Francis B. Stephens

New insights concerning the role of carnitine in the regulation of fuel metabolism in skeletal muscle

In skeletal muscle, carnitine plays an essential role in the translocation of long‐chain fatty‐acids into the mitochondrial matrix for subsequent β‐oxidation, and in the regulation of the mitochondrial acetyl‐CoA/CoASH ratio. Interest in these vital metabolic roles of carnitine in skeletal muscle appears to have waned over the past 25 years. However, recent research has shed new light on the importance of carnitine as a regulator of muscle fuel selection. It has been established that muscle free carnitine availability may be limiting to fat oxidation during high intensity submaximal exercise. Furthermore, increasing muscle total carnitine content in resting healthy humans (via insulin‐mediated stimulation of muscle carnitine transport) reduces muscle glycolysis, increases glycogen storage and is accompanied by an apparent increase in fat oxidation. By increasing muscle pyruvate dehydrogenase complex (PDC) activity and acetylcarnitine content at rest, it has also been established that PDC flux and acetyl group availability limits aerobic ATP re‐synthesis at the onset of exercise (the acetyl group deficit). Thus, carnitine plays a vital role in the regulation of muscle fuel metabolism. The demonstration that its availability can be readily manipulated in humans, and impacts on physiological function, will result in renewed business and scientific interest in this compound.

BJSM reviews: A–Z of nutritional supplements: dietary supplements, sports nutrition foods and ergogenic aids for health and performance Part 4

Welcome to Part 4 in our A–Z of nutritional supplement series. One of the enjoyable aspects of reviewing the many supplements in a fairly logical order from A to Z means that, every now and then, a review comprises a real assortment of supplements, and Part 4 is certainly one of them! In Part 4, the review takes us from aspartame, a sweetening ingredient found in many foods and drinks, to BCAA, the three branched chain essential amino acids; then onto bee pollen, marketed as a “superfood,” followed by the ultra trace element boron, known more for its association with bone health, and finally to l-carnitine, a supplement which is widely used in the sporting arena and seems to be gaining in popularity. We are grateful for the excellent contributions from our invited reviewers, which facilitate the provision of access to impartial advice on the value of these individual ingredients and supplements. These contributions are establishing that, for some, the performance evidence is limited or simply does not yet exist. In the January 2010 issue, we will be turning our attention to “buffers,” where we will be covering the key ones used by many athletes, such as beta-alanine and carnosine, together with the two more established compounds of sodium bicarbonate and sodium citrate. ### PJ Rogers Aspartame (C14H18N2O5) is an example of an intense or non-nutritive sweetener and an ingredient of many thousands of drink and food products consumed worldwide. It is a methyl ester of a dipeptide composed of the amino acids aspartic acid and phenylalanine, which are constituents of all protein-containing foods. Aspartame is about 180 times sweeter than sucrose with, for most individuals, minimal bitterness and a good quality of sweet taste. Being composed of amino acids, it has an energy value …

Chronic oral ingestion of l‐carnitine and carbohydrate increases muscle carnitine content and alters muscle fuel metabolism during exercise in humans

Non‐technical summary After 30 years of endeavour, this is the first study to show that muscle carnitine content can be increased in humans by dietary means and, perhaps more importantly, that carnitine plays a dual role in skeletal muscle fuel metabolism that is exercise intensity dependent. Specifically, we have shown that increasing muscle total carnitine content reduces muscle carbohydrate use during low intensity exercise, consistent with an increase in muscle lipid utilisation. However, during high intensity exercise muscle carnitine loading results in a better matching of glycolytic, pyruvate dehydrogenase complex and mitochondrial flux, thereby reducing muscle anaerobic energy generation. Collectively, these metabolic effects resulted in a reduced perception of effort and increased work output during a validated exercise performance test. These findings have significant implications for athletic performance and pathophysiological conditions where fat oxidation is impaired or anaerobic ATP production is increased during exercise.

Insulin stimulates l-carnitine accumulation in human skeletal muscle

Increasing skeletal muscle carnitine content may alleviate the decline in muscle fat oxidation seen during intense exercise. Studies to date, however, have failed to increase muscle carnitine content, in healthy humans, by dietary or intravenous L‐carnitine administration. We hypothesized that insulin could augment Na+‐dependent skeletal muscle carnitine transport. On two randomized visits, eight healthy men underwent 5 h of intravenous L‐carnitine infusion with serum insulin maintained at fasting (7.4±0.4 mIU⋅l−1) or physiologically high (149.2±6.9 mIU⋅l−1) concentrations. The combination of hypercarnitinemia (~500 μmol⋅l−1) and hyperinsulinemia increased muscle total carnitine (TC) content from 22.0 ± 0.9 to 24.7 ± 1.4 mmol⋅(kg dm)−1 (P<0.05) and was associated with a 2.3 ± 0.3‐fold increase in carnitine transporter protein (OCTN2) mRNA expression (P<0.05). Hypercarnitinemia in the presence of a fasting insulin concentration had no effect on either of these parameters. This study demonstrates that insulin can acutely increase muscle TC content in humans during hypercarnitinemia, which is associated with an increase in OCTN2 transcription. These novel findings may be of importance to the regulation of muscle fat oxidation during exercise, particularly in obesity and type 2 diabetes where it is known to be impaired.

Skeletal muscle carnitine loading increases energy expenditure, modulates fuel metabolism gene networks and prevents body fat accumulation in humans

•  Carnitine is a substrate for the carnitine palmitoyltransferase 1 enzyme, a rate‐limiting step in fatty acid oxidation within skeletal muscle. •  Insulin stimulates carnitine transport into skeletal muscle. •  A 20% increase in muscle carnitine content, achieved via 12 weeks of twice daily supplementation of a beverage containing 1.36 g of l‐carnitine and 80 g of carbohydrate (in order to stimulate insulin‐mediated muscle carnitine transport), prevented an 18% increase in body fat mass associated with carbohydrate supplementation alone in healthy young men. •  A novel finding of the present study was that this prevention of fat gain was associated with a greater energy expenditure and fat oxidation during low‐intensity physical activity, and an adaptive increase in expression of gene networks involved in muscle insulin signalling and fatty acid metabolism. •  Implications to health warrant further investigation, particularly in obese individuals who have a reduced reliance on muscle fat oxidation during exercise.

Lipid induced insulin resistance is associated with an impaired skeletal muscle protein synthetic response to amino acid ingestion in healthy young men

The ability to maintain skeletal muscle mass appears to be impaired in insulin-resistant conditions, such as type 2 diabetes, that are characterized by muscle lipid accumulation. The current study investigated the effect of acutely increasing lipid availability on muscle protein synthesis. Seven healthy young male volunteers underwent a 7-h intravenous infusion of l-[ring-2H5]phenylalanine on two randomized occasions combined with 0.9% saline or 10% Intralipid at 100 mL/h. After a 4-h “basal” period, a 21-g bolus of amino acids was administered and a 3-h hyperinsulinemic-euglycemic clamp was commenced (“fed” period). Muscle biopsy specimens were obtained from the vastus lateralis at 1.5, 4, and 7 h. Lipid infusion reduced fed whole-body glucose disposal by 20%. Furthermore, whereas the mixed muscle fractional synthetic rate increased from the basal to the fed period during saline infusion by 2.2-fold, no change occurred during lipid infusion, despite similar circulating insulin and leucine concentrations. This “anabolic resistance” to insulin and amino acids with lipid infusion was associated with a complete suppression of muscle 4E-BP1 phosphorylation. We propose that increased muscle lipid availability may contribute to anabolic resistance in insulin-resistant conditions by impairing translation initiation.

The Role of FOXO and PPAR Transcription Factors in Diet-Mediated Inhibition of PDC Activation and Carbohydrate Oxidation During Exercise in Humans and the Role of Pharmacological Activation of PDC in Overriding These Changes

High-fat feeding inhibits pyruvate dehydrogenase complex (PDC)–controlled carbohydrate (CHO) oxidation, which contributes to muscle insulin resistance. We aimed to reveal molecular changes underpinning this process in resting and exercising humans. We also tested whether pharmacological activation of PDC overrides these diet-induced changes. Healthy males consumed a control diet (CD) and on two further occasions an isocaloric high-fat diet (HFD). After each diet, subjects cycled for 60 min after intravenous infusion with saline (CD and HFD) or dichloroacetate (HFD+DCA). Quadriceps muscle biopsies obtained before and after 10 and 60 min of exercise were used to estimate CHO use, PDC activation, and mRNAs associated with insulin, fat, and CHO signaling. Compared with CD, HFD increased resting pyruvate dehydrogenase kinase 2 (PDK2), PDK4, forkhead box class O transcription factor 1 (FOXO1), and peroxisome proliferator–activated receptor transcription factor α (PPARα) mRNA and reduced PDC activation. Exercise increased PDC activation and whole-body CHO use in HFD, but to a lower extent than in CD. Meanwhile PDK4 and FOXO1, but not PPARα or PDK2, mRNA remained elevated. HFD+DCA activated PDC throughout and restored whole-body CHO use during exercise. FOXO1 appears to play a role in HFD-mediated muscle PDK4 upregulation and inhibition of PDC and CHO oxidation in humans. Also, pharmacological activation of PDC restores HFD-mediated inhibition of CHO oxidation during exercise.

Post-exercise ingestion of a unique, high molecular weight glucose polymer solution improves performance during a subsequent bout of cycling exercise

Abstract The aim of the present study was to determine the effect of post-exercise ingestion of a unique, high molecular weight glucose polymer solution, known to augment gastric emptying and post-exercise muscle glycogen re-synthesis, on performance during a subsequent bout of intense exercise. On three randomized visits, eight healthy men cycled to exhaustion at 73.0% (s = 1.3) maximal oxygen uptake (90 min, s = 15). Immediately after this, participants consumed a one-litre solution containing sugar-free flavoured water (control), 100 g of a low molecular weight glucose polymer or 100 g of a very high molecular weight glucose polymer, and rested on a bed for 2 h. After recovery, a 15-min time-trial was performed on a cycle ergometer, during which work output was determined. Post-exercise ingestion of the very high molecular weight glucose polymer solution resulted in faster and greater increases in blood glucose (P < 0.001) and serum insulin (P < 0.01) concentrations than the low molecular weight glucose polymer solution, and greater work output during the 15-min time-trial (164.1 kJ, s = 21.1) than both the sugar-free flavoured water (137.5 kJ, s = 24.2; P < 0.05) and the low molecular weight glucose polymer (149.4 kJ, s = 21.8; P < 0.05) solutions. These findings could be of practical importance for athletes wishing to optimize performance by facilitating rapid re-synthesis of the muscle glycogen store during recovery following prolonged sub-maximal exercise.

Transient transcriptional events in human skeletal muscle at the outset of concentric resistance exercise training

We sought to ascertain the time course of transcriptional events that occur in human skeletal muscle at the outset of resistance exercise (RE) training in RE naive individuals and determine whether the magnitude of response was associated with exercise-induced muscle damage. Sixteen RE naive men were recruited; eight underwent two sessions of 5 × 30 maximum isokinetic knee extensions (180°/s) separated by 48 h. Muscle biopsies of the vastus lateralis, obtained from different sites, were taken at baseline and 24 h after each exercise bout. Eight individuals acted as nonexercise controls with biopsies obtained at the same time intervals. Transcriptional changes were assessed by microarray and protein levels of heat shock protein (HSP) 27 and αB-crystallin in muscle cross sections by immunohistochemistry as a proxy measure of muscle damage. In control subjects, no probe sets were significantly altered (false discovery rate < 0.05), and HSP27 and αB-crystallin protein remained unchanged throughout the study. In exercised subjects, significant intersubject variability following the initial RE bout was observed in the muscle transcriptome, with greatest changes occurring in subjects with elevated HSP27 and αB-crystallin protein. Following the second bout, the transcriptome response was more consistent, revealing a cohort of probe sets associated with immune activation, the suppression of oxidative metabolism, and ubiquitination, as differentially regulated. The results reveal that the initial transcriptional response to RE is variable in RE naive volunteers, potentially associated with muscle damage and unlikely to reflect longer term adaptations to RE training. These results highlight the importance of considering multiple time points when determining the transcriptional response to RE and associated physiological adaptation.

Effect of whey protein- and carbohydrate-enriched diet on glycogen resynthesis during the first 48 h after a soccer game

The effect of a whey protein‐ and carbohydrate (CHO)‐enriched diet on the rate of muscle glycogen resynthesis after a soccer match was examined. Sixteen elite soccer players were randomly assigned to a group ingesting a diet rich in carbohydrates and whey protein [CHO, protein, and fat content was 71, 21, and 8E%, respectively; high content of carbohydrates and whey protein (HCP), n = 9] or a group ingesting a normal diet (55, 18, and 26E%; control [CON], n = 7) during a 48‐h recovery period after a soccer match. CON and three additional players carried out a 90‐ and 60‐min simulated match without body contacts (SIM90 and SIM60). Muscle glycogen was lowered (P < 0.05) by 54, 48, 53, and 38% after the matches in CON, HCP, SIM90, and SIM60, respectively. Glycogen resynthesis during the first 48 h after the match was not different between CON and HCP, whereas glycogen resynthesis was slower (P < 0.05) during the first 24 h after SIM60 than SIM90 (2.88 ± 0.84 vs 4.32 ± 0.54 mmol/kg dw/h). In HCP, glycogen content in type II muscle fibers was still lowered 48 h after the match. In conclusion, glycogen resynthesis 48 h after a soccer match is not elevated by ingestion of a HCP diet. Furthermore, glycogen resynthesis does not appear to be impaired by body contacts during a match.