What should you know about L-carnitine?

L-carnitine is a molecule naturally present in the body, derived from the amino acids lysine and methionine. It plays a key role in energy metabolism by facilitating the transport of long-chain fatty acids into the mitochondria—the cell’s powerhouses—where they are used to produce ATP. L-carnitine is therefore essential for the proper functioning of the body. From a biochemical perspective, carnitine corresponds to 3-hydroxy-4-trimethylammoniobutanoate. It exists in two stereoisomeric forms: D-carnitine, the inactive form, and L-carnitine, the biologically active form.

The human body can synthesize L-carnitine in sufficient quantities under normal conditions, thanks to the contributions of the liver, kidneys, and brain. It is also found in the diet, mainly in red meat and certain animal-based products. However, its levels can vary depending on age, health status, diet, or exercise habits. The L-carnitine therefore attracts particular interest in both medical and nutritional fields, and its use in the form of dietary supplements is becoming increasingly common.

Did you know? Carnitine derives its name from the Latin word carnis, meaning "meat".

L-carnitine can be obtained in two main ways: through endogenous synthesis within the body or by industrial production for use in dietary supplements and cosmetic applications.

• Endogenous synthesis of L-carnitine : It depends on the presence of lysine and methionine, two amino acids, as well as several cofactors such as vitamin C, vitamin B6, niacin, and iron. These components act at various enzymatic stages, enabling the stepwise conversion of methylated lysine into functional L-carnitine. This process is especially active in the liver and kidneys, from which the molecule is then transported to peripheral tissues.

• Industrial production of L-carnitine : It is generally carried out by microbial fermentation or by stereospecific chemical synthesis. Fermentation uses microorganisms capable of converting precursors such as trimethyllysine into L-carnitine through an enzymatic cascade, such asEscherichia coli. Chemical synthesis, on the other hand, relies on assembling basic organic building blocks but must be carefully controlled to avoid formation of the D-enantiomer, as only the L-form is biologically active.

If the L-carnitine is best known for its role in cellular energy metabolism, it is also increasingly sought after in cosmetics, whether for skin or hair care.

• The L-carnitine for caring for oily skin : L-carnitine could initially regulate sebum production. Indeed, by promoting the transport of long-chain fatty acids into mitochondria, it reduces the availability of intracellular lipids required for sebum synthesis in sebaceous glands. This effect has been confirmed in several clinical trials, including the one presented below.

• L-carnitine may help maintain firm skin : The L-carnitine is also promising for slowing the onset of wrinkles due to its antioxidant properties, which help limit the degradation of collagen and elastin fibers by free radicals. Furthermore, studies in vitro have shown that it inhibits the activity of the metalloproteinases MMP-1 and MMP-2, enzymes involved in the breakdown of the dermal matrix. These mechanisms have clinical relevance: a study conducted on 50 women demonstrated that after 12 weeks of applying a cream containing 1% L-carnitine, skin roughness was significantly reduced compared to placebo, with improved texture and a visible reduction in wrinkles.

• L-carnitine could calm skin inflammation : L-carnitine may also help soothe sensitized skin. Studies in vitro have shown that it can reduce levels of pro-inflammatory cytokines such as TNF-α and the p-NFκB marker, highlighting an overall anti-inflammatory effect. Moreover, a clinical trial evaluated a treatment combining L-carnitine, biotin, panthenol, bisabolol, and α-tocopherol in 30 patients with eczema. For three weeks, participants applied the product or a placebo twice daily. The results showed a significant improvement in lesions (erythema, vesicles, scaling) and a reduction in pruritus in the treated group. However, it is difficult to determine how pivotal the inclusion of L-carnitine in the formulation was.

• L-carnitine could promote skin healing : The L-carnitine could support skin healing by acting on multiple stages of the repair process. It promotes fibroblast proliferation, stimulates collagen synthesis, and accelerates re-epithelialization, thereby contributing to a faster and more robust reconstruction of skin tissue. At the same time, its antioxidant and anti-inflammatory properties help limit oxidative stress and local inflammation, two factors that can slow down the healing process.

• L-carnitine may be able to reduce cellulite : The L-carnitine could help reduce the appearance of cellulite by promoting the mobilization of stored fats in adipocytes. By facilitating the entry of fatty acids into mitochondria via the carnitine palmitoyltransferase 1 (CPT1) complex, it stimulates their oxidation and causes a decrease in cell volume. This is why topical application of L-carnitine, particularly in combination with caffeine, has been associated with reductions in thigh circumference and fat mass in certain clinical trials, including the one presented below conducted with 10 volunteers over 28 days.

• L-carnitine could stimulate hair growth : The L-carnitine could also help prevent hair loss. An in vitro study showed that L-carnitine-L-tartrate (CT) applied to human hair follicles in the anagen phase promoted hair shaft elongation: follicles treated with concentrations ranging from 0.5 to 5 µM exhibited a 37% increase in hair length compared with the control group. However, although these results are promising, only a few clinical trials have been conducted to date, and further research is needed to confirm the effectiveness of L-carnitine on hair growth.

L-carnitine is considered a well-tolerated active ingredient, even on sensitive skin.

Studies conducted on L-carnitine at concentrations typically ranging from 0.5% to 2% have reported no adverse effects (no redness, irritation, or itching). Regarding vulnerable populations such as pregnant or breastfeeding women or young children, data are limited. However, nothing suggests that the topical use of L-carnitine poses a danger for these groups. As a precaution, women planning a pregnancy are advised to consult their physician before use.

• HOPPEL C. The role of carnitine in normal and altered fatty acid metabolism. American Journal of Kidney Diseases (2003).

• BUM-CHUN L. & al. Anti-aging effects of L-carnitine on human skin. Journal of the Society of Cosmetic Scientists of Korea (2004).

• GÜLÇIN I. Antioxidant and antiradical activities of L-carnitine. Life Sciences (2006).

• PAUS R. & al. L-carnitine-L-tartrate promotes human hair growth in vitro. Experimental Dermatology (2007).

• TUZUN S. & al. Preliminary study comparing the effects of locally and systemically applied L-carnitine on the healing of full-thickness skin defects. Scandinavian Journal of Surgery (2010).

• LIBROWSKI T. & al. L-carnitine - Metabolic functions and meaning in humans life. Current Drug Metabolism (2011).

• EFSA Panel on Additives and Products or Substances used in Animal Feed (FEEDAP). Scientific Opinion on the safety and efficacy of L-carnitine and L-carnitine L-tartrate as feed additives for all animal species based on a dossier submitted by Lonza Benelux BV. EFSA Journal (2012).

• WENCK H. & al. Topically applied L-carnitine effectively reduces sebum secretion in human skin. Journal of Cosmetic Dermatology (2012).

• MESSINESE S. et al. Topical treatment of contact dermatitis with natural active principles. Journal of Plastic Dermatology (2013).

• RODRIGUEZ J. & al. Effects of a topical lotion containing aminophylline, caffeine, yohimbe, L-carnitine, and gotu kola on thigh circumference, skinfold thickness, and fat mass in sedentary females. Journal of Cosmetic Dermatology (2019).

• DETUDOM P. & al. Efficacy of anti-sebum moisturizing cream containing 2% l-carnitine and 5% epigallocatechin gallate in seborrhea: A randomized clinical trial. Journal of Cosmetic Dermatology (2023).

• ALI F. & al. Anti-pruritic effect of L-carnitine against chloroquine-induced pruritus mediated via nitric oxide pathway. BMC Pharmacology and Toxicology (2024).