsible for female infertility. Multiple studies have been carried out to find supplements to enhance oocyte quality and mitigate infertility problems. l-carnitine and its derivatives have diverse roles in developing oocytes and early embryos. This review focuses on the in vitro and in vivo studies that using l-carnitine alone or in combination with other supplements for oocyte quality enhancement. The key roles of l-carnitine in oocyte quality and embryo growth were summarized, and the underlying mechanism was also elucidated. l-carnitine helps in the lipid metabolism process by controlling the transfer of fatty acids to mitochondria for β-oxidation. l-carnitine modulates glucose metabolism and enhances respiratory chain enzyme activity. Furthermore, it acts as an antioxidant to prevent oxidative damage and inhibit apoptosis, a signal in response to oxidative stress. Results show the potential of l-carnitine as a potential agent in assisted reproductive technology to improve oocyte quality and the subsequent embryonic development.

Lipid and Lipoprotein Dysregulation in Sepsis: Clinical and Mechanistic Insights into Chronic Critical Illness

In addition to their well-characterized roles in metabolism, lipids and lipoproteins have pleiotropic effects on the innate immune system. These undergo clinically relevant alterations during sepsis and acute inflammatory responses. High-density lipoprotein (HDL) plays an important role in regulating the immune response by clearing bacterial toxins, supporting corticosteroid release, decreasing platelet aggregation, inhibiting endothelial cell apoptosis, reducing the monocyte inflammatory response, and inhibiting expression of endothelial cell adhesion molecules. It undergoes quantitative as well as qualitative changes which can be measured using the HDL inflammatory index (HII). Pro-inflammatory, or dysfunctional HDL (dysHDL) lacks the ability to perform these functions, and we have also found it to independently predict adverse outcomes and organ failure in sepsis. Another important class of lipids known as specialized pro-resolving mediators (SPMs) positively affect the escalation and resolution of inflammation in a temporal fashion. These undergo phenotypic changes in sepsis and differ significantly between survivors and non-survivors. Certain subsets of sepsis survivors go on to have perilous post-hospitalization courses where this inflammation continues in a low grade fashion. This is associated with immunosuppression in a syndrome of persistent inflammation, immunosuppression, and catabolism syndrome (PICS). The continuous release of tissue damage-related patterns and viral reactivation secondary to immunosuppression feed this chronic cycle of inflammation. Animal data indicate that dysregulation of endogenous lipids and SPMs play important roles in this process. Lipids and their associated pathways have been the target of many clinical trials in recent years which have not shown mortality benefit. These results are limited by patient heterogeneity and poor animal models. Considerations of sepsis phenotypes and novel biomarkers in future trials are important factors to be considered in future research. Further characterization of lipid dysregulation and chronic inflammation during sepsis will aid mortality risk stratification, detection of sepsis, and inform individualized pharmacologic therapies.

L-Carnitine and Acylcarnitines: Mitochondrial Biomarkers for Precision Medicine

Biomarker discovery and implementation are at the forefront of the precision medicine movement. Modern advances in the field of metabolomics afford the opportunity to readily identify new metabolite biomarkers across a wide array of disciplines. Many of the metabolites are derived from or directly reflective of mitochondrial metabolism. L-carnitine and acylcarnitines are established mitochondrial biomarkers used to screen neonates for a series of genetic disorders affecting fatty acid oxidation, known as the inborn errors of metabolism. However, L-carnitine and acylcarnitines are not routinely measured beyond this screening, despite the growing evidence that shows their clinical utility outside of these disorders. Measurements of the carnitine pool have been used to identify the disease and prognosticate mortality among disorders such as diabetes, sepsis, cancer, and heart failure, as well as identify subjects experiencing adverse drug reactions from various medications like valproic acid, clofazimine, zidovudine, cisplatin, propofol, and cyclosporine. The aim of this review is to collect and interpret the literature evidence supporting the clinical biomarker application of L-carnitine and acylcarnitines. Further study of these metabolites could ultimately provide mechanistic insights that guide therapeutic decisions and elucidate new pharmacologic targets.

The bright and the dark sides of L-carnitine supplementation: a systematic review

Background

L-carnitine (LC) is used as a supplement by recreationally-active, competitive and highly trained athletes. This systematic review aims to evaluate the effect of prolonged LC supplementation on metabolism and metabolic modifications.

Methods

A literature search was conducted in the MEDLINE (via PubMed) and Web of Science databases from the inception up February 2020. Eligibility criteria included studies on healthy human subjects, treated for at least 12 weeks with LC administered orally, with no drugs or any other multi-ingredient supplements co-ingestion.

Results

The initial search retrieved 1024 articles, and a total of 11 studies were finally included after applying inclusion and exclusion criteria. All the selected studies were conducted with healthy human subjects, with supplemented dose ranging from 1 g to 4 g per day for either 12 or 24 weeks. LC supplementation, in combination with carbohydrates (CHO) effectively elevated total carnitine content in skeletal muscle. Twenty-four-weeks of LC supplementation did not affect muscle strength in healthy aged women, but significantly increased muscle mass, improved physical effort tolerance and cognitive function in centenarians. LC supplementation was also noted to induce an increase of fasting plasma trimethylamine-N-oxide (TMAO) levels, which was not associated with modification of determined inflammatory nor oxidative stress markers.

Conclusion

Prolonged LC supplementation in specific conditions may affect physical performance. On the other hand, LC supplementation elevates fasting plasma TMAO, compound supposed to be pro-atherogenic. Therefore, additional studies focusing on long-term supplementation and its longitudinal effect on the cardiovascular system are needed.

Nutraceutical Supplements in the Thyroid Setting: Health Benefits beyond Basic Nutrition

In recent years, there has been a growing interest in nutraceuticals, which may be considered as an efficient, preventive, and therapeutic tool in facing different pathological conditions, including thyroid diseases. Although iodine remains the major nutrient required for the functioning of the thyroid gland, other dietary components play important roles in clinical thyroidology-these include selenium, l-carnitine, myo-inositol, melatonin, and resveratrol-some of which have antioxidant properties. The main concern regarding the appropriate and effective use of nutraceuticals in prevention and treatment is due to the lack of clinical data supporting their efficacy. Another limitation is the discrepancy between the concentration claimed by the label and the real concentration. This paper provides a detailed critical review on the health benefits, beyond basic nutrition, of some popular nutraceutical supplements, with a special focus on their effects on thyroid pathophysiology and aims to distinguish between the truths and myths surrounding the clinical use of such nutraceuticals.

Effects of L-carnitine supplementation on respiratory distress syndrome development and prognosis in premature infants: A single blind randomized controlled trial

The aim of the present study was to investigate the efficacy of L-carnitine therapy on the occurrence and prognosis of respiratory distress syndrome (RDS). A single blind, randomized controlled trial study was conducted on 130 infants with gestational ages of 28-36 weeks. Infants were assigned to experimental groups (groups 1 and 2) and control groups (groups 3 and 4). Groups 1 and 3 consisted of infants with RDS, and groups 2 and 4 groups were composed of infants without RDS. The experimental groups were treated with carnitine. No statistically significant differences in serum carnitine levels were detected between the study and the control groups on day 1 of treatment (P=0.06). However, on day 7 of treatment, serum carnitine levels in the experimental groups were significantly increased (P=0.02), as compared with the control groups. The surfactant requirement value, which is how many rounds of surfactant therapy were required, was 1.56±0.97 in group 1, and 2.12±0.99 in group 3 (P<0.001). The mean duration of mechanical ventilation required was 3.04±3.60 days in group 1, and 4.73±5.63 days in group 3 (P<0.001). The present results indicate that carnitine supplementation in premature infants with RDS may help to increase carnitine levels, thus decreasing the duration of mechanical ventilation and surfactant requirement.

Creatine, L-carnitine, and ω3 polyunsaturated fatty acid supplementation from healthy to diseased skeletal muscle

Myopathies are chronic degenerative pathologies that induce the deterioration of the structure and function of skeletal muscle. So far a definitive therapy has not yet been developed and the main aim of myopathy treatment is to slow the progression of the disease. Current nonpharmacological therapies include rehabilitation, ventilator assistance, and nutritional supplements, all of which aim to delay the onset of the disease and relieve its symptoms. Besides an adequate diet, nutritional supplements could play an important role in the treatment of myopathic patients. Here we review the most recent in vitro and in vivo studies investigating the role supplementation with creatine, L-carnitine, and ω3 PUFAs plays in myopathy treatment. Our results suggest that these dietary supplements could have beneficial effects; nevertheless continued studies are required before they could be recommended as a routine treatment in muscle diseases.

Effective dosing of L-carnitine in the secondary prevention of cardiovascular disease: a systematic review and meta-analysis

Background

L-carnitine supplementation has been associated with a significant reduction in all-cause mortality, ventricular arrhythmia, and angina in the setting of acute myocardial infarction (MI). However, on account of strict homeostatic regulation of plasma L-carnitine concentrations, higher doses of L-carnitine supplementation may not provide additional therapeutic benefits. This study aims to evaluate the effects of various oral maintenance dosages of L-carnitine on all-cause mortality and cardiovascular morbidities in the setting of acute MI.

Methods

After a systematic review of several major electronic databases (PubMed, EMBASE, and the Cochrane Library) up to November 2013, a meta-analysis of five controlled trials (n = 3108) was conducted to determine the effects of L-carnitine on all-cause mortality and cardiovascular morbidities in the setting of acute MI.

Results

The interaction test yielded no significant differences between the effects of the four daily oral maintenance dosages of L-carnitine (i.e., 2 g, 3 g, 4 g, and 6 g) on all-cause mortality (risk ratio [RR] = 0.77, 95% CI [0.57-1.03], P = 0.08) with a statistically insignificant trend favoring the 3 g dose (RR = 0.48) over the lower 2 g dose (RR = 0.62), which was favored over the higher 4 g and 6 g doses (RR = 0.78, 0.78). There was no significant differences between the effects of the daily oral maintenance dosages of 2 g and 6 g on heart failure (RR = 0.53, 95% CI [0.25-1.13], P = 0.10), unstable angina (RR = 0.90, 95% CI [0.51-1.58], P = 0.71), or myocardial reinfarction (RR = 0.74, 95% CI [0.30-1.80], P = 0.50).

Conclusions

There appears to be no significant marginal benefit in terms of all-cause mortality, heart failure, unstable angina, or myocardial reinfarction in the setting of acute MI for oral L-carnitine maintenance doses of greater or less than 3 g per day.

Effect of carnitine, acetyl-, and propionylcarnitine supplementation on the body carnitine pool, skeletal muscle composition, and physical performance in mice

PURPOSE

Pharmacokinetics and effects on skeletal muscle and physical performance of oral acetylcarnitine and propionylcarnitine are not well characterized. We therefore investigated the influence of oral acetylcarnitine, propionylcarnitine, and carnitine on body carnitine homeostasis, energy metabolism, and physical performance in mice and compared the findings to non-supplemented control animals.

METHODS

Mice were supplemented orally with 2 mmol/kg/day carnitine, acetylcarnitine, or propionylcarnitine for 4 weeks and studied either at rest or after exhaustive exercise.

RESULTS

In the supplemented groups, total plasma and urine carnitine concentrations were significantly higher than in the control group receiving no carnitine, whereas the skeletal muscle carnitine content remained unchanged. The supplemented acylcarnitines were hydrolyzed in intestine and liver and reached the systemic circulation as carnitine. Bioavailability of carnitine and acylcarnitines, determined as the urinary excretion of total carnitine, was in the range of 19 %. Skeletal muscle morphology, including fiber-type composition, was not affected, and oxygen consumption by soleus or gastrocnemius fibers was not different between the groups. Supplementation with carnitine or acylcarnitines had no significant impact on the running capacity, but was associated with lower plasma lactate levels and a higher glycogen content in white skeletal muscle after exhaustive exercise.

CONCLUSIONS

Oral supplementation of carnitine, acetylcarnitine, or propionylcarnitine in mice is associated with increased plasma and urine total carnitine concentrations, but does not affect the skeletal muscle carnitine content. Despite better preservation of skeletal muscle glycogen and lower plasma lactate levels, physical performance was not improved by carnitine or acylcarnitine supplementation.

Glycine propionyl-L-carnitine produces enhanced anaerobic work capacity with reduced lactate accumulation in resistance trained males

Background

Recent research has indicated that short term administration of glycine propionyl-L-carnitine (GPLC) significantly elevates levels of nitric oxide metabolites at rest and in response to reactive hyperaemia. However, no scientific evidence exists that suggests such supplementation enhances exercise performance in healthy, trained individuals. The purpose of this study was to examine the effects of GPLC on the performance of repeated high intensity stationary cycle sprints with limited recovery periods in resistance trained male subjects.

Methods

In a double-blind, placebo-controlled, cross-over design, twenty-four male resistance trained subjects (25.2 ± 3.6 years) participated in two test sessions separated by one week. Testing was performed 90 minutes following oral ingestion of either 4.5 grams GPLC or 4.5 grams cellulose (PL), in randomized order. The exercise testing protocol consisted of five 10-second Wingate cycle sprints separated by 1-minute active recovery periods. Peak (PP) and mean values (MP) of sprint power output and percent decrement of power (DEC) were determined per bout and standardized relative to body masss. Heart rate (HR) and blood lactate (LAC) were measured prior to, during and following the five sprint bouts.

Results

Significant main effects (p < 0.001) were observed for sprint bout order in values of PP, MP, DEC, and HR. There were significant main effects detected for condition in PP and MP (p < 0.05), with values across the five sprint bouts 2.6 – 15% greater with GPLC. Significant statistical interactions were detected between bout order and condition for both PP and MP (p < 0.05). There was a significant main effect of condition for LAC, LAC values 15.7% lower 4 min post-exercise with GPLC (p = 0.09) and with GPLC resulting in 16.2% less LAC at 14 min post-exercise (p < 0.05).

Conclusion

These findings indicate that short-term oral supplementation of GPLC can enhance peak power production in resistance trained males with significantly less LAC accumulation.

Maternal dietary L-carnitine supplementation influences fetal carnitine status and stimulates carnitine palmitoyltransferase and pyruvate dehydrogenase complex activities in swine

Effects of increasing maternal L-carnitine on carnitine status and energy metabolism in the fetus were evaluated by feeding pregnant swine a corn-soybean-based diet containing either 0 or 50 mg/kg added L-carnitine (n = 10/treatment) during the first 70 d of gestation. Carnitine, carnitine palmitoyltransferase (CPT), and pyruvate dehydrogenase complex (PDHC) activities were analyzed in tissues collected from fetuses on d 55 and 70. Maternal L-carnitine supplementation increased both fetal free and long-chain carnitine concentrations by 45% in liver and free carnitine by 31% in heart tissues but did not affect kidney tissue. Elevations in free and acylcarnitines increased with gestational age from 55 to 70 d in liver but not in heart and kidney. The increased carnitine concentrations resulted in a 45% increase in PDHC activity in heart and liver on d 70 of gestation but did not affect kidney and liver on d 55 of gestation. The increases in carnitine concentrations were accompanied by a 70% increase in hepatic CPT activity in 70-d-old fetuses, but activities in heart and kidney were unaffected. The Michaelis constant (K(m)) of CPT for carnitine in fetal tissues was not influenced by carnitine supplementation (P > 0.1). Notably, the concentrations of carnitine measured on d 70 were only 25-40% of the K(m) values in liver, 60-70% in heart, and 30-40% in kidney (P < 0.001). We conclude that carnitine ingestion during pregnancy increases fetal carnitine concentrations and stimulates heart PDHC and liver CPT activity without altering carnitine K(m).

Effects of oral l-carnitine supplementation in racing Greyhounds

l-Carnitine supplementation can stimulate erythropoiesis, reduce exercise-induced plasma lactate concentrations and decrease post-exercise muscle damage. Next to horses, Greyhounds represent the premier animal racing species and perform short-duration, very high-intensity exercise that has the potential to incur substantial muscle damage. Under resting and standard racing conditions (5/16 mile), we tested the novel hypotheses that l-carnitine supplementation in Greyhounds would: (1) elevate haematocrit at rest and immediately post-exercise; (2) reduce peak post-exercise plasma lactate; and (3) reduce indices of muscle damage (plasma creatine phosphokinase, CPK and aspartate aminotransferase, AST). Six conditioned Greyhounds (30.1 ± 1.6 kg) underwent a randomized placebo-controlled crossover study to determine the effects of 6 weeks of l-carnitine supplementation (100 mg kg− 1 of body weight/day) at rest and following a maximal speed 5/16 mile race. In accordance with our hypotheses, l-carnitine elevated resting and immediately post-race haematocrit (control, 60.1 ± 1.7, l-carnitine, 63.6 ± 1.7; P < 0.05) and reduced peak post-race plasma CPK and AST concentrations (both P < 0.05). Those dogs with the highest peak post-exercise plasma CPK concentrations under placebo conditions evidenced the greatest reduction with l-carnitine supplementation (r = 0.99, P < 0.01). However, contrary to our hypotheses, l-carnitine did not change peak post-exercise plasma lactate concentrations (control, 27.0 ± 2.1, l-carnitine, 27.7 ± 1.3; P>0.05). We conclude that l-carnitine supplementation increases the potential for oxygen transport and reduces plasma indicators of muscle damage, CPK and AST in racing Greyhounds.

The effect of supplementation with n-3 fatty acids on the physical performance in subjects with spinal cord injury

A global physical evaluation was performed in 21 males with spinal cord injury (SCI), at the beginning and at three and six months of omega-3 fatty acid (FA) supplementation. A significant increase in the proportion of eicosapentaenoic acid and docosahexanoic acid in plasma was observed in response to the supplementation (p<0.05). After six months of FA supplementation, strength endurance time increased from 127.7+/-19.0 s to 215.2+/-45.6 s in the right arm, and from 139+/-27.6 s to 237.7+/-48.7 s, in the left arm. The time to perform 20 repetitions of 70% maximum workload showed a reduction of 41% between the first and the third test. The time taken to cover a 90 meter long track, with a 6% slope, was reduced from 66.9+/-8.0 s to 59.3+/-6.7 s, at the end of the study (p<0.05). In conclusion, omega-3 FA supplementation could contribute to improve the functional capabilities in SCI subjects.

Supplementation of L-carnitine in athletes: does it make sense?

Studies in athletes have shown that carnitine supplementation may foster exercise performance. As reported in the majority of studies, an increase in maximal oxygen consumption and a lowering of the respiratory quotient indicate that dietary carnitine has the potential to stimulate lipid metabolism. Treatment with L-carnitine also has been shown to induce a significant postexercise decrease in plasma lactate, which is formed and used continuously under fully aerobic conditions. Data from preliminary studies have indicated that L-carnitine supplementation can attenuate the deleterious effects of hypoxic training and speed up recovery from exercise stress. Recent data have indicated that L-carnitine plays a decisive role in the prevention of cellular damage and favorably affects recovery from exercise stress. Uptake of L-carnitine by blood cells may induce at least three mechanisms: 1) stimulation of hematopoiesis, 2) a dose-dependent inhibition of collagen-induced platelet aggregation, and 3) the prevention of programmed cell death in immune cells. As recently shown, carnitine has direct effects in regulation of gene expression (i.e., carnitine-acyltransferases) and may also exert effects via modulating intracellular fatty acid concentration. Thus there is evidence for a beneficial effect of L-carnitine supplementation in training, competition, and recovery from strenuous exercise and in regenerative athletics.

Kinetics, pharmacokinetics, and regulation of L-carnitine and acetyl-L-carnitine metabolism

In mammals, the carnitine pool consists of nonesterified L-carnitine and many acylcarnitine esters. Of these esters, acetyl-L-carnitine is quantitatively and functionally the most significant. Carnitine homeostasis is maintained by absorption from diet, a modest rate of synthesis, and efficient renal reabsorption. Dietary L-carnitine is absorbed by active and passive transfer across enterocyte membranes. Bioavailability of dietary L-carnitine is 54-87% and is dependent on the amount of L-carnitine in the meal. Absorption of L-carnitine dietary supplements (0.5-6 g) is primarily passive; bioavailability is 14-18% of dose. Unabsorbed L-carnitine is mostly degraded by microorganisms in the large intestine. Circulating L-carnitine is distributed to two kinetically defined compartments: one large and slow-turnover (presumably muscle), and another relatively small and rapid-turnover (presumably liver, kidney, and other tissues). At normal dietary L-carnitine intake, whole-body turnover time in humans is 38-119 h. In vitro experiments suggest that acetyl-L-carnitine is partially hydrolyzed in enterocytes during absorption. In vivo, circulating acetyl-L-carnitine concentration was increased 43% after oral acetyl-L-carnitine supplements of 2 g/day, indicating that acetyl-L-carnitine is absorbed at least partially without hydrolysis. After single-dose intravenous administration (0.5 g), acetyl-L-carnitine is rapidly, but not completely hydrolyzed, and acetyl-L-carnitine and L-carnitine concentrations return to baseline within 12 h. At normal circulating l-carnitine concentrations, renal l-carnitine reabsorption is highly efficient (90-99% of filtered load; clearance, 1-3 mL/min), but displays saturation kinetics. Thus, as circulating L-carnitine concentration increases (as after high-dose intravenous or oral administration of L-carnitine), efficiency of reabsorption decreases and clearance increases, resulting in rapid decline of circulating L-carnitine concentration to baseline. Elimination kinetics for acetyl-L-carnitine are similar to those for L-carnitine. There is evidence for renal tubular secretion of both L-carnitine and acetyl-L-carnitine. Future research should address the correlation of supplement dosage, changes and maintenance of tissue L-carnitine and acetyl-L-carnitine concentrations, and metabolic and functional changes and outcomes.

Long-term administration of L-carnitine to humans: effect on skeletal muscle carnitine content and physical performance

BACKGROUND

Long-term administration of high oral doses of L-carnitine on the skeletal muscle composition and the physical performance has not been studied in humans.

METHODS

Eight healthy male adults were treated with 2 x 2 g of L-carnitine per day for 3 months. Muscle biopsies and exercise tests were performed before, immediately after, and 2 months after the treatment. Exercise tests were performed using a bicycle ergometer for 10 min at 20%, 40%, and 60% of the individual maximal workload (P(max)), respectively, until exhaustion.

RESULTS

There were no significant differences between V(O(2)max), RER(max), and P(max) between the three time points investigated. At submaximal intensities, the only difference to the pretreatment values was a 5% increase in V(O(2)) at 20% and 40% of P(max) 2 months after the cessation of the treatment. The total carnitine content in the skeletal muscle was 4.10 +/- 0.82 micromol/g before, 4.79 +/- 1.19 micromol/g immediately after, and 4.19 +/- 0.61 micromol/g wet weight 2 months after the treatment (no significant difference). Activities of the two mitochondrial enzymes citrate synthase and cytochrome oxidase, as well as the skeletal muscle fiber composition also remained unaffected by the administration of L-carnitine.

CONCLUSIONS

Long-term oral treatment of healthy adults with L-carnitine is not associated with a significant increase in the muscle carnitine content, mitochondrial proliferation, or physical performance. Beneficial effects of the long-term treatment with L-carnitine on the physical performance of healthy adults cannot be explained by an increase in the carnitine muscle stores.

Supplemental carnitine and exercise

Carnitine is an endogenous compound with well-established roles in intermediary metabolism. An obligate for optimal mitochondrial fatty acid oxidation, it is a critical source of energy and also protects the cell from acyl-CoA accretion through the generation of acylcarnitines. Carnitine homeostasis is affected by exercise in a well-defined manner because of the interaction of the carnitine-acylcarnitine pool with key metabolic pathways. Carnitine supplementation has been hypothesized to improve exercise performance in healthy humans through various mechanisms, including enhanced muscle fatty acid oxidation, altered glucose homeostasis, enhanced acylcarnitine production, modification of training responses, and altered muscle fatigue resistance. Available experimental clinical studies designed to assess the effect of carnitine on exercise metabolism or performance in healthy humans do not permit definitive conclusions to be drawn. In the aggregate, however, these studies suggest that carnitine supplementation does not improve maximal oxygen uptake or metabolic status during exercise in healthy humans. Carnitine administration for </=1 mo in humans increases plasma carnitine concentrations but does not increase muscle carnitine content. Additional clinical trials integrating physiologic, biochemical, and pharmacologic assessments are needed to definitively clarify any effects of carnitine on exercise performance in healthy persons.

Sensitivity of CPT I to malonyl-CoA in trained and untrained human skeletal muscle

The present study examined the sensitivity of carnitine palmitoyltransferase I (CPT I) activity to its inhibitor malonyl-CoA (M-CoA), and simulated metabolic conditions of rest and exercise, in aerobically trained and untrained humans. Maximal CPT I activity was measured in mitochondria isolated from resting human skeletal muscle. Mean CPT I activity was 492.8 +/- 72.8 and 260.8 +/- 33.6 micromol. min(-1). kg wet muscle(-1) in trained and untrained subjects, respectively (pH 7.0, 37 degrees C). The sensitivity to M-CoA was greater in trained muscle; the IC(50) for M-CoA was 0.17 +/- 0.04 and 0.49 +/- 0.17 microM in trained and untrained muscle, respectively. The presence of acetyl-CoA, free coenzyme A (CoASH), and acetylcarnitine, in concentrations simulating rest and exercise conditions did not release the M-CoA-induced inhibition of CPT I activity. However, CPT I activity was reduced at pH 6.8 vs. pH 7.0 in both trained and untrained muscle in the presence of physiological concentrations of M-CoA. The results of this study indicate that aerobic training is associated with an increase in the sensitivity of CPT I to M-CoA. Accumulations of acetyl-CoA, CoASH, and acetylcarnitine do not counteract the M-CoA-induced inhibition of CPT I activity. However, small decreases in pH produce large reductions in the activity of CPT I and may contribute to the decrease in fat metabolism that occurs during moderate and intense aerobic exercise intensities.

Biological roles of L-carnitine in perinatal metabolism

Carnitine performs a crucial role in the energy supply of tissues during fetal life and in the neonatal period by controlling the influx of fatty acids into mitochondria. Carnitine also facilitates the oxidation of pyruvate and branched chain amino acids, and contributes to the protection of cells from the deleterious actions of acyl CoAs. Carnitine further acts as a secondary antioxidant, favouring fatty acid replacement within previously oxidatively damaged membrane phospholipids. Availability of L-carnitine is essential in the developing fetus for processes underlying fetal maturation. L-carnitine is also essential for development of hepatic ketone synthesis, a central pathway for neonatal energy metabolism. Ketone bodies inhibit the oxidation of both glucose and lactate, sparing these metabolic substrates for biosynthetic functions.

Human skeletal muscle carnitine palmitoyltransferase I activity determined in isolated intact mitochondria

This study was designed to compare the activity of skeletal muscle carnitine palmitoyltransferase I (CPT I) in trained and inactive men (n = 14) and women (n = 12). CPT I activity was measured in intact mitochondria, isolated from needle biopsy vastus lateralis muscle samples ( approximately 60 mg). The variability of CPT I activity determined on two biopsy samples from the same leg on the same day was 4.4, whereas it was 7.0% on two biopsy samples from the same leg on different days. The method was sensitive to the CPT I inhibitor malonyl-CoA (88% inhibition) and therefore specific for CPT I activity. The mean CPT I activity for all 26 subjects was 141.1 +/- 10.6 micromol . min-1 . kg wet muscle (wm)-1 and was not different when all men vs. all women (140.5 +/- 15.7 and 142.2 +/- 14.5 micromol . min-1 . kg wm-1, respectively) were compared. However, CPT I activity was significantly higher in trained vs. inactive subjects for both men (176.2 +/- 21.1 vs. 104.1 +/- 13.6 micromol . min-1 . kg wm-1) and women (167.6 +/- 14.1 vs. 91.2 +/- 9.5 micromol . min-1 . kg wm-1). CPT I activity was also significantly correlated with citrate synthase activity (all subjects, r = 0.76) and maximal oxygen consumption expressed in milliliters per kilogram per minute (all subjects, r = 0.69). The results of this study suggest that CPT I activity can be accurately and reliably measured in intact mitochondria isolated from human muscle biopsy samples. CPT I activity was not affected by gender, and higher activities in aerobically trained subjects appeared to be the result of increased mitochondrial content in both men and women.

The role of carnitine and carnitine supplementation during exercise in man and in individuals with special needs

Carnitine is critical for normal skeletal muscle bioenergetics. Carnitine has a dual role as it is required for long-chain fatty acid oxidation, and also shuttles accumulated acyl groups out of the mitochondria. Muscle requires optimization of both of these metabolic processes during peak exercise performance. Theoretically, carnitine availability may become limiting for either fatty acid oxidation or the removal of acyl-CoAs during exercise. Despite the theoretical basis for carnitine supplementation in otherwise healthy persons to improve exercise performance, clinical data have not demonstrated consistent benefits of carnitine administration. Additionally, most of the anticipated metabolic effects of carnitine supplementation have not been observed in healthy persons. The failure to demonstrate clinical efficacy of carnitine may reflect the complex pharmacokinetics and pharmacodynamics of carnitine supplementation, the challenges of clinical trial design for performance endpoints, or the adequacy of endogenous carnitine content to meet even extreme metabolic demands in the healthy state. In patients with end stage renal disease there is evidence of impaired cellular metabolism, the accumulation of metabolic intermediates and increased carnitine demands to support acylcarnitine production. Years of nutritional changes and dialysis therapy may also lower skeletal muscle carnitine content in these patients. Preliminary data have demonstrated beneficial effects of carnitine supplementation to improve muscle function and exercise capacity in these patients. Peripheral arterial disease (PAD) is also associated with altered muscle metabolic function and endogenous acylcarnitine accumulation. Therapy with either carnitine or propionylcarnitine has been shown to increase claudication-limited exercise capacity in patients with PAD. Further clinical research is needed to define the optimal use of carnitine and acylcarnitines as therapeutic modalities to improve exercise performance in disease states, and any potential benefit in healthy individuals.

The association of acetyl-L-carnitine with glucose and lipid metabolism in human muscle in vivo: the effect of hyperinsulinemia

We examined whether hyperinsulinemia is associated with changes in the amount of L-carnitine and acetyl-L-carnitine in the muscle and whether the source of acetyl-coenzyme A (CoA) (glucose or free fatty acids [FFAs]) influences its further metabolism to acetyl-L-carnitine or through tricarboxylic acid in the skeletal muscle of man in vivo. Twelve healthy men (aged 45 +/- 2 years; body mass index, 25.2 +/- 1.0 kg/m2) were studied using a 4-hour euglycemic-hyperinsulinemic clamp (1.5 mU/kg/min) and indirect calorimetry. Although the mean muscle free L-carnitine and acetyl-L-carnitine concentrations remained unchanged during hyperinsulinemia in the group as a whole, the individual changes in muscle free L-carnitine and acetyl-L-carnitine concentrations were inversely related (r = -.72, P < .02). The basal level of acetyl-L-carnitine was inversely related to the rate of lipid oxidation (r = -.70, P < .02). In a stepwise linear regression analysis, 77% of the variation in the change of acetyl-L-carnitine concentrations was explained by the basal muscle glycogen level (inversely) and nonoxidative glucose disposal rate (directly) during hyperinsulinemia (P < .001); by adding the final FFA concentration (inverse correlation) to the model, 88% of the variation was explained (P < .001). In conclusion, (1) hyperinsulinemia does not enhance skeletal muscle free L-carnitine or acetyl-L-carnitine concentrations in-man, and (2) the acetyl group of acetyl-L-carnitine in human skeletal muscle in vivo is probably mostly derived from glucose and not through beta-oxidation from fatty acids.

Carnitine and its derivatives in cardiovascular disease

Carnitine and its derivative propionyl-L-carnitine are endogenous cofactors which enhance carbohydrate metabolism and reduce the intracellular buildup of toxic metabolites in ischemic conditions. The carnitines have been, and are being used in a spectrum of diseases including multiple cardiovascular conditions. These include angina, acute myocardial infarction, postmyocardial infarction, congestive heart failure, peripheral vascular disease, dyslipidemia, and diabetes. Most published data on carnitine, propionyl-L-carnitine, and other carnitine congeners are favorable but the clinical trials have been relatively small. In currently used doses, these substances are virtually devoid of significant side effects.

Dietary fatty acids influence the activity and metabolic control of mitochondrial carnitine palmitoyltransferase I in rat heart and skeletal muscle

The fatty acid composition of the diet has been found to influence the activity and sensitivity of mitochondrial carnitine palmitoyltransferase I (CPT I; EC 2.3.1.21) to inhibition by malonyl CoA in rat heart and skeletal muscle. The nutritional state of rats has been shown to have less influence on the activity and metabolic control of mitochondrial CPT I in heart and skeletal muscle tissue than in the liver, a tissue in which CPT I activity and sensitivity to inhibition by malonyl CoA can be shown to be regulated acutely under different nutritional conditions. However, because manipulation of the nutritional state in these previous studies was restricted mainly to examining the effect of starvation, this study was undertaken to determine whether, as in liver, the fatty acid content and composition of the diet can regulate the activity and metabolic control of CPT I in heart and skeletal muscle. Rats were fed for up to 10 wk either a nonpurified low fat diet (30 g fat/kg) or a high fat diet (200 g fat/kg) containing one of the following five oil types: hydrogenated coconut oil (HCO), olive oil (OO), safflower oil (SO), evening primrose oil (EPO) or menhaden (fish) oil (MO). Feeding a diet enriched in MO had the most pronounced effect. Rats fed MO had a significantly greater skeletal muscle CPT I specific activity and tissue capacity, and a lower sensitivity of CPT I to malonyl CoA inhibition compared with rats fed a low fat diet, but the duration of feeding required to modulate this sensitivity was longer than that observed previously for the liver enzyme. Progressively greater sensitivity of heart CPT I to malonyl CoA occurred with feeding duration in all groups. These studies indicate that the fatty acid composition of the diet is involved in the regulation of mitochondrial CPT I activity in heart and skeletal muscle.

L-carnitine stimulation of mitochondrial oxidative phosphorylation rate in isolated rat skeletal muscle mitochondria

L-carnitine effects on mitochondrial adenosine triphosphate (ATP) production rate were investigated in mitochondria isolated from rat extensor digitorum longus (EDL) muscle. Kinetic parameters, apparent Michaelis constant for free adenosine diphosphate (ADP) (K(m)) and apparent maximal ATP production rate (Vmax), were determined in absence and presence of L-carnitine. Although the K(m) remained unchanged in response to L-carnitine addition when pyruvate + malate was oxidized, the affinity of isolated mitochondria towards ADP was decreased when pyruvate + palmitoylcarnitine + malate was used as substrate. As a result of L-carnitine addition, a slight non-significant (P < 0.07) increase in Vmax was observed with pyruvate + malate while the increase reached the level of significance with pyruvate + palmitoylcarnitine + malate. Moreover, a positive correlation was obtained when mitochondrial ATP production rate was plotted against the difference between values measured with and without L-carnitine. Our data suggest that addition of L-carnitine to isolated rat skeletal muscle mitochondria can stimulate mitochondrial oxidative phosphorylation rate under conditions where the inhibitory effect of acetyl-CoA accumulation on pyruvate dehydrogenase complex activity is optimized. A change in the control of mitochondrial ATP flux by pyruvate dehydrogenase (PDH) complex might also occur with increasing mitochondrial ATP production rate, reinforcing the L-carnitine effects.

Abnormal carnitine distribution in the muscles of patients with idiopathic inflammatory myopathy

OBJECTIVE

To analyze the levels of free carnitine and carnitine esters in the muscles of patients with inflammatory myopathies.

METHODS

Six men and 7 women with inflammatory myopathy and 25 age-matched healthy controls were studied. Free carnitine and carnitine esters in muscle homogenates were measured by a radiochemical procedure. Muscle histochemical staining and measurement of respiratory chain enzyme activity were also performed.

RESULTS

Eleven patients had muscle carnitine insufficiency. Five of them had subsarcolemmal oxidative accumulations, 5 had lipid droplets, and 4 had defects of the respiratory chain enzyme complexes.

CONCLUSION

Abnormal distribution of muscle carnitine is present in patients with inflammatory myopathies and could impair muscle function. Coexistent mitochondrial dysfunction may contribute to carnitine insufficiency.

Effect of training and nutrition on the development of skeletal muscle

Recent data suggest that an increased muscle mitochondrial oxidative capacity is not a prerequisite for a marked metabolic effect of endurance training, but instead may represent an adaptive phenomenon to the new metabolic situation. The muscle content of the glucose transporter GLUT-4 increases after one or only a few exercise sessions. The ensuring enhancement of the insulin sensitivity allows faster replenishment of muscle glycogen stores following exercise bouts. There is evidence that the muscle content of GLUT-4 declines more slowly with inactivity than the muscle oxidative capacity. This may be a sign that, as in the chronically stimulated rabbit muscle, adaptive changes in human skeletal muscle follow a "first-in, last-out' sequence. There is evidence that muscle (and possibly also plasma) triacylglycerol is more important as an energy source during exercise than was previously recognized. Endurance training increases the content of slow isoforms of myosin in the muscle, and information on changes in muscle shortening velocity and on the molecular regulation of muscle volume is emerging. beta 2-agonists are well documented to enhance muscle mass, whereas creatine supplementation appears to enhance performance during high-intensity exercise.