CLA vs L-Carnitine: What's the Difference
CLA and L-carnitine have for decades stood side by side in the “fat burners” section, although these are substances of entirely different nature. CLA is a group of fatty acids that are themselves fat, while L-carnitine is a compound that helps deliver fatty acids into the mitochondria for oxidation. The editorial team breaks down how each of them works and what of this is confirmed in humans.
Chemical nature and origin
CLA (conjugated linoleic acid) is a collective name for isomers of linoleic acid in which the double bonds are located next to each other, separated by one single bond, that is, “conjugated.” The most important isomers are cis-9,trans-11 (rumenic acid) and trans-10,cis-12.
In nature, CLA is formed mainly by bacteria in the rumen of ruminant animals, so the main dietary sources are dairy products and beef and lamb meat. There the cis-9,trans-11 isomer predominates. Supplements, however, are produced by chemical isomerization of oils (safflower or sunflower), and they usually contain roughly equal shares of both main isomers.
L-carnitine is a small water-soluble compound, a derivative of amino acids. The body synthesizes it from lysine and methionine mainly in the liver and kidneys with the participation of vitamin C, iron and vitamin B6. The main dietary source is red meat; in plant products there is very little carnitine.
So the first key difference: CLA is a fatty acid that enters cell membranes and tissues as a lipid, whereas L-carnitine is a transport molecule that is not itself a source of energy but participates in the metabolism of other substances.
| Parameter | CLA | L-carnitine |
|---|---|---|
| Class of substance | Polyunsaturated fatty acid (isomers) | Derivative of amino acids, quaternary ammonium compound |
| Synthesis in the body | Practically none (small conversion from vaccenic acid) | Yes, from lysine and methionine |
| Dietary sources | Dairy products, ruminant meat | Red meat |
| Main role | Signaling and membrane effects of lipids | Transport of long-chain fatty acids into mitochondria |
| Medical use | None | Treatment of carnitine deficiency (levocarnitine) |
How L-carnitine works
Long-chain fatty acids cannot pass on their own through the inner mitochondrial membrane, where their oxidation takes place. For this there is the so-called carnitine shuttle: the enzyme CPT1 on the outer membrane attaches a fatty acid to carnitine, forming acylcarnitine; a special translocase protein carries it inside, and the enzyme CPT2 releases the fatty acid for beta-oxidation.
The second function of carnitine is buffering acetyl groups. During intense work, acetyl-CoA accumulates in the mitochondria, and carnitine binds the excess acetyl groups, forming acetylcarnitine. A review by Stephens, Constantin-Teodosiu and Greenhaff (2007) describes how this function affects the ratio of carbohydrate and fat use in muscle.
From this arose the idea: if you add carnitine, cells will burn more fat. However, in a healthy person carnitine is rarely a limiting factor for fat oxidation. About 95% of the body's carnitine is concentrated in skeletal muscle, and oral intake on its own poorly raises its content in muscle tissue.
A study by Wall et al. (2011) showed that the carnitine content in muscle could be raised by about 21% only with prolonged intake, over 24 weeks, of L-carnitine together with a large amount of carbohydrates, which stimulate insulin secretion and carnitine uptake by muscle. Without the carbohydrate accompaniment, no such effect was observed.
How CLA is supposed to work
Interest in CLA arose after studies in mice, in which a mix of isomers, in particular trans-10,cis-12, caused a significant reduction of adipose tissue. The proposed mechanisms included inhibition of lipoprotein lipase (the enzyme that helps adipocytes capture fats from the blood), an effect on stearoyl-CoA desaturase, enhancement of fat oxidation and even the death of fat cells.
However, the effects in rodents turned out to be much stronger than in humans. Mice have a different metabolism and respond to the trans-10,cis-12 isomer especially markedly - along with significant adverse changes, in particular fat accumulation in the liver and insulin resistance.
The cis-9,trans-11 isomer, which predominates in natural products, is considered less active with respect to fat mass in this context. That is, “natural” CLA from milk and meat and CLA from supplements are not quite the same thing in terms of isomer ratio.
Thus, the mechanism of action of CLA is a set of effects on lipid metabolism and signaling in adipose tissue, whereas L-carnitine has a clear, well-described transport function. However, as studies show, the clarity of the mechanism does not yet guarantee a practical result.
What the meta-analyses show
A meta-analysis by Whigham, Watras and Schoeller (2007) evaluated studies of CLA in humans and concluded that at a dose of about 3.2 g per day a reduction of fat mass of roughly 0.09 kg per week compared with placebo is observed. The effect is statistically noticeable, but on a practical scale modest.
A later meta-analysis by Onakpoya et al. (2012), devoted to long-term CLA intake in people with overweight and obesity, also found a small difference in favor of CLA, which the authors assessed as doubtful from a clinical point of view.
For L-carnitine, a meta-analysis by Pooyandjoo et al. (2016) showed a moderate reduction of body weight in adults compared with control, with the effect weakening over time. The authors noted the heterogeneity of the studies and the need for further work.
So in neither case do the data support the image of a “fat burner.” Both substances at best give a small additional effect against the background of a calorie deficit, which is the main factor of weight loss.
- CLA: a small reduction of fat mass in meta-analyses, clinical significance doubtful.
- L-carnitine: a moderate reduction of body weight, heterogeneous data.
- Both: an effect much smaller than that of a calorie deficit and physical activity.
Safety, forms, medical use
For CLA, the main caution concerns the trans-10,cis-12 isomer. A study by Risérus et al. (2002) showed that in men with obesity and metabolic syndrome, intake of this isomer reduced insulin sensitivity. Among other reported side effects are digestive disorders. People with carbohydrate metabolism disorders should discuss CLA intake with a doctor.
For L-carnitine, a link with trimethylamine-N-oxide (TMAO) is discussed. A study by Koeth et al. (2013) showed that the gut microbiota converts carnitine into trimethylamine, which in the liver is oxidized to TMAO, and TMAO was associated with atherosclerosis in models and observational data. The clinical significance of this for supplements in humans is still being debated.
Forms of carnitine differ: L-carnitine base, L-carnitine tartrate (often in sports products), acetyl-L-carnitine (better studied in the context of the nervous system) and glycine propionyl-L-carnitine. One should use precisely the L-form: D-carnitine is biologically inactive and can compete with the L-form.
In medicine, levocarnitine is a medicinal product for the treatment of primary and secondary carnitine deficiency, including in patients on hemodialysis. CLA has no medical use. This difference well illustrates the status of the substances: carnitine is a physiologically necessary compound with clear medical indications, CLA is a dietary lipid with experimental effects.
Editorial conclusions
CLA is a group of fatty acid isomers that affects lipid metabolism through signaling mechanisms; L-carnitine is a transport molecule without which the oxidation of long-chain fatty acids in the mitochondria is impossible.
Despite the different mechanisms, the result for weight loss in both is similar: a small effect in meta-analyses, much smaller than the impact of a calorie deficit and physical activity.
In terms of safety, both substances have their nuances: the CLA trans-10,cis-12 isomer may worsen insulin sensitivity, and carnitine is linked to the formation of TMAO.
The editorial team broke down the practical choice between them in the article “CLA vs L-carnitine: what to choose and for whom.” We also recommend the materials “Taurine vs betaine: what's the difference” and “EAA vs whey protein: what's the difference.”
References
- Whigham LD, Watras AC, Schoeller DA. Efficacy of conjugated linoleic acid for reducing fat mass: a meta-analysis in humans. Am J Clin Nutr. 2007;85(5):1203–1211.
- Onakpoya IJ, Posadzki PP, Watson LK, Davies LA, Ernst E. The efficacy of long-term conjugated linoleic acid (CLA) supplementation on body composition in overweight and obese individuals: a systematic review and meta-analysis of randomized clinical trials. Eur J Nutr. 2012;51(2):127–134.
- Risérus U, Arner P, Brismar K, Vessby B. Treatment with dietary trans10cis12 conjugated linoleic acid causes isomer-specific insulin resistance in obese men with the metabolic syndrome. Diabetes Care. 2002;25(9):1516–1521.
- Stephens FB, Constantin-Teodosiu D, Greenhaff PL. New insights concerning the role of carnitine in the regulation of fuel metabolism in skeletal muscle. J Physiol. 2007;581(Pt 2):431–444.
- Wall BT, Stephens FB, Constantin-Teodosiu D, et al. Chronic oral ingestion of L-carnitine and carbohydrate increases muscle carnitine content and alters muscle fuel metabolism during exercise in humans. J Physiol. 2011;589(Pt 4):963–973.
- Pooyandjoo M, Nouhi M, Shab-Bidar S, Djafarian K, Olyaeemanesh A. The effect of (L-)carnitine on weight loss in adults: a systematic review and meta-analysis of randomized controlled trials. Obes Rev. 2016;17(10):970–976.
- 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(5):576–585.
Andriy Melnyk
A strength-sports coach and author of programs for beginner and intermediate levels. Writes about training planning.
