L-Carnitine - a Sports Nutrition Perspective

What is L-Carnitine?

L-Carnitine is a nitrogenous compound naturally found in human skeletal muscle, heart, liver, kidneys and plasma. An average human body contains 20-25g of L-Carnitine, of which over 95% is in skeletal muscles.

Carnitine exists in two isomeric forms: L-Carnitine and D-Carnitine, of which L-Carnitine is the physiological active form².

L-Carnitine is a vitamin-like nutrient essential for energy production and fat metabolism. It is used by athletes as an ergogenic aid. Supplementing diet with L-Carnitine increases maximal aerobic power, and promotes glycogen sparing in the course of prolonged exercise^{3, 4}. Supplementation of L-Carnitine raises the performance of submaximal exercise and resistance to fatique.

L-Carnitine is present in foods, especially red meat and some dairy foods. Animal products are generally rich sources of Carnitine, while plant foods contain considerably lower levels⁴.

L-Carnitine – Fate in Human Body

Carnitine is primarily synthesized from two essential amino acids, methionine and lysine in the liver and kidneys. Its formation requires the presence of co-factors such as, vitamin C, niacin, vitamin B₆ and iron⁴. In addition to endogenous synthesis, dietary intake of Carnitine serves to maintain tissue Carnitine stores. Meat, poultry, fish and dairy products are important sources of Carnitine.

In resting muscle, L-Carnitine is found to be approximately 80-90% as free L-Carnitine, whereas the rest is in esterified form. Carnitine also forms long and short chain acyl derivatives, with predominant short chain acyl derivative, acetyl-L-Carnitine (acylcarnitine). Carnitine is normally eliminated via urine as free Carnitine and acylcarnitines⁵. The daily urinary excretion of Carnitine is 15-50mg. Carnitine and acylcarnitine due to their smaller molecular sizes are significantly removed from the blood during filtration in kidneys but approximately 90% of them are reabsorbed⁶.

Is Carnitine available in different isomeric forms?

Carnitine exists as two distinct isomers, L-Carnitine (naturally occurring Carnitine) and D-Carnitine (synthetic Carnitine). L-Carnitine is the physiologically active form, while the non-physiological form, D-Carnitine, may replace the natural intracellular L-Carnitine. D-Carnitine depletes endogenous L-Carnitine stores by competitive antagonism^2,5. D-Carnitine or mixtures of D-Carnitine and L-Carnitine are to be avoided. Pure L-Carnitine or its salts should be only sources for Carnitine supplementation.

Biochemical Role of L-Carnitine

L-Carnitine possesses following different biochemical functions, which are very important for athletic performance.

A. Fat Transport and Mitochondrial b-Oxidation of Long Chain Fatty Acids

The primary function of Carnitine in the body is to facilitate the burning of fatty acids for metabolic energy. L-Carnitine has been shown to have two metabolic functions (Figure 1), as follows^{5, 7}:

1. It transports long-chain fatty acids across the inner mitochondrial membrane to the mitochondrial matrix, where b-oxidation of fatty acids takes place.
2. It also transports the potentially toxic metabolites (by-products of lipid catabolism) from the mitochondrial matrix back into the cytoplasm, in the form of Carnitine esters.

L-Carnitine acts as a carrier in fatty acid transport and its supplementation increases the utilisation of free fatty acids for energy production in endurance trained athletes^{8, 9}. Since, L-Carnitine involves in energy production and utilisation, it can be considered as an ergogenic aid for athletes.

L-Carnitine supplementation can protect endurance athlete carbohydrate stores³, which are at risk during periods of increased exercise.

B.  Carbohydrate Metabolism

Although, Carnitine is famous for its fat-burning properties, growing evidence indicates that it is also important in carbohydrate metabolism³. It is evident that, there is a strong correlation between muscle Carnitine and the activity of Krebs cycle, a pivotal cycle in carbohydrate metabolism⁴.

Carnitine concentration in muscle is found to be directly proportional to muscle glycogen store. Carnitine can act as anti-catabolic agent because of its ‘Glycogen Sparing’ effect to improve energy production from fats and effectively reduces the need to burn glycogen³.

C. Lactate Clearance

Intensive, acute exercise leads to lactate accumulation and an associated drop in serum pH. High levels of lactic acid increase the acidity of blood and tissues, lead to fatigue and decrease in ATP production. Since, L-Carnitine inhibits key anaerobic enzyme phosphofructokinase (PFK), maximum rate of glycolysis is reduced. L-Carnitine reduces acetyl CoA/CoA quotient and as a consequence, less lactate is formed. Its supplementation is useful in intense physical exercise to improve the performance by clearing extra lactic acid from blood and tissues^{10, 11}.

D. Peripheral Blood Supply

L-Carnitine shows a demonstrable increase in peripheral blood supply and capillary dilatation. This can lead to more oxygen absorption and substrate turn over, especially during intense execise⁷.

E. Branched Chain Amino Acid (BCAA) Metabolism

Carnitine levels are linked to the metabolism of BCAAs, considered the most important amino acids for sports performance¹². In skeletal muscles BCAAs are partially metabolised to branched chain keto acids (BCKA’s), which subsequently get linked up to Carnitine and released into general circulation (blood). These BCKA Carnitine complexes are taken up by the liver for further oxidation to yield energy or for production of glucose (Gluconeogenesis)⁴.

F. Ammonia Detoxification

Ammonia, a by-product of protein degradation, is a recognised factor in exercise-induced fatigue, and is toxic at low amounts. Carnitine works to lower ammonia levels by increasing its incorporation into urea, which is subsequently excreted in the urine¹³. Carnitine supplementation has protective effect against ammonia poisoning caused due to intense work-outs¹².

L-Carnitine also protects the immune system and stabilises membrane during intense physical exercise⁷. More recently, studies have shown that Carnitine and its acyl esters are radical scavengers and iron-chelators, and act as antioxidants¹⁴.

Why is L-Carnitine Supplementation Important?

Although, Carnitine is both biosynthesized and supplied in the diet, there is a need for supplementary Carnitine in some conditions, as follows:

In competitive athletes, intensive endurance efforts (cycling stage races, repeated long-distance runs, etc.) cause a clearly detectable L-Carnitine deficiency in the muscles. Its supplementation can have a positive effect on athletic exercise capacity¹⁵ and can help to maintain fitness, enhance detoxification and aid faster recovery.

An unbalanced diet with a low intake of L-Carnitine and/or of one of its precursors often leads to a deficiency in L-Carnitine. It is evident that vegetarian competitive athletes are Carnitine deficient.

To produce L-Carnitine endogenously, lysine, methionine, niacin, vitamin B₆, Vitamin C and iron are required. L-Lysine is the backbone of Carnitine and diets low in lysine can lead to Carnitine deficiency¹⁶. In addition, methionine, vitamin C¹⁷ and iron deficiency can cause significant drop in Carnitine levels in the body.

Individuals, with different physiological conditions (fasting, malnutrition, obesity, infertility in males, dialysis patients, muscle diseases, and hospitalised patients receiving total parental nutrition), may need supplementary L-Carnitine.

What are the Benefits of L-Carnitine Supplementation in Athletes?

L-Carnitine acts as an ergogenic and anti-catabolic agent for sports performance in athletes. Its supplementation can be benefited as follows:

• Increased utilisation of free fatty acids for energy for increase muscle and heart energy in endurance sports (ergogenic aid)
• Increased ‘Glycogen Sparing’ effect in endurance athletes, can provide protection for their muscle glycogen stores, which are at risk during periods of increased exercise (anti-catabolic)
• Reduced lactic acid concentration in the muscle leads to a delay in the onset of muscular fatigue and cramps
• Increased peripheral blood supply provides more oxygen absorption and leads to more energy production in aerobic exercise
• Increased BCAA metabolism to yield more energy and prodcution of glucose
• Enhanced removal of ammonia and its detoxification
• Increased protection to immune system and improved membrane stability during intense physical exercise.

More recently, studies have shown that Carnitine and its acyl are radical scavengers and ironchelators, and hence act as antioxidants.

L-Carnitine supplementation can improve mental and physical performance and can be used to achieve optimum health. Currently, L-Carnitine does not appear on the International Olympic Committee (IOC) banned substances list as it is naturally occurring constituent of a normal diet.

Carnitine Supplementation - Is it Safe?

L-Carnitine is completely safe, with the possible exception of mild diarrhoea at very high doses^4,7.  As a vitaminoid, L-Carnitine is practically non-toxic and produces no side effects even after the intake of large doses (up to 15g). L-Carnitine causes neither physical nor psychological dependency even after long-term use. Supplementation may be discontinued or resumed at any time. There is no evidential published data on any intolerance and adverse side effects of L-Carnitine.

D-Carnitine or mixtures of D-Carnitine and L-Carnitine are to be avoided.

Horleys L-Carnitine Capsules: How Much and when

L-Carnitine is very hygroscopic (absorbs atmospheric moisture) in nature and quickly becomes sticky in humid conditions whereas, L-Carnitine L-tartrate (Tartrate salt of L-Carnitine) is a white, crystalline powder and does not lose its consistency in humid conditions. Horleys L-Carnitine capsule is a very stable product due to following reasons:

1. It contains L-Carnitine L-tartrate, a stable salt of L-Carnitine.
2. It is presented in the capsule form to avoid contact with atmospheric moisture.

It has been found that a dose of 1-2g/day^{7 is} effective during periods of high exercise load prior to competitive events.

1-2 capsules should be taken approximately 30 minutes prior to training. For additional effect take one additional capsule in the morning and at night.

REFERENCES:

1. Lennon, D.F.L. et.al, 1983, Effect of acute moderate intensity exercise on Carnitine metabolism in men and women, J. Appl. Physiol. 55: 489-495.
2. Negrao, C.E. et.al. 1987, Carnitine supplementation and depletion: tissue Carnitines and enzymes in fatty acid oxidation, J. Appl. Physiol, 63: 315-321.
3. Cederblad, G. et.al. 1976, Carnitine concentration in relation to enzyme activities and substrate utilization in human skeletal muscle, Scand. J. Clin. Lab. Invest., 36 : 547-552
4. Leibovitz, B.E.1993, L-Carnitine: Dietary sources, Lonza Ltd. Basel, Pg.10
5. Heinonen, O.J. 1996, Carnitine and physical exercise; A review article, Sports Med., 22(2): 109-132
6. Rebouche, C. and Mark, D. 1984, Sodium gradient-stimulated transport of L-Carnitine into renal brush border membrane vesicles: kinetics, specificity, and regulation by dietary Carnitine, Archiv. Biochem & Biophys, 235: 393-402
7. Neumann, G. 1998, L-Carnitine and the athlete: mode of action, Lonza Ltd., Basel, Pg.7
8. Gorostiaga, E.E. et.al.1989, Decrease in respiratory quotient during exercise following L-Carnitine supplementation. Int. J. Sport Med., 10: 169-174
9. Marconi, C. et.al. 1985, Effects of L-Carnitine loading on the aerobic and anaerobic performance of endurance athletes, Eur. J. Appl. Physiol., 54: 131-135.
10. Brevetti, et.al.1988, Increases in walking distance in patients with peripheral vascular disease treated with L-Carnitine: a double blind, cross-over study, Circulation, 77: 767-773.
11. Vecchiet, L. et.al. 1990, Influence of L-Carnitine administration on maximal physical exercise, Eur. J. Appl. Physiol. 61: 486-490
12. Pepine, C. 1991, The Therapeutic potential of Carnitine in cardiovascular disorders, Clin. Therapeutics.13: 2-20.
13. Costell, M. et.al. 1984, Effects of L-Carnitine on urea synthesis following acute ammonia intoxication in mice. Biochem. Biophy. Res. Commun, 120: 726-733.
14. Arduini, A. 1992,Carnitine and its acyl  esters as secondary antioxidants?, Am. Heart J., 123: 1726-1727
15. Cerretelli, P. and Marconi, C. 1990, L-Carnitine Supplementation in humans. The effects on physical performance, Int. J. Sports. Med., 11: 1-14.
16. Broquist, H.P. and Borum, P.R. 1982, Carnitine Biosynthesis. Nutritional Implications. Advances in Nutr. Res., 4: 181-204.
17. Hughes, R. et.al. 1980, Dietary ascorbic acid and muscle Carnitine in guinea pigs, Brit. J. Nutr., 43: 385-387.