L-carnitine is often mistaken as an amino acid, when in fact it is a charged molecule known as a ‘quat’. Most people tout carnitine as an effective fat loss agent, since it is necessary for fatty acid oxidation; yet the literature on carnitine and performance enhancement remains a little underwhelming (and contradictory).

Thus, this article will dive into the mechanisms of carnitine, its purported efficacy as a dietary supplement, and how to make the most of this popular ingredient.

Carnitine: Structure and Function

Carnitine is synthesized from the amino acids lysine and methionine; it is involved in fatty acid metabolism across mitochondrial membranes (it breaks parts of those acids down which contributes to the formation of ATP).

The average male body holds about 20g to 25g of carnitine, most of which (~95%) resides in muscle tissue.[1] A typical Western diet contains  roughly 100mg to 300mg of carnitine per day, mostly from meat and dairy. In vegan diets, carnitine intake may be as low as a few milligrams per day.

The Major Drawback of Carnitine: Bioavailability

Carnitine supplements often purport that they enhance cardiovascular performance and/or increase fat loss. Yet, there isn’t much compelling evidence that this is the case (particularly with plain L-carnitine). The premise behind L-carnitine supplementation is sound, but a major flaw is its exceptionally poor bioavailability.

Oral L-carnitine supplementation in itself does not have any research-supported ergogenic effects, mainly due to the aforementioned poor absorption.[2] See below for summary of the lackluster results in research:

  • At 6g/day for 10 days, carnitine has no significant effects on resting oxygen consumption in normal subjects[3]
  • 5g/day for 5 days, administered before 2hr cycling, had no effects on muscle energy metabolism[4]
  • Studies that did find an effect on VO2max and respiratory quotient could not be replicated/supported by further studies[5],[6]
  • 2x2g carnitine per day for 3 months had no significant effect on muscle creatine content[7]
  • Even intravenous administration does not help increase carnitine stores; a series of I.V. infusion studies by discuss how this method of administration, similarly to oral supplementation, fails to increase muscle carnitine stores.[8]

The benefits to carnitine supplementation, with the oral route and without a mechanism to increase transport of carnitine into mitochondria, are only seen in individuals who have deficiencies. In healthy individuals these benefits are absent. Similar to many nutrients, there are thresholds or ceilings where particular metabolic processes are saturated. 

Benefits of Carnitine Loading

Increased muscle carnitine content increases fatty acid oxidation, due to carnitine levels causing muscle tissue to favor fat as an energy source. This metabolic change has a host of benefits for improved body composition: It can stave off fat gain, boost fat loss, help increase lean mass, improve brain health, blood flow and leptin levels.

Some of the extant literature suggests that increased carnitine levels may also aid post-exercise recovery via increased blood flow and reduced accumulation of ammonia.

Enter Choline

Choline is a constituent of acetylcholine and phosphatidylcholine. The former is an important neurotransmitter and the latter is a phospholipid. Phospholipids are components of cell membranes and critical for cell metabolism and integrity; they are often supplemented with via lecithin granules.

Many people get about 400mcg to 900mcg per day of choline, mostly via lecithin (found in egg yolk, liver, meat and peanuts). Too much choline in your diet can make you smell like fish (literally) and cause diarrhea. Choline is considered a lipotropic agent (prevents deposition of fat in the liver) and appears to act as a ‘shuttle’ for carnitine.

Choline as a ‘Shuttle’ for Carnitine

The only way to reap the (metabolic) benefits of carnitine supplementation is to circumvent its poor bioavailability. One way of doing this via oral administration in combination with supplemental choline:

  • 3-4g/day (higher dose presumed for men) of choline only increased carnitine levels by reducing excretion. More importantly, choline does not retain carnitine circulating but rather adds it into the muscle tissue pool of carnitine.[9]

Effective dosage using choline would range from 3g to 4g of choline bitartrate (or a lower amount of alpha-GPC) + 1.5g to 2g of pure L-carnitine. After only a week of this protocol, muscle carnitine content will increase significantly. Choline alone for seven days increases carnitine retention.

Increased muscle carnitine levels, caused by choline+carnitine supplementation were maintained even after two weeks no supplementation. At this point it’s important to point out that the analogy or imagery of a ‘shuttle’ that carries carnitine into the mitochondria of muscle cells is inaccurate.

Carnitine is not co-transported with choline in that sense.[10] A likely mechanism is that choline causes an up regulation of OCTN2 (sodium-dependent carnitine transporter), which is present not only in muscle tissues.

Insulin as a ‘Shuttle’

Increased insulin levels concomitant with carnitine ingestion will also lead to greater carnitine retention.
In a study by Stephens et al., 80g of carbs (vitargo) + 1.36g carnitine (as 2g l-carnitine l-tartrate were administered twice per day for 12 weeks, administered after an overnight fast once in the morning and once four hours later).[11] The idea here was to maximize the time period during which both plasma carnitine and circulating insulin levels were elevated (subjects were permitted to eat).

The following are their core findings:

  • Controls gain 2kg of body fat determined via DEXA scans (that which the treatment group did not muscle carnitine content was significantly increased only in treatment group by ~20% after 12 weeks)
  • During low-intensity cycling, energy expenditure increased by 6% only in the treatment group after 12 weeks.
  • PPAR signalling was increased by upwards of 50% (amongst many other pathways, most of which indicate a metabolic shift towards greater utilization of fat and decreased use of muscle glycogen)

Take-Home Points

Taking carnitine with hypercaloric diets could prevent potential fat gain. Glucose ingestion prior to or during “[…]exercise typically blunts exercise-induced gene expression of skeletal muscle fat oxidative genes”.[12]

This effect seems to be prevented by chronic coadministration of carnitine. Moreover, simultaneous supplementation with choline will boost absorption of L-carnitine significantly.

[1] Feller AG, Rudman D. Role of carnitine in human nutrition. Journal of Nutrition 118: 541-547, 1988.

[2] Greig C, Finch KM, Jones DA, Cooper M, Sargeant AJ, et al. The effect of oral supplementation with L-carnitine on maximum and submaximal exercise capacity. European Journal of Applied Physiology 56: 457-460, 1987.

[3] Dal Negro R, Pomari G, Zoccatelli 0, Turco P. Changes in physical performance of untrained volunteers: effects of L-carnitine. Clinical Trials Journal 23: 242-248, 1986.

[4] Soop M, Bjorkman 0, Cederblad G, Hagenfeldt L, Wahren J. Influence of carnitine supplementation on muscle substrate and carnitine metabolism during exercise. Journal ofApplied Physiology 64: 2394-2399, 1988

[5] Gorostiaga EM, Maurer CA, Eclache JP. Decrease in respiratory quotient during exercise following L-carnitine supplementa- tion. International Journal ofSports Medicine 10: 169-174, 1989 

[6] Cerretelli P, Marconi C. L-carnitine supplementation in humans. The effects on physical performance. International Journal of Sports Medicine II: 1-14, 1990

[7] Wächter S, Vogt M, Kreis R, Boesch C, Bigler P, Hoppeler H, Krähenbühl S. Clin Chim Acta. 2002 Apr; 318(1-2):51-61.

[8] Brass, EP, Hoppel, CL & Hiatt, WR (1994). Effect of intravenous l-carnitine on carnitine homeostasis and fuel metabolism during exercise in humans, Clinical Pharmacology Therapy, 55, 681-692

[9] Hongu, N & Sachan, D. S. (2003). Carnitine and choline supplementation with exercise alter carnitine profiles, biochemical markers of fat metabolism and serum leptin concentration in healthy women. American Society for Nutritional Sciences Journal of Nutrition, 133(1), 84-89.

[10] Dodson, W. L. & Sachan, D. S. (1996) Choline supplementation reduces urinary carnitine excretion in humans. Am. J. Clin. Nutr. 63:904-910.

[11] Stephens F, Constantin-Teodosiu D, Laithwaite D, Simpson E, PL G. An acute increase in skeletal muscle carnitine content alters fuel metabolism in resting human skeletal muscle. J Clin Endocrinol Metab. 2006; 91:5013–5018.

[12] Civitarese AE, Hesselink MK, Russell AP, Ravussin E, Schrauwen P. Glucose ingestion during exercise blunts exercise-induced gene expression of skeletal muscle fat oxidative genes. Am J Physiol Endocrinol Metab. 2005;289:E1023–E1029.