The "Fat-Burning" Supplement L-Carnitine

The amino acid L-Carnitine (or just carnitine) has a key role in the metabolism of fat. Carnitine is essential to the fat burning process, it transports fat into the mitochondria (the little furnace in every cell) to be oxidized or “burnt” to provide an important source of energy. During any low intensity activity (walking, sitting, sleeping) fat is the dominant fuel. Large molecules of fat (long-chain fatty acids ) are particularly dependent on carnitine. Fat cannot penetrate the inner mitochondial membrane to be utilized unless it is paired to carnitine. In fact, every cell’s fat burning capacity is carnitine-dependant! Carnitine is critical for burning fat and therefore, weight loss and getting lean. All this information is well established in science. So if you are into “toning up,” “cutting up,” or losing fat you’re probably ready to rush out and buy a bucket full.

Well, let me welcome you to one of the best supplement scams that’s been around for over 15 years and still going strong. If you hoped that a carnitine supplement would help “cut you up” and get you lean, it is a sad, unfortunate fact that the only thing that will be reduced by taking carnitine is your pocket book.

After you wipe away the tears of disappointment and frustration I want you to re-read that first paragraph again, for this is how supplement con men make their money. They take a tiny piece of science such as I have showed you and then wrap it up in a thick blanket of hype. I’m going to provide you with the real truth; the science on carnitine in supplement form, simply and quickly, so you can decide for your self.

However, lets get one thing straight, right from the start about scientific research on supplements. Advertisements in muscle magazines showing pictures of bodybuilders in bad fitting lab coats while standing next to computer equipment that isn’t even switched on, are not scientific research. That’s just deceitful marketing (and bad dress sense). If you believe the “scientific information” provided in these ads, come down to Sydney, Australia for I’ve got this big bridge I want to sell you! Just remember that supplement companies don’t own research laboratories, only universities do. Now, let’s get on with the legitimate stuff.

Carnitine is a non-essential amino acid synthesized from the carbon chain and nitrogen of lysine and the methyl groups of methionine.(1) Carnitine originates from the Latin word for meat or flesh, which are its best dietary sources.(2) Yes, carnitine is vital to the fat burning process. It is required for the transportation of fatty acids into the mitochondria (the fat burning part of the cell).(2-5) In this little chamber, b-oxidation (burning) of fat occurs to provide energy.(3) For those that are interested in the biochemistry; during this oxidation process in the michochondrial matrix, the esterified form of carnitine (acetylcarnitine) forms acetyl-CoA. In muscle, this is able to enter the citric acid cycle to ultimately produce ATP (energy). In the liver during low calorie diets, this can also form ketones.(20)

Unfortunately for fat loss enthusiasts, time and time again research has demonstrated that extra carnitine does not enable more fat than usual to be burnt for energy. Nor do healthy bodies ever become deficient in carnitine, not even hard training athletes. This information is also well established in research but you won’t find the supplement marketers telling you about these other “critical facts” on carnitine.

The total carnitine pool of a healthy 70kg male is about 100mmol. In humans about 98% of this is found in muscle.(20) Some non-essential amino acids (like glutamine) are devoured ravenously by the body and indeed are required in supplement form because of this. However, carnitine is one non-essential amino acid that does not fit this profile. Carnitine has no degradation pathways, and only 1-2% is lost in urine, sweat etc.(20) Carnitine actually undergoes extensive reabsorption in the kidneys; up to 99%.(2)

Some vitamins or minerals such as chromium need to be supplemented in the diet because they are lost (excreted) in large amounts as a result of exercise. However urinary excretion of carnitine does not change as a result of exercise, either during or after.(1,6) An entire series of studies demonstrate that muscle carnitine levels change little and are easily maintained during exercise training(1-6) and many researchers have gone so far as to state that there is no risk of athletes developing a carnitine deficiency.(20) In fact the worlds leading authority on L-Carnitine research Olli Heinonen of Turku University Hospital in Finland, has reviewed the research on high intensity exercise and carnitine metabolism extensively. He has concluded that carnitine levels seem to be resistant to changes induced by all kinds of exercise.(20) Heinonen; is one of the most progressive exercise physiologists in the world, he recommends that due to the fact exercise does not induce a loss in carnitine or impact on carnitine levels, there is no physiological reason for supplementation.(20)

However, for those that choose to look past all this research (the marketers of carnitine supplements) let’s examine the information that has directly assessed carnitine supplementation.

Fat is an important source of energy during low intensity and endurance events. Because of carnitine’s essential role in fat metabolism the sales pitch on carnitine supplementation goes something like this; adding carnitine to the diet would stimulate increased fat oxidation during exercise and enhance fat utilization. This would help shed the excess pounds while also sparing critical glycogen reserves. This may also enhance sports performance, especially in endurance events. For a long time, using a gas exchange apparatus fitted to the athlete’s mouth and nose, exercise physiologists have been able to quantify the type of fuel that is actually burnt during exercise. This measure is called a respiratory quotient (R value). The value provides clear information as to whether fat, carbohydrate or protein is being “burnt” during aerobic or anaerobic exercise. Repeated exercise physiology tests demonstrate very clearly that loading up on carnitine does not alter this R-value in any way, nor does it “shift” fuel oxidation toward fat burning.

The research is quite conclusive; carnitine supplementation does not increase or enhance fat burning, nor does it enhance performance during any type of exercise. (2, 10,12,14,15,20)

Does carnitine spare valuable muscle glycogen? Nope, carnitine supplementation shows zero effect on energy metabolism. (7,15,16)

Some research has examined carnitine supplementation to see if it would enhance high intensity exercise. It was hoped that carnitine administration may “prime” anaerobic pathways by stimulating an enzyme complex called pyruvate dehydrogenase activity, and therefore modifying the acetyl-CoA/free CoA ratio. Theoretically, this may divert lactic acid accumulation away from the working muscle. Diverting lactic acid build up is a sure way to increase work capacity before fatigue sets in. But, this theory was soon debunked.(8) The fact is, carnitine loading only increases muscle carnitine levels by 1-2% (13) and anaerobic energy pathways work exactly the same in the presence of extra carnitine or not.(20)

What about research that has directly examined body fat or weight loss and carnitine supplementation? Glad you asked. Once again despite all the hype, proper scientific investigation reveals carnitine supplementation has failed to deliver the goods in getting rid of the pudge. With several examples that show subjects on low calorie diets undertaking exercise programs and supplementing with carnitine show no difference in fat or weight loss to those that did not use carnitine.(17,18,19)

In point form, this is the research to date on L-carnitine supplementation.
As a result of exercise there appears to be a redistribution of carnitine, but no loss of total carnitine stores in the body. The body recycles carnitine extremely well. After stringent assessment it is concluded that athletes are not at risk of developing a carnitine deficiency and unlike other amino acids such as glutamine; hard training does not produce an increased demand.(4)

Carnitine supplementation does not reduce body fat or help weight loss. There is no theoretical basis for these assumptions and much less clinical support.(20)

Carnitine does not enhance anaerobic energy pathways (short duration, intense activity) to reduce lactic acid levels. The evidence shows carnitine has no effect on lactic acid formation.(20)

Carnitine neither enhances fat burning (oxidation) nor spares muscle glycogen or enhances performance during exercise.(1-12)
In a world of confusion one thing appears certain; that carnitine in any form, will not enhance fat burning or any type of exercise performance. The weight of evidence against the effectiveness of carnitine has been staking up for quite some time and it is data that is freely accessible to all. Therefore, you would have to question very seriously, the honesty and integrity of any company or marketer that still insists on promoting or incorporating carnitine into fat-loss supplements. If these people are trying to bullshit you on an aspect as “clear-cut” as the carnitine issue, what else would they lie about?
Check out the Science for yourself:

Lemon DLF, et al. Effects of acute moderate-intensity exercise on carnitine metabolism in men and women. J.Appl.Physiol.1983;55:489-95.

Snoop M, et al.,Influence of carnitine supplementation on muscle substrate and carnitine metabolism during exercise. J.Appl.Physiol. 1988;64:2394-9.

Haitt WR, et al. Carnitine and acetylcarnitine metabolism during exercise in humans: dependent on skeletal muscle metabolic state. J.Clin.Invest.1989;84:1167-73.

Sahalin K. Muscle carnitine metabolism during incremental, dynamic exercise in humans. Acta Physiol. Scand. 1990;138:259-62.

Arenas J, et al. Carnitine in muscle, serum and urine of nonprofessional athletes: effects of physiological exercise, training, and L-carnitine administration. Muscle Nerve. 1991;14;598-604.

Decombaz J, et al. Muscle carnitine after strenuous endurance exercise.J.Appl.Physiol.1992;72:423-7.

Vukovich MD, Costill DL, Fink WJ. Carnitine supplementation: effect on muscle carnitine and glycogen content during exercise. Med.Sci.Sports Exerc.1994;26:1122-9.

Constantin-Teodosiu D. Regulation of pyruvate dehydrogenase complex activity and acetyl group formation in skeletal muscle during exercise. Huddinge: Huddinge University Hospital, 1992.

Marconi C, et al. Effects on L-carnitine loading on the aerobic and anaerobic performance of endurance athletes. Eur.J.Appl Physiol. 1985;54:131-5.

Cooper MD, et al. The effect on marathon running on carnitine metabolism and on some aspects of muscle mitochondrial activities and antioxidant mechanisms. J. Sports Sci. 1986;4:79-87.

Oyono-Enguelle S, et al. Prolonged submaximal exercise and L-carnitine in humans. Eur. J. Appl Physiol. 1988.58:53-61.

Trappe SW, et al. The effects of L-carnitine supplementation on performance during interval swimming. Int. J. Sports Med 1994;15:181-5.

Siliprandi NA et al. Effect of carnitine on muscle metabolism. [letter]. Eur.J.Appl.Physiol. 1992;64:278.

Wyss V, Ganzit GP, Rienzi A. Effects of L-carnitine administration on VO2 max and the aerobic-anaerobic threshold in normoxia and acute hypoxia. Eur.J.Appl.Physiol. 1990;601-6.

Decombanz J, et al. Effect of L-carnitine on submaximal exercise metabolism after depletion of muscle glycogen. Med Sci Sports Exerc 1993;25:733-40.

Brass EP, Hoppel CL, Haitt WR. Effect of intravenous L-carnitine on carnitine homeostasis and fuel metabolism during exercise in humans. Clin. Pharma. Ther. 1994;55:681-92.

Grunewald KK, Bailey RS. Commercially marketed supplements for bodybuilding athletes. Sports Med. 1993;15:90-103.

Williams MH. Ergogenic and ergolytic substances. Med Sci Sports Exerc 1992:24Suppl.:S344-8.

Villanti RG, et al. L-carnitine supplementation combined with aerobic training does not promote weight loss in moderately obese women. Int. J.Sport Nutr Exerc Metab 2000;2:199-207.

Heinonen OJ. Carnitine and physical exercise. Sports Med. 1996;2:109-132.


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