ICURT PROCEEDINGS
Carnitine in Maintenance Hemodialysis Patients Gianfranco Guarnieri, MD Carnitine is a conditionally essential metabolite that plays a critical role in cell physiology. Carnitine is necessary for fatty acid transport to sites of beta-oxidation in the mitochondria, where it also helps to prevent organic acid accumulation. Because of these key regulatory functions, carnitine represents a crucial determinant of mitochondrial energy metabolism, whose deficiency may lead to metabolic and clinical disturbances. Loss of carnitine through dialytic membranes occurs in maintenance hemodialysis, resulting in potential carnitine depletion and relative increments of esterified carnitine forms. Carnitine supplementation has been reported to counteract some of these alterations and has been associated with some clinical benefits, such as enhanced response to erythropoietin as well as improvement in exercise tolerance, intradialytic symptom, hyperparathyroidism, insulin resistance, inflammatory and oxidant status, protein balance, lipid profile, cardiac function, and quality of life. Carnitine supplementation has an attractive theoretical rationale; however, there are no definitive supportive studies and conclusive evidence that L-carnitine supplementation in maintenance hemodialysis patients could improve these conditions. A trial of carnitine administration could be attempted for 6 to 12 months only in selected patients on dialysis who do not adequately respond to standard therapies, in the presence of symptomatology, and in conjunction with patient dialysis age and documented L-carnitine deficiency. Ó 2015 by the National Kidney Foundation, Inc. All rights reserved.
Carnitine Metabolism in the Normal Individual
C
ARNITINE IS A water soluble zwitterionic quaternary amine (beta-hydroxy-gamma trimethylaminobutyric acid; molecular weight, 161.2 Da), biologically active only in the ‘‘L’’ isoform, which is a natural constituent of cells of animal origin (from prokaryotic to eukaryotic ones).1-10 L-Carnitine is involved in the transesterification reactions, catalyzed by a number of carnitine acyltransferase enzymes, which allow the reversible transfer of fatty acids from coenzyme A (CoA) to the carnitine hydroxyl: Carnitine1Acyl-CoA%Acylcarnitine1CoA
By these reactions, carnitine can exert its roles in the regulation of several metabolic pathways, in the maintenance of a normal structure of cell membrane phospholipids (as has been shown in erythrocytes), and in the removal of excess acyl moieties. Disease states characterized by inborn errors of carnitine metabolism were first described in 1973. Normal carnitine metabolism is illustrated in Figure 1. Skeletal and heart muscles cannot synthesize carnitine as they lack the final enzyme butyrobetaine hydroxylase. Therefore, these tissues are entirely dependent on carnitine uptake from the blood. Department of Internal Medicine, Cattinara General Hospital, University of Trieste, Trieste, Italy. Financial Disclosure: The author declares that he has no relevant financial interests. Address correspondence to Professor Gianfranco Guarnieri, MD, Salita Cedassamare, 25/3, Trieste 34136, Italy. E-mail: [email protected] Ó 2015 by the National Kidney Foundation, Inc. All rights reserved. 1051-2276/$36.00 http://dx.doi.org/10.1053/j.jrn.2014.10.025
Journal of Renal Nutrition, Vol -, No - (-), 2015: pp 1-7
Normally, about 60% to 75% of the carnitine present in a mixed diet is rapidly absorbed from the intestinal lumen across the mucosal membrane by passive and active transport mechanisms. However, in the case of oral carnitine supplementation, the bioavailability of this compound may decrease to 15% or less. Plasma levels of free and esterified carnitine are mainly regulated by the kinetics of their reabsorption in the kidney. The renal clearance of acylcarnitines is 4 to 8 times higher than that of free carnitine because of the preferential tubular reabsorption of free carnitine; therefore, the acyl-to-free carnitine ratio in the urine of healthy subjects is around 1, whereas the same ratio in plasma is about 0.2 to 0.3.
The Physiological Role of Carnitine The carnitine system is involved in many physiological functions1-10 such as: 1. Transport of long-chain fatty acids into the mitochondrial matrix for beta-oxidation to provide cellular energy. A most relevant physiological role of carnitine is to regulate fatty acid metabolism (Fig. 2). In particular, carnitine is a crucial regulator of fatty acid transport into the mitochondria, a ratelimiting step in their beta-oxidation. Carnitine is therefore a key regulator of mitochondrial energy metabolism and adenosine triphosphate production. 2. Transport of partially oxidized fatty acid from peroxisomes to mitochondria 3. Modulation of the intramitochondrial acyl-CoAto-free CoA ratio and increased glucose oxidative metabolism. Increased free CoA and reduced intramitochondrial acyl moieties result in increased free CoA-to-acetyl-CoA ratio (‘‘buffering of the acylto-free CoA ratio’’). A related relevant carnitine 1
2
GUARNIERI Figure 1. Carnitine metabolism in normal patients. Car, carnitine; Fe, iron; Lys, Lysine; met, methionine; RBC, red blood cell; vit, vitamin.
effect is represented by modulation of tissue levels of acyl-CoA compounds, which in turn may affect activities of enzymes (e.g., pyruvate dehydrogenase [PDH] complex) involved in the regulation of master metabolic pathways. Importantly, enhanced PDH activity could result in increased glucose oxidative metabolism (oxidative decarboxylation of pyruvate to acetyl-CoA) and enhanced insulinmediated glucose use. The increased PDH activity as also reflected by commonly reported decrements of both lactate and pyruvate, with resulting reduction of their ratio, after carnitine supplementation in vivo. Excess acyl-CoA levels may also result in inhibition of adenine nucleotide translocase (linking mitochondrial energy production with the sarcomeric adenosine triphosphatase contraction mechanism), increased mitochondrial permeability, which
may initiate apoptosis, and impaired insulin action through decreased GLUT4 receptor activity. Modulation of fatty acids and/or acyl-CoA’s availability by carnitine and carnitine acyltransferases may further influence transcriptional regulation of a wide variety of genes related to the control of oxidative metabolism, adipogenesis, glycolysis, and gluconeogenesis. 4. Export from the mitochondria of the branched chain acyl groups. 5. Detoxifying action. Among other functions, the carnitine system contributes to the trapping and excretion of unphysiological acyl groups (such as valproic acid) and acyl-CoA compounds with a detoxifying effect. 6. Storage and transport of energy between different organs. Figure 2. Metabolic functions of carnitine. ATP, adenosine triphosphate; PDH, pyruvate dehydrogenase.
CARNITINE IN HEMODIALYSIS PATIENTS
3
Figure 3. Potential causes and clinical consequences of carnitine deficiency or insufficiency (relative lack of free carnitine compared to metabolic needs) in MHD patients.
7. Regulation of membrane phospholipids. 8. Other functions: Carnitine has also osmolyte functions which might be relevant in the protection of cells and tissues from osmotic stress. In the endoplasmic reticulum, carnitine is involved in the acylation-deacylation of lipoproteins such as very low density lipoproteins (VLDLs) or of phospholipids. Carnitine might have a regulatory role on cytokine production and functions and be potentially useful in systemic inflammatory conditions. Experimental evidence also suggests that carnitine may improve sperm counts and sperm motility. Carnitine can be detected at the nuclear level, where it could influence gene transcription by modulating acyl-CoA content.
Carnitine Deficiency and Insufficiency
Carnitine ‘‘deficiency’’1-10 indicates conditions in which carnitine concentrations in plasma or tissues are below the requirements for the normal functions of the organism, with values commonly lower than 25 mmol/L for total serum carnitine or ,10 mmol/g of noncollagen protein
for muscle total carnitine. Carnitine deficiencies can be primary (systemic and myopathic) or secondary. Primary carnitine deficiencies are the consequence of alterations in the intracellular transport of carnitine. Secondary deficiency can be the consequence of genetically determined metabolic errors involving excessive production of short-chain organic acids or of acquired conditions. These may involve a decreased biosynthesis (liver cirrhosis, chronic renal disease, extreme prematurity, and so forth), a decreased intake (malnutrition, vegetarian diets, and so forth), increased requirements (pregnancy, puerperium, severe trauma or infections, burns, and so forth), or increased losses (Fanconi syndrome, renal tubular acidosis, hypercatabolism, Fanconi syndrome). Iatrogenic conditions of carnitine deficiency or insufficiency include hemodialysis and administration of drugs such as valproic acid, pivalic acid derivatives, and zidovudine. Carnitine insufficiency (Fig. 3) indicates secondary acquired conditions in which there is a relative lack of carnitine compared to an increased metabolic need. In conditions of carnitine insufficiency, an excess of acyl groups, arising from either a decreased b-oxidation or an increased fatty acid administration or production, overwhelms the
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GUARNIERI
availability of free CoA. Consequently, the acyl-to-free carnitine ratio increases because a larger amount of free carnitine is esterified to acylcarnitines to ‘‘buffer’’ the excess of acyl groups. In these situations, carnitine supplements may contribute to regenerate the sequestrated free CoA, thereby maintaining normal metabolic processes. The principal potential consequences of carnitine deficiency or insufficiency are the following: (1) accumulation in the cells of toxic acyl moieties that may have a detergent-like action; (2) accumulation of acetyl-CoA in the mitochondria, which has been shown to inhibit several enzymes (refer the previous sections); (3) impaired export of the excess of organic acids; (4) reduced efficiency of mitochondrial energy production from lipids and glucose; and (5) impaired amino acid metabolism (mainly of the branched chain) and, possibly, worsening of nitrogen balance.
Carnitine in Uremia and Hemodialysis Alterations in carnitine metabolism and in carnitine plasma and tissue levels differ profoundly in different stages of renal disease.1-14 In patients with chronic renal failure on conservative treatment, plasma concentration of acylcarnitines is enhanced by reduced glomerular filtration rate (GFR). Although all carnitine fractions increase, altered proportion of acyl-to-free carnitine also occurs, with relative increment of acylcarnitines. Because of inadequate energy intake, acylcarnitines may further increase, possibly because of the production of ketone bodies. In agreement with this observation, we previously observed a blunted ability in handling exogenously administered acetate in chronically uremic patients.1 Plasma levels of L-carnitine decline by approximately 60 to 70% during the course of a single dialysis session. After 12 months of hemodialysis, endogenous plasma and muscle L-carnitine levels are approximately half of those measured before the initiation of hemodialysis treatment. Acylcarnitine levels are significantly higher in chronic hemodialysis patients, particularly in patients who have been receiving dialysis treatment for at least 12 months. In fact, acylcarnitine levels make up approximately 50% of the total plasma carnitine pool in these patients (of which only half is accounted for by acetyl L-carnitine); this compares with 15% in healthy subjects (consisting primarily of acetyl L-carnitine and negligible levels of the nonacetyl acylcarnitines). Importantly, free carnitine concentrations are also reported to be reduced in skeletal muscles, and this decline appears to be proportional to the dialysis duration.11 Accumulation of the long-chain esters has been noted. A significant inverse correlation between carbon chain length of the acyl group and dialytic removal of the acylcarnitines has been observed, with a notable reduction in the removal of medium- and long-chain esters. Omission of a single dosage of supplemental carnitine in long-term administration schemes results in dramatic decrease and reprofiling of carnitine esters even after the usual 44 hours of interdialytic
period.13,14 Intradialytic loss of carnitine is not the only potential cause of carnitine depletion in hemodialysis patients as additional causes may include reduced carnitine intake or intestinal absorption as well as impaired carnitine de novo renal synthesis. In summary, patients on maintenance hemodialysis (MHD) treatment have low free carnitine plasma concentrations, with increased acylcarnitine-to-carnitine ratio, possibly because of preferential removal of free carnitine during dialysis sessions. The aforementioned alterations have important potential adverse metabolic effects, especially in skeletal muscle. Such metabolic disturbances might contribute to relevant clinical abnormalities often observed in MHD patients (Fig. 3), including muscle weakness, myopathy, and cardiomyopathy; loss of body protein and cachexia; insulin resistance (IR); altered plasma lipid profile; anemia and resistance to erythropoietin; and intradialytic disturbances and symptoms such as muscle cramps, hypotension, and cardiac arrhythmias.1-10
Carnitine Supplementation to Hemodialysis Patients Available biochemical and clinical information provides a rationale for carnitine supplementation to hemodialysis patients. Indeed, several effects of carnitine supplementation have been reported in hemodialysis patients: hypertrophy of type I muscle fiber and increased arm circumference, improvement of dyslipidemia, glucose and protein metabolism, nutritional state and nitrogen retention, skeletal muscle (intradialytic cramps, fatigue) and heart (intradialytic arrhythmias, hypotension) function, oxidative stress and inflammation, anemia and erythropoietin (EPO) sensitivity, exercise capacity and maximum oxygen consumption, sense of well-being, physical endurance, and so forth.1-10 Recently, the issue of carnitine, intestinal microbiota, and cardiovascular disease (CVD) has been raised.15,16 Chronic dietary L-carnitine supplementation in mice significantly altered cecal microbial composition, markedly enhanced synthesis of trimethylamine-N-oxide (TMAO), and increased atherosclerosis but not after suppression of intestinal microbiota. It was concluded that intestinal microbiota may thus participate in the well-established link between increased red meat consumption and CVD risk. Increased TMAO levels were associated with an increased risk of incident major adverse cardiovascular events during the 3 years of follow-up in 4,007 patients undergoing elective coronary angiography. McCarty17 pointed out that the values of TMAO achieved in the TMAO-fed mice were at least an order of magnitude higher than those occurring in humans. The TMAO could simply be functioning as a marker for habitual ingestion of foods rich in carnitine and choline, which include animal products such as red meat and eggs, known to be directly linked to increased vascular risk. Besides, human exposure to TMAO will be approximately an order of magnitude higher from ingesting a given weight
CARNITINE IN HEMODIALYSIS PATIENTS
of fish than of beef; yet, as is well known, fish-rich diets are far more compatible with vascular health than those high in red meats. And finally, supplemental carnitine slowed or stopped the progression of atherosclerotic lesions in rabbits. Indeed, carnitine administration has been reported to be beneficial in patients with angina, ischemia-induced cardiac insufficiency, cardiogenic shock, cardiomyopathy, and myocardial infarction.18 Plasma levels of adipokines and related hormones are greatly elevated in patients on regular hemodialysis. L-carnitine administration further augmented the plasma levels of protective adiponectin; therefore, it may have a role in preventing cardiovascular complications of uremia.19 Compared with placebo or control, L-carnitine is associated with a 27% reduction in all-cause mortality, a 65% reduction in ventricular arrhythmias, and a 40% reduction in anginal symptoms in patients experiencing an acute myocardial infarction.20 L-carnitine is essential for cardiac mitochondria to attenuate the membrane permeability transition, and to maintain the ultrastructure and membrane stabilization, in the presence of high fatty acid b-oxidation. Consequently, the cells may be protected against apoptosis by L-carnitine through inhibition of the fatty acid–induced cytochrome c release. On release of cytochrome c to the cytoplasm, the protein binds apoptotic protease-activating factor 1.21 Advanced glycation end products (AGEs) contribute to CVD in patients with hemodialysis. Oral L-carnitine supplementation significantly decreased skin AGE levels in hemodialysis patients with carnitine deficiency. Carnitine is reported to inhibit the formation of AGEs in vitro, and the studies by Adachi et al.22 and Fukami et al.23 suggest that supplementation of carnitine may be a therapeutic target for preventing the accumulation of tissue AGEs and subsequently reducing the risk of CVD in hemodialysis patients. L-carnitine supplementation was associated with protein-sparing effects in MHD patients.7 Insulinmediated glucose disappearance was improved by L-carnitine only in those patients with greater baseline IR. IR, which is highly prevalent in patients receiving chronic hemodialysis, has been proposed to play a critical role in the development of sarcopenia.24 The severity of IR was associated a lesser decline in plasma leucine concentrations, suggesting a similar resistance to protein anabolism. Evidence from both animal and clinical studies exists that L-carnitine supplementation causes an improved nitrogen balance either via increased protein synthesis or reduced protein degradation, an inhibition of apoptosis, and an abrogation of inflammatory processes under pathologic conditions.25 Carnitine decreased the transcript levels of several genes involved in the ubiquitin proteasome system (UPS) in skeletal muscle and liver of piglets.26 These data suggest that the inhibitory effect of carnitine on the expression of genes of the UPS is mediated indirectly, probably via modulating the release of inhibitors of the UPS such as
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insulin-like growth factor 1. The inhibitory effect of carnitine on the expression of genes of the UPS might explain, at least partially, the increased protein accretion in piglets supplemented with carnitine. The same authors reported that supplementation of carnitine markedly decreases the concentrations of ubiquitinated proteins in skeletal muscle of rats, indicating a diminished degradation of myofibrillar proteins by the UPS.27 Both lipid and glucose oxidation funnel into acetylcarnitine (Fig. 2). The mitochondrial enzyme carnitine acetylCoA transferase (CrAT) converts acetyl-CoA to the membrane-permeable acetylcarnitine and permits mitochondrial efflux of excess acetyl-CoA that otherwise could inhibit PDH.28,29 Carnitine supplementation improves glucose homeostasis in insulin-resistant humans. The authors identified an essential role for the mitochondrial matrix enzyme, CrAT, in regulating substrate switching and glucose tolerance. By converting acetyl-CoA to its membrane permeant acetylcarnitine ester, CrAT regulates mitochondrial and intracellular carbon trafficking. Reduced muscle mitochondrial function occurs in chronic uremia and it could cause insulin resistance.9,21 As a consequence, mitochondrial dysfunction contributes to the pathophysiology of insulin resistance, obesity, diabetes, vascular disease, and chronic heart failure. Lcarnitine is intrinsically involved in mitochondrial metabolism and function as it plays a key role in fatty acid oxidation and energy metabolism. In conclusion, carnitine could positively affect glucose metabolism by10,30-32: - Relieving the accumulation of unmetabolized and potentially toxic acyl derivatives from the mitochondria (scavenger, detoxifying effect) - Reducing the acyl-to-free CoA ratio and increasing the activity of PDH complex activity (buffering effect). By converting acetyl-CoA to its membrane permeant acetylcarnitine ester, CrAT regulates mitochondrial acyl-CoA–CoA balance and PDH activity: YAc(et)yl-CoA/CoA / [PDH activity / [Glycolytic flux / [Glucose metabolism.
- Improving the mitochondrial function through a reduction of myocellular lipid content (‘‘lipotoxicity’’) and a modulation of the systemic inflammation and the oxidative stress. Strong evidence has been provided, at least from animal studies, that L-carnitine supplementation prevents oxidative stress and ameliorates mitochondrial function, whereas results from a very low number of available clinical studies in this regard are inconclusive.25 Several studies reported in MHD patients supplemented with carnitine an improvement of anemia and of the response to erythropoietin (recombinant human EPO).
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In 1999, the US Food and Drug Administration approved L-carnitine for use in dialysis-related carnitine deficiency, as defined by low L-carnitine levels. The American Centers for Medicare and Medicaid Services issued in 2002 and 2006 a national coverage determination for intravenous L-carnitine for treatment of EPO-resistant anemia and dialysis hypotension. A recent meta-analysis 33 failed to confirm the previous findings regarding the effects of Lcarnitine on hemoglobin and the EPO dose but showed that L-carnitine significantly decreased serum C-reactive protein levels. The extent of the decrease was both statistically and clinically relevant. However, the relevance of decrease in C-reactive protein level with hard end points such as all-cause mortality and cardiovascular complications still remains to be clarified. In addition, Mercadal et al.34 reported that the treatment of incident hemodialysis patients with L-carnitine did not improve their response to recombinant human EPO. These results can be generalized only to unselected hemodialysis patients new to hemodialysis. They cannot be extended to long-term dialysis patients with more profound carnitine deficiency. Very negative conclusions were reported in a Cochrane review by Badve et al.35: ‘‘Titles and abstracts of 521 records were screened, of which we reviewed 99 from the full text. Only two studies matched our inclusion criteria. Because interventions differed, data could not be combined for quantitative meta-analysis. There was inadequate evidence identified to inform recommendation of any intervention to ameliorate ESA hyporesponsiveness. Adequately powered randomised controlled trials (RCTs) are required to establish the safety and efficacy of interventions to improve responsiveness to ESA therapy.’’ According to Yee et al.,36 L-carnitine should be used to treat anemia only in dialysis patients with documented L-carnitine deficiency in whom all other conventional measures to raise hemoglobin level have been attempted and failed. In a systematic review and meta-analysis by Huang et al.,37 no effect of L-carnitine was found on serum total cholesterol, high density lipoproteins-cholesterol, VLDLcholesterol, and serum triglycerides. By contrast, that meta-analysis suggested a promising effect of L-carnitine on low density lipoproteins-cholesterol, but further largescale, well-designed, randomized, controlled, trials were suggested. In a review article, Cal o et al.38 concluded that heterogeneous clinical response to carnitine therapy in dialysis patients, reported by other studies, and the lack of largescale randomized trials are the rationale for the reluctance regarding a widespread use of carnitine supplements in dialysis patients. According to Wasserstein,39 in spite of an attractive theoretical rationale of carnitine supplementation and some suggestive but not definitive supportive studies, the expense of L-carnitine in the absence of definitive studies is hard to justify.
In conclusion, carnitine supplementation in MHD patients is in a deadlock because of its great importance in human physiology and of the inconclusive placebo-controlled randomized trials documenting benefits for a wide variety of clinical conditions.40 Differences in experimental design, sample size, patient selection, and carnitine dosage could account for these discrepancies. In particular, carnitine supplementation could be more beneficial for selected patient subgroups with more profound carnitine depletion such as older patients, female patients, and patients with longer disease duration. Responders and nonresponders have been reported in some studies. Given the safety profile of Lcarnitine and the high incidence of carnitine deficiency in hemodialysis patients, it could be reasonable to try to treat the patients in the presence of symptomatology and in conjunction with patient dialysis age. The response to therapy should be monitored at 3-month intervals and discontinued after 9 to 12 months if the patients do not improve.41
References 1. Guarnieri G, Toigo G, Crapesi L, et al. Carnitine metabolism in chronic renal failure. Kidney Int. 1987;32:S116-S127. 2. Di Lisa F, Barbato R, Menab o R, Siliprandi N. Carnitine and carnitine esters in mitochondial metabolism and function. In: De Jong J, Ferrari R, eds. The Carnitine System: A New Therapeutical Approach to Cardiovascular Diseases. Dordrecht, The Netherlands: Kluwer Academic Pub; 1995:21-38. 3. Rebouche CH. Carnitine. In: Shills ME, Olson JA, Shike M, eds. Modern Nutrition in Health and Disease. 9th ed. Philadelphia: Lea & Fabiger; 2000:505-512. 4. Guarnieri G, Situlin R, Biolo G. Carnitine metabolism in uremia. Am J Kidney Dis. 2001;38:S63-S67. 5. Guarnieri G, Biolo G, Toigo G, Situlin R. Carnitine in renal failure. In: Kopple J, Massry S, eds. Nutritional Management of Renal Disease. Philadelphia, PA: Lippincott Williams & Wilkins; 2001. 6. Guarnieri G, Biolo G, Vinci P, Massolino B, Barazzoni R. Advances in carnitine in chronic uremia. J Ren Nutr. 2007;17:23-29. 7. Biolo G, Stulle M, Bianco F, et al. Insulin action on glucose and protein metabolism during L-carnitine supplementation in maintenance haemodialysis patients. Nephrol Dial Transplant. 2008;23:991-997. 8. Reuter SE, Evans AM. Carnitine and acylcarnitines pharmacokinetic, pharmacological and clinical aspects. Clin Pharmacokinet. 2012;51:553-572. 9. Marcovina SM, Sirtori C, Peracino A, et al. Translating the basic knowledge of mitochondrial functions to metabolic therapy: role of L-carnitine. Transl Res. 2013;161:73-84. 10. Malaguarnera G, Giordano M, Malaguarnera M. L-Carnitine in Patients with Diabetes. In: Watson RR, Preedy VR, eds. Bioactive Food as Dietary Interventions for Diabetes. chapter 34. San Diego, CA, London, UK: Academic Press-Elsevier; 2013. 11. Evans AM, Faull RJ, Nation RL, et al. Impact of hemodialysis on endogenous plasma and muscle carnitine levels in patients with end-stage renal disease. Kidney Int. 2004;66:1527-1534. 12. Reuter ES, Evans AM. What is the value of L-carnitine level determination for carnitine supplementation in hemodialysis patients? Hemodial Int. 2013;17:335-338. 13. Bene J, Csiky B, Wittmann I, Sulyok E, Melegh B. Dramatic decrease of carnitine esters after interruption of exogenous carnitine supply in hemodialysis patients. Ren Fail. 2012;34:555-558. 14. Csiky B, Bene J, Wittmann I, Sulyok E, Melegh B. Effect of hemodialysis session on the dynamics of carnitine ester profile changes in L-carnitine pretreated end-stage renal disease patients. Int Urol Nephrol. 2013;45:847-855.
CARNITINE IN HEMODIALYSIS PATIENTS 15. Koeth RA, Wang Z, Levison BS, et al. Intestinal microbiota metabolism of L-carnitine, a nutrient in red meat, promotes atherosclerosis. Nat Med. 2013;19:576-585. 16. Wilson Tang WH, Wang Z, Levison BS, et al. Intestinal microbial metabolism of phosphatidylcholine and cardiovascular risk. N Engl J Med. 2013;368:17. 17. McCarty MF. L-carnitine consumption, its metabolism by intestinal microbiota, and cardiovascular health. Mayo Clin Proc. 2013;88:78i-789. 18. Kudoh Y, Aoyama S, Torii T, et al. Hemodynamic stabilizing effects of L-carnitine in chronic hemodialysis patients. Cardiorenal Med. 2013;3: 200-207. 19. Csiky B, Nyul Z, T oth G, et al. L-carnitine supplementation and adipokines in patients with end-stage renal disease on regular hemodialysis. Exp Clin Endocrinol Diabetes. 2010;118:735-740. 20. DiNicolantonio JJ, Lavie CJ, Fareset H, Menezes AR, O’Keefe JH. L-carnitine in the secondary prevention of cardiovascular disease: systematic review and meta-analysis. Mayo Clin Proc. 2013;88:544-551. 21. Oyanagi E, Yano H, Uchida M, Utsumi K, Sasaki J. Protective action of L-carnitine on cardiac mitochondrial function and structure against fatty acid stress. Biochem Biophys Res Commun. 2011;412:61-67. 22. Adachi T, Fukami K, Yamagishi S, et al. Decreased serum carnitine is independently correlated with increased tissue accumulation levels of advanced glycation end products in haemodialysis patients. Nephrology. 2012;17:689-694. 23. Fukami K, Yamagishi S, Sakai K, et al. Potential inhibitory effects of L-carnitine supplementation on tissue advanced glycation end products in patients with hemodialysis. Rejuvenation Res. 2013;16:460-466. 24. Deger SM, Egbert P, Ellis CD, Sha F, Ikizler TA, Hung AM. Insulin resistance and protein metabolism in chronic hemodialysis patients. J Ren Nutr. 2013;23:e59-e66. 25. Ringseis R, Keller J, Eder K. Mechanisms underlying the antiwasting effect of L-carnitine supplementation under pathologic conditions: evidence from experimental and clinical studies. Eur J Nutr. 2013;52: 1421-1442. 26. Keller J, Ringseis R, Koc A, Lukas I, Kluge H, Eder K. Supplementation with L-carnitine downregulates genes of the ubiquitin proteasome system in the skeletal muscle and liver of piglets. Animal. 2012;6:70-78. 27. Keller J, Couturier A, Haferkamp M, Most E, Eder K. Supplementation of carnitine leads to an activation of the IGF-1/PI3K/Akt signalling pathway and down regulates the E3 ligase MuRF1 in skeletal muscle of rats. Nutr Metab. 2013;10:28.
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28. Schooneman MG, Vaz FM, Houten SM, Soeters MR. Acylcarnitines. Reflecting or inflicting insulin resistance? Diabetes. 2013;62:1-8. 29. Muoio DM, Noland RC, Kovalik JP, et al. Muscle-specific deletion of carnitine acetyltransferase compromises glucose tolerance and metabolic flexibility. Cell Metab. 2012;15:764-777. 30. Galloway SDR, Craig TP, Clelandet SJ. Effects of oral L-carnitine supplementation on insulin sensitivity indices in response to glucose feeding in lean and overweight/obese males. Amino Acids. 2011;41:507-515. 31. Ringseis R, Keller J, Eder K. Role of carnitine in the regulation of glucose homeostasis and insulin sensitivity: evidence from in vivo and in vitro studies with carnitine supplementation and carnitine deficiency. Eur J Nutr. 2012;51:1-18. 32. Vidal-Casariego A, Burgos-Pelaez R, Martınez-Faedo C, et al. Metabolic effects of L-carnitine on type 2 diabetes mellitus: systematic review and meta-analysis. Exp Clin Endocrinol Diabetes. 2013;121:234-238. 33. Chen Y, Abbate M, Tang L, et al. L-Carnitine supplementation for adults with end-stage kidney disease requiring maintenance hemodialysis: a systematic review and meta-analysis. Am J Clin Nutr. 2014;99:408-422. 34. Mercadal L, Coudert M, Vassault A, et al. L-carnitine treatment in incident hemodialysis patients: the multicenter, randomized, doubleblinded, placebo-controlled CARNIDIAL trial. Clin J Am Soc Nephrol. 2012;7:1836-1842. 35. Badve SV, Beller EM, Cass A, et al. Interventions for erythropoietinresistant anaemia in dialysis patients. Cochrane Database Syst Rev. 2013: CD006861. 36. Yee J. L-carnitine for anemia in hemodialysis patients: a last resort. Clin J Am Soc Nephrol. 2012;7:1746-1748. 37. Huang H, Song L, Zhang H, Zhang H, Zhang J, Zhao W. Influence of L-carnitine supplementation on serum lipid profile in hemodialysis patients: a systematic review and meta-analysis. Kidney Blood Press Res. 2014;38:31-41. 38. Cal o LA, Vertolli U, Davis PA, Savica V. L carnitine in hemodialysis patients. Hemodial Int. 2012;16:428-434. 39. Wasserstein AG. L-carnitine supplementation in dialysis: treatment in quest of disease. Semin Dial. 2013;26:11-15. 40. Bloomer RJ, Farney TM, McAllister MJ. An Overview of Carnitine. In: Bagchi D, Nair S, Sen CK, eds. Nutrition and Enhanced Sports Performance, Chapter 41. San Diego, CA, London, UK: Academic Press-Elsevier; 2013:405-413. 41. Molyneux R, Seymour AM, Bhandari S. Value of carnitine therapy in kidney dialysis patients and effects on cardiac function from human and animal studies. Curr Drug Targets. 2012;13:285-293.
Carnitine in Maintenance Hemodialysis Patients Gianfranco Guarnieri, MD Carnitine is a conditionally essential metabolite that plays a critical role in cell physiology. Carnitine is necessary for fatty acid transport to sites of beta-oxidation in the mitochondria, where it also helps to prevent organic acid accumulation. Because of these key regulatory functions, carnitine represents a crucial determinant of mitochondrial energy metabolism, whose deficiency may lead to metabolic and clinical disturbances. Loss of carnitine through dialytic membranes occurs in maintenance hemodialysis, resulting in potential carnitine depletion and relative increments of esterified carnitine forms. Carnitine supplementation has been reported to counteract some of these alterations and has been associated with some clinical benefits, such as enhanced response to erythropoietin as well as improvement in exercise tolerance, intradialytic symptom, hyperparathyroidism, insulin resistance, inflammatory and oxidant status, protein balance, lipid profile, cardiac function, and quality of life. Carnitine supplementation has an attractive theoretical rationale; however, there are no definitive supportive studies and conclusive evidence that L-carnitine supplementation in maintenance hemodialysis patients could improve these conditions. A trial of carnitine administration could be attempted for 6 to 12 months only in selected patients on dialysis who do not adequately respond to standard therapies, in the presence of symptomatology, and in conjunction with patient dialysis age and documented L-carnitine deficiency. Ó 2015 by the National Kidney Foundation, Inc. All rights reserved.
Carnitine Metabolism in the Normal Individual
C
ARNITINE IS A water soluble zwitterionic quaternary amine (beta-hydroxy-gamma trimethylaminobutyric acid; molecular weight, 161.2 Da), biologically active only in the ‘‘L’’ isoform, which is a natural constituent of cells of animal origin (from prokaryotic to eukaryotic ones).1-10 L-Carnitine is involved in the transesterification reactions, catalyzed by a number of carnitine acyltransferase enzymes, which allow the reversible transfer of fatty acids from coenzyme A (CoA) to the carnitine hydroxyl: Carnitine1Acyl-CoA%Acylcarnitine1CoA
By these reactions, carnitine can exert its roles in the regulation of several metabolic pathways, in the maintenance of a normal structure of cell membrane phospholipids (as has been shown in erythrocytes), and in the removal of excess acyl moieties. Disease states characterized by inborn errors of carnitine metabolism were first described in 1973. Normal carnitine metabolism is illustrated in Figure 1. Skeletal and heart muscles cannot synthesize carnitine as they lack the final enzyme butyrobetaine hydroxylase. Therefore, these tissues are entirely dependent on carnitine uptake from the blood. Department of Internal Medicine, Cattinara General Hospital, University of Trieste, Trieste, Italy. Financial Disclosure: The author declares that he has no relevant financial interests. Address correspondence to Professor Gianfranco Guarnieri, MD, Salita Cedassamare, 25/3, Trieste 34136, Italy. E-mail: [email protected] Ó 2015 by the National Kidney Foundation, Inc. All rights reserved. 1051-2276/$36.00 http://dx.doi.org/10.1053/j.jrn.2014.10.025
Journal of Renal Nutrition, Vol -, No - (-), 2015: pp 1-7
Normally, about 60% to 75% of the carnitine present in a mixed diet is rapidly absorbed from the intestinal lumen across the mucosal membrane by passive and active transport mechanisms. However, in the case of oral carnitine supplementation, the bioavailability of this compound may decrease to 15% or less. Plasma levels of free and esterified carnitine are mainly regulated by the kinetics of their reabsorption in the kidney. The renal clearance of acylcarnitines is 4 to 8 times higher than that of free carnitine because of the preferential tubular reabsorption of free carnitine; therefore, the acyl-to-free carnitine ratio in the urine of healthy subjects is around 1, whereas the same ratio in plasma is about 0.2 to 0.3.
The Physiological Role of Carnitine The carnitine system is involved in many physiological functions1-10 such as: 1. Transport of long-chain fatty acids into the mitochondrial matrix for beta-oxidation to provide cellular energy. A most relevant physiological role of carnitine is to regulate fatty acid metabolism (Fig. 2). In particular, carnitine is a crucial regulator of fatty acid transport into the mitochondria, a ratelimiting step in their beta-oxidation. Carnitine is therefore a key regulator of mitochondrial energy metabolism and adenosine triphosphate production. 2. Transport of partially oxidized fatty acid from peroxisomes to mitochondria 3. Modulation of the intramitochondrial acyl-CoAto-free CoA ratio and increased glucose oxidative metabolism. Increased free CoA and reduced intramitochondrial acyl moieties result in increased free CoA-to-acetyl-CoA ratio (‘‘buffering of the acylto-free CoA ratio’’). A related relevant carnitine 1
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GUARNIERI Figure 1. Carnitine metabolism in normal patients. Car, carnitine; Fe, iron; Lys, Lysine; met, methionine; RBC, red blood cell; vit, vitamin.
effect is represented by modulation of tissue levels of acyl-CoA compounds, which in turn may affect activities of enzymes (e.g., pyruvate dehydrogenase [PDH] complex) involved in the regulation of master metabolic pathways. Importantly, enhanced PDH activity could result in increased glucose oxidative metabolism (oxidative decarboxylation of pyruvate to acetyl-CoA) and enhanced insulinmediated glucose use. The increased PDH activity as also reflected by commonly reported decrements of both lactate and pyruvate, with resulting reduction of their ratio, after carnitine supplementation in vivo. Excess acyl-CoA levels may also result in inhibition of adenine nucleotide translocase (linking mitochondrial energy production with the sarcomeric adenosine triphosphatase contraction mechanism), increased mitochondrial permeability, which
may initiate apoptosis, and impaired insulin action through decreased GLUT4 receptor activity. Modulation of fatty acids and/or acyl-CoA’s availability by carnitine and carnitine acyltransferases may further influence transcriptional regulation of a wide variety of genes related to the control of oxidative metabolism, adipogenesis, glycolysis, and gluconeogenesis. 4. Export from the mitochondria of the branched chain acyl groups. 5. Detoxifying action. Among other functions, the carnitine system contributes to the trapping and excretion of unphysiological acyl groups (such as valproic acid) and acyl-CoA compounds with a detoxifying effect. 6. Storage and transport of energy between different organs. Figure 2. Metabolic functions of carnitine. ATP, adenosine triphosphate; PDH, pyruvate dehydrogenase.
CARNITINE IN HEMODIALYSIS PATIENTS
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Figure 3. Potential causes and clinical consequences of carnitine deficiency or insufficiency (relative lack of free carnitine compared to metabolic needs) in MHD patients.
7. Regulation of membrane phospholipids. 8. Other functions: Carnitine has also osmolyte functions which might be relevant in the protection of cells and tissues from osmotic stress. In the endoplasmic reticulum, carnitine is involved in the acylation-deacylation of lipoproteins such as very low density lipoproteins (VLDLs) or of phospholipids. Carnitine might have a regulatory role on cytokine production and functions and be potentially useful in systemic inflammatory conditions. Experimental evidence also suggests that carnitine may improve sperm counts and sperm motility. Carnitine can be detected at the nuclear level, where it could influence gene transcription by modulating acyl-CoA content.
Carnitine Deficiency and Insufficiency
Carnitine ‘‘deficiency’’1-10 indicates conditions in which carnitine concentrations in plasma or tissues are below the requirements for the normal functions of the organism, with values commonly lower than 25 mmol/L for total serum carnitine or ,10 mmol/g of noncollagen protein
for muscle total carnitine. Carnitine deficiencies can be primary (systemic and myopathic) or secondary. Primary carnitine deficiencies are the consequence of alterations in the intracellular transport of carnitine. Secondary deficiency can be the consequence of genetically determined metabolic errors involving excessive production of short-chain organic acids or of acquired conditions. These may involve a decreased biosynthesis (liver cirrhosis, chronic renal disease, extreme prematurity, and so forth), a decreased intake (malnutrition, vegetarian diets, and so forth), increased requirements (pregnancy, puerperium, severe trauma or infections, burns, and so forth), or increased losses (Fanconi syndrome, renal tubular acidosis, hypercatabolism, Fanconi syndrome). Iatrogenic conditions of carnitine deficiency or insufficiency include hemodialysis and administration of drugs such as valproic acid, pivalic acid derivatives, and zidovudine. Carnitine insufficiency (Fig. 3) indicates secondary acquired conditions in which there is a relative lack of carnitine compared to an increased metabolic need. In conditions of carnitine insufficiency, an excess of acyl groups, arising from either a decreased b-oxidation or an increased fatty acid administration or production, overwhelms the
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availability of free CoA. Consequently, the acyl-to-free carnitine ratio increases because a larger amount of free carnitine is esterified to acylcarnitines to ‘‘buffer’’ the excess of acyl groups. In these situations, carnitine supplements may contribute to regenerate the sequestrated free CoA, thereby maintaining normal metabolic processes. The principal potential consequences of carnitine deficiency or insufficiency are the following: (1) accumulation in the cells of toxic acyl moieties that may have a detergent-like action; (2) accumulation of acetyl-CoA in the mitochondria, which has been shown to inhibit several enzymes (refer the previous sections); (3) impaired export of the excess of organic acids; (4) reduced efficiency of mitochondrial energy production from lipids and glucose; and (5) impaired amino acid metabolism (mainly of the branched chain) and, possibly, worsening of nitrogen balance.
Carnitine in Uremia and Hemodialysis Alterations in carnitine metabolism and in carnitine plasma and tissue levels differ profoundly in different stages of renal disease.1-14 In patients with chronic renal failure on conservative treatment, plasma concentration of acylcarnitines is enhanced by reduced glomerular filtration rate (GFR). Although all carnitine fractions increase, altered proportion of acyl-to-free carnitine also occurs, with relative increment of acylcarnitines. Because of inadequate energy intake, acylcarnitines may further increase, possibly because of the production of ketone bodies. In agreement with this observation, we previously observed a blunted ability in handling exogenously administered acetate in chronically uremic patients.1 Plasma levels of L-carnitine decline by approximately 60 to 70% during the course of a single dialysis session. After 12 months of hemodialysis, endogenous plasma and muscle L-carnitine levels are approximately half of those measured before the initiation of hemodialysis treatment. Acylcarnitine levels are significantly higher in chronic hemodialysis patients, particularly in patients who have been receiving dialysis treatment for at least 12 months. In fact, acylcarnitine levels make up approximately 50% of the total plasma carnitine pool in these patients (of which only half is accounted for by acetyl L-carnitine); this compares with 15% in healthy subjects (consisting primarily of acetyl L-carnitine and negligible levels of the nonacetyl acylcarnitines). Importantly, free carnitine concentrations are also reported to be reduced in skeletal muscles, and this decline appears to be proportional to the dialysis duration.11 Accumulation of the long-chain esters has been noted. A significant inverse correlation between carbon chain length of the acyl group and dialytic removal of the acylcarnitines has been observed, with a notable reduction in the removal of medium- and long-chain esters. Omission of a single dosage of supplemental carnitine in long-term administration schemes results in dramatic decrease and reprofiling of carnitine esters even after the usual 44 hours of interdialytic
period.13,14 Intradialytic loss of carnitine is not the only potential cause of carnitine depletion in hemodialysis patients as additional causes may include reduced carnitine intake or intestinal absorption as well as impaired carnitine de novo renal synthesis. In summary, patients on maintenance hemodialysis (MHD) treatment have low free carnitine plasma concentrations, with increased acylcarnitine-to-carnitine ratio, possibly because of preferential removal of free carnitine during dialysis sessions. The aforementioned alterations have important potential adverse metabolic effects, especially in skeletal muscle. Such metabolic disturbances might contribute to relevant clinical abnormalities often observed in MHD patients (Fig. 3), including muscle weakness, myopathy, and cardiomyopathy; loss of body protein and cachexia; insulin resistance (IR); altered plasma lipid profile; anemia and resistance to erythropoietin; and intradialytic disturbances and symptoms such as muscle cramps, hypotension, and cardiac arrhythmias.1-10
Carnitine Supplementation to Hemodialysis Patients Available biochemical and clinical information provides a rationale for carnitine supplementation to hemodialysis patients. Indeed, several effects of carnitine supplementation have been reported in hemodialysis patients: hypertrophy of type I muscle fiber and increased arm circumference, improvement of dyslipidemia, glucose and protein metabolism, nutritional state and nitrogen retention, skeletal muscle (intradialytic cramps, fatigue) and heart (intradialytic arrhythmias, hypotension) function, oxidative stress and inflammation, anemia and erythropoietin (EPO) sensitivity, exercise capacity and maximum oxygen consumption, sense of well-being, physical endurance, and so forth.1-10 Recently, the issue of carnitine, intestinal microbiota, and cardiovascular disease (CVD) has been raised.15,16 Chronic dietary L-carnitine supplementation in mice significantly altered cecal microbial composition, markedly enhanced synthesis of trimethylamine-N-oxide (TMAO), and increased atherosclerosis but not after suppression of intestinal microbiota. It was concluded that intestinal microbiota may thus participate in the well-established link between increased red meat consumption and CVD risk. Increased TMAO levels were associated with an increased risk of incident major adverse cardiovascular events during the 3 years of follow-up in 4,007 patients undergoing elective coronary angiography. McCarty17 pointed out that the values of TMAO achieved in the TMAO-fed mice were at least an order of magnitude higher than those occurring in humans. The TMAO could simply be functioning as a marker for habitual ingestion of foods rich in carnitine and choline, which include animal products such as red meat and eggs, known to be directly linked to increased vascular risk. Besides, human exposure to TMAO will be approximately an order of magnitude higher from ingesting a given weight
CARNITINE IN HEMODIALYSIS PATIENTS
of fish than of beef; yet, as is well known, fish-rich diets are far more compatible with vascular health than those high in red meats. And finally, supplemental carnitine slowed or stopped the progression of atherosclerotic lesions in rabbits. Indeed, carnitine administration has been reported to be beneficial in patients with angina, ischemia-induced cardiac insufficiency, cardiogenic shock, cardiomyopathy, and myocardial infarction.18 Plasma levels of adipokines and related hormones are greatly elevated in patients on regular hemodialysis. L-carnitine administration further augmented the plasma levels of protective adiponectin; therefore, it may have a role in preventing cardiovascular complications of uremia.19 Compared with placebo or control, L-carnitine is associated with a 27% reduction in all-cause mortality, a 65% reduction in ventricular arrhythmias, and a 40% reduction in anginal symptoms in patients experiencing an acute myocardial infarction.20 L-carnitine is essential for cardiac mitochondria to attenuate the membrane permeability transition, and to maintain the ultrastructure and membrane stabilization, in the presence of high fatty acid b-oxidation. Consequently, the cells may be protected against apoptosis by L-carnitine through inhibition of the fatty acid–induced cytochrome c release. On release of cytochrome c to the cytoplasm, the protein binds apoptotic protease-activating factor 1.21 Advanced glycation end products (AGEs) contribute to CVD in patients with hemodialysis. Oral L-carnitine supplementation significantly decreased skin AGE levels in hemodialysis patients with carnitine deficiency. Carnitine is reported to inhibit the formation of AGEs in vitro, and the studies by Adachi et al.22 and Fukami et al.23 suggest that supplementation of carnitine may be a therapeutic target for preventing the accumulation of tissue AGEs and subsequently reducing the risk of CVD in hemodialysis patients. L-carnitine supplementation was associated with protein-sparing effects in MHD patients.7 Insulinmediated glucose disappearance was improved by L-carnitine only in those patients with greater baseline IR. IR, which is highly prevalent in patients receiving chronic hemodialysis, has been proposed to play a critical role in the development of sarcopenia.24 The severity of IR was associated a lesser decline in plasma leucine concentrations, suggesting a similar resistance to protein anabolism. Evidence from both animal and clinical studies exists that L-carnitine supplementation causes an improved nitrogen balance either via increased protein synthesis or reduced protein degradation, an inhibition of apoptosis, and an abrogation of inflammatory processes under pathologic conditions.25 Carnitine decreased the transcript levels of several genes involved in the ubiquitin proteasome system (UPS) in skeletal muscle and liver of piglets.26 These data suggest that the inhibitory effect of carnitine on the expression of genes of the UPS is mediated indirectly, probably via modulating the release of inhibitors of the UPS such as
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insulin-like growth factor 1. The inhibitory effect of carnitine on the expression of genes of the UPS might explain, at least partially, the increased protein accretion in piglets supplemented with carnitine. The same authors reported that supplementation of carnitine markedly decreases the concentrations of ubiquitinated proteins in skeletal muscle of rats, indicating a diminished degradation of myofibrillar proteins by the UPS.27 Both lipid and glucose oxidation funnel into acetylcarnitine (Fig. 2). The mitochondrial enzyme carnitine acetylCoA transferase (CrAT) converts acetyl-CoA to the membrane-permeable acetylcarnitine and permits mitochondrial efflux of excess acetyl-CoA that otherwise could inhibit PDH.28,29 Carnitine supplementation improves glucose homeostasis in insulin-resistant humans. The authors identified an essential role for the mitochondrial matrix enzyme, CrAT, in regulating substrate switching and glucose tolerance. By converting acetyl-CoA to its membrane permeant acetylcarnitine ester, CrAT regulates mitochondrial and intracellular carbon trafficking. Reduced muscle mitochondrial function occurs in chronic uremia and it could cause insulin resistance.9,21 As a consequence, mitochondrial dysfunction contributes to the pathophysiology of insulin resistance, obesity, diabetes, vascular disease, and chronic heart failure. Lcarnitine is intrinsically involved in mitochondrial metabolism and function as it plays a key role in fatty acid oxidation and energy metabolism. In conclusion, carnitine could positively affect glucose metabolism by10,30-32: - Relieving the accumulation of unmetabolized and potentially toxic acyl derivatives from the mitochondria (scavenger, detoxifying effect) - Reducing the acyl-to-free CoA ratio and increasing the activity of PDH complex activity (buffering effect). By converting acetyl-CoA to its membrane permeant acetylcarnitine ester, CrAT regulates mitochondrial acyl-CoA–CoA balance and PDH activity: YAc(et)yl-CoA/CoA / [PDH activity / [Glycolytic flux / [Glucose metabolism.
- Improving the mitochondrial function through a reduction of myocellular lipid content (‘‘lipotoxicity’’) and a modulation of the systemic inflammation and the oxidative stress. Strong evidence has been provided, at least from animal studies, that L-carnitine supplementation prevents oxidative stress and ameliorates mitochondrial function, whereas results from a very low number of available clinical studies in this regard are inconclusive.25 Several studies reported in MHD patients supplemented with carnitine an improvement of anemia and of the response to erythropoietin (recombinant human EPO).
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In 1999, the US Food and Drug Administration approved L-carnitine for use in dialysis-related carnitine deficiency, as defined by low L-carnitine levels. The American Centers for Medicare and Medicaid Services issued in 2002 and 2006 a national coverage determination for intravenous L-carnitine for treatment of EPO-resistant anemia and dialysis hypotension. A recent meta-analysis 33 failed to confirm the previous findings regarding the effects of Lcarnitine on hemoglobin and the EPO dose but showed that L-carnitine significantly decreased serum C-reactive protein levels. The extent of the decrease was both statistically and clinically relevant. However, the relevance of decrease in C-reactive protein level with hard end points such as all-cause mortality and cardiovascular complications still remains to be clarified. In addition, Mercadal et al.34 reported that the treatment of incident hemodialysis patients with L-carnitine did not improve their response to recombinant human EPO. These results can be generalized only to unselected hemodialysis patients new to hemodialysis. They cannot be extended to long-term dialysis patients with more profound carnitine deficiency. Very negative conclusions were reported in a Cochrane review by Badve et al.35: ‘‘Titles and abstracts of 521 records were screened, of which we reviewed 99 from the full text. Only two studies matched our inclusion criteria. Because interventions differed, data could not be combined for quantitative meta-analysis. There was inadequate evidence identified to inform recommendation of any intervention to ameliorate ESA hyporesponsiveness. Adequately powered randomised controlled trials (RCTs) are required to establish the safety and efficacy of interventions to improve responsiveness to ESA therapy.’’ According to Yee et al.,36 L-carnitine should be used to treat anemia only in dialysis patients with documented L-carnitine deficiency in whom all other conventional measures to raise hemoglobin level have been attempted and failed. In a systematic review and meta-analysis by Huang et al.,37 no effect of L-carnitine was found on serum total cholesterol, high density lipoproteins-cholesterol, VLDLcholesterol, and serum triglycerides. By contrast, that meta-analysis suggested a promising effect of L-carnitine on low density lipoproteins-cholesterol, but further largescale, well-designed, randomized, controlled, trials were suggested. In a review article, Cal o et al.38 concluded that heterogeneous clinical response to carnitine therapy in dialysis patients, reported by other studies, and the lack of largescale randomized trials are the rationale for the reluctance regarding a widespread use of carnitine supplements in dialysis patients. According to Wasserstein,39 in spite of an attractive theoretical rationale of carnitine supplementation and some suggestive but not definitive supportive studies, the expense of L-carnitine in the absence of definitive studies is hard to justify.
In conclusion, carnitine supplementation in MHD patients is in a deadlock because of its great importance in human physiology and of the inconclusive placebo-controlled randomized trials documenting benefits for a wide variety of clinical conditions.40 Differences in experimental design, sample size, patient selection, and carnitine dosage could account for these discrepancies. In particular, carnitine supplementation could be more beneficial for selected patient subgroups with more profound carnitine depletion such as older patients, female patients, and patients with longer disease duration. Responders and nonresponders have been reported in some studies. Given the safety profile of Lcarnitine and the high incidence of carnitine deficiency in hemodialysis patients, it could be reasonable to try to treat the patients in the presence of symptomatology and in conjunction with patient dialysis age. The response to therapy should be monitored at 3-month intervals and discontinued after 9 to 12 months if the patients do not improve.41
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CARNITINE IN HEMODIALYSIS PATIENTS 15. Koeth RA, Wang Z, Levison BS, et al. Intestinal microbiota metabolism of L-carnitine, a nutrient in red meat, promotes atherosclerosis. Nat Med. 2013;19:576-585. 16. Wilson Tang WH, Wang Z, Levison BS, et al. Intestinal microbial metabolism of phosphatidylcholine and cardiovascular risk. N Engl J Med. 2013;368:17. 17. McCarty MF. L-carnitine consumption, its metabolism by intestinal microbiota, and cardiovascular health. Mayo Clin Proc. 2013;88:78i-789. 18. Kudoh Y, Aoyama S, Torii T, et al. Hemodynamic stabilizing effects of L-carnitine in chronic hemodialysis patients. Cardiorenal Med. 2013;3: 200-207. 19. Csiky B, Nyul Z, T oth G, et al. L-carnitine supplementation and adipokines in patients with end-stage renal disease on regular hemodialysis. Exp Clin Endocrinol Diabetes. 2010;118:735-740. 20. DiNicolantonio JJ, Lavie CJ, Fareset H, Menezes AR, O’Keefe JH. L-carnitine in the secondary prevention of cardiovascular disease: systematic review and meta-analysis. Mayo Clin Proc. 2013;88:544-551. 21. Oyanagi E, Yano H, Uchida M, Utsumi K, Sasaki J. Protective action of L-carnitine on cardiac mitochondrial function and structure against fatty acid stress. Biochem Biophys Res Commun. 2011;412:61-67. 22. Adachi T, Fukami K, Yamagishi S, et al. Decreased serum carnitine is independently correlated with increased tissue accumulation levels of advanced glycation end products in haemodialysis patients. Nephrology. 2012;17:689-694. 23. Fukami K, Yamagishi S, Sakai K, et al. Potential inhibitory effects of L-carnitine supplementation on tissue advanced glycation end products in patients with hemodialysis. Rejuvenation Res. 2013;16:460-466. 24. Deger SM, Egbert P, Ellis CD, Sha F, Ikizler TA, Hung AM. Insulin resistance and protein metabolism in chronic hemodialysis patients. J Ren Nutr. 2013;23:e59-e66. 25. Ringseis R, Keller J, Eder K. Mechanisms underlying the antiwasting effect of L-carnitine supplementation under pathologic conditions: evidence from experimental and clinical studies. Eur J Nutr. 2013;52: 1421-1442. 26. Keller J, Ringseis R, Koc A, Lukas I, Kluge H, Eder K. Supplementation with L-carnitine downregulates genes of the ubiquitin proteasome system in the skeletal muscle and liver of piglets. Animal. 2012;6:70-78. 27. Keller J, Couturier A, Haferkamp M, Most E, Eder K. Supplementation of carnitine leads to an activation of the IGF-1/PI3K/Akt signalling pathway and down regulates the E3 ligase MuRF1 in skeletal muscle of rats. Nutr Metab. 2013;10:28.
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28. Schooneman MG, Vaz FM, Houten SM, Soeters MR. Acylcarnitines. Reflecting or inflicting insulin resistance? Diabetes. 2013;62:1-8. 29. Muoio DM, Noland RC, Kovalik JP, et al. Muscle-specific deletion of carnitine acetyltransferase compromises glucose tolerance and metabolic flexibility. Cell Metab. 2012;15:764-777. 30. Galloway SDR, Craig TP, Clelandet SJ. Effects of oral L-carnitine supplementation on insulin sensitivity indices in response to glucose feeding in lean and overweight/obese males. Amino Acids. 2011;41:507-515. 31. Ringseis R, Keller J, Eder K. Role of carnitine in the regulation of glucose homeostasis and insulin sensitivity: evidence from in vivo and in vitro studies with carnitine supplementation and carnitine deficiency. Eur J Nutr. 2012;51:1-18. 32. Vidal-Casariego A, Burgos-Pelaez R, Martınez-Faedo C, et al. Metabolic effects of L-carnitine on type 2 diabetes mellitus: systematic review and meta-analysis. Exp Clin Endocrinol Diabetes. 2013;121:234-238. 33. Chen Y, Abbate M, Tang L, et al. L-Carnitine supplementation for adults with end-stage kidney disease requiring maintenance hemodialysis: a systematic review and meta-analysis. Am J Clin Nutr. 2014;99:408-422. 34. Mercadal L, Coudert M, Vassault A, et al. L-carnitine treatment in incident hemodialysis patients: the multicenter, randomized, doubleblinded, placebo-controlled CARNIDIAL trial. Clin J Am Soc Nephrol. 2012;7:1836-1842. 35. Badve SV, Beller EM, Cass A, et al. Interventions for erythropoietinresistant anaemia in dialysis patients. Cochrane Database Syst Rev. 2013: CD006861. 36. Yee J. L-carnitine for anemia in hemodialysis patients: a last resort. Clin J Am Soc Nephrol. 2012;7:1746-1748. 37. Huang H, Song L, Zhang H, Zhang H, Zhang J, Zhao W. Influence of L-carnitine supplementation on serum lipid profile in hemodialysis patients: a systematic review and meta-analysis. Kidney Blood Press Res. 2014;38:31-41. 38. Cal o LA, Vertolli U, Davis PA, Savica V. L carnitine in hemodialysis patients. Hemodial Int. 2012;16:428-434. 39. Wasserstein AG. L-carnitine supplementation in dialysis: treatment in quest of disease. Semin Dial. 2013;26:11-15. 40. Bloomer RJ, Farney TM, McAllister MJ. An Overview of Carnitine. In: Bagchi D, Nair S, Sen CK, eds. Nutrition and Enhanced Sports Performance, Chapter 41. San Diego, CA, London, UK: Academic Press-Elsevier; 2013:405-413. 41. Molyneux R, Seymour AM, Bhandari S. Value of carnitine therapy in kidney dialysis patients and effects on cardiac function from human and animal studies. Curr Drug Targets. 2012;13:285-293.
