L-carnitine and acetyl-L-carnitine (often sold as ALCAR) sound interchangeable, and both are forms of carnitine, a compound the body uses to move fatty acids into mitochondria to be burned for energy. But the research on them has gone in different directions. Most L-carnitine studies are about exercise and recovery. Most acetyl-L-carnitine studies are about memory and thinking in older adults, usually people with cognitive impairment. Which one makes sense depends on which of those questions you are asking.

This article is educational and is not medical advice. Supplements are not intended to diagnose, treat, cure, or prevent any disease. Talk to a qualified clinician before starting anything new, especially if you take medication, are pregnant or breastfeeding, or manage a health condition.

The chemical difference

Acetyl-L-carnitine is L-carnitine with an acetyl group attached. The body makes both, and red meat is the main dietary source of carnitine. The acetyl group matters because it can be handed off inside cells: it feeds the pool of acetyl units the brain uses in energy metabolism and in making the neurotransmitter acetylcholine (Pettegrew 2000; Nałecz 2004).

Does ALCAR reach the brain better?

This is the most repeated claim about ALCAR, and it is weaker than it sounds. Carnitine's entry into the brain is limited by the blood-brain barrier and depends on specific transporters (Nałecz 2004), and the case for the acetyl form rests mostly on that kind of preclinical and mechanistic work. We found no human study that directly compares how much of each form reaches the brain. The Cochrane review of ALCAR in dementia itself calls for more work on how ALCAR behaves in the human body (Hudson 2003). The stronger reason to pick ALCAR for a cognitive goal is simpler: it is the form the cognition trials actually used.

What each is studied for

  • Acetyl-L-carnitine and cognition: a 2003 meta-analysis of trials in mild cognitive impairment and early Alzheimer's disease (1.5 to 3 g/day for 3 to 12 months) found a small advantage over placebo (Montgomery 2003). The Cochrane review of trials in Alzheimer's disease found a benefit on clinicians' overall impression at 12 and 24 weeks, but no benefit on objective measures of cognition, severity or daily function, and concluded there was no evidence to recommend routine use (Hudson 2003). There is no comparable trial base in healthy adults, which is why our profile grades it C.
  • L-carnitine and exercise: a 2018 review reported that L-carnitine supplementation reduced markers of muscle damage and soreness after exercise (Fielding 2018). That review was sponsored by a manufacturer of L-carnitine, which is worth weighing.
  • Neither form has good evidence as a same-day focus aid. The cognition trials measured effects over months, not hours.

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Doses used in studies

  • Acetyl-L-carnitine: 1.5 to 3 g/day in the cognition trials, usually split into two doses.
  • L-carnitine: the thyroid trial below used 2 or 4 g/day. Sports products often list L-carnitine L-tartrate, a salt form.
  • Check which form a label lists and how much of it. A gram of a salt such as L-carnitine L-tartrate is not a gram of carnitine.

Cautions that apply to both

  • Thyroid: in a randomized trial, L-carnitine at 2 or 4 g/day acted as a peripheral antagonist of thyroid hormone, reversing symptoms of excess thyroid hormone in women on high-dose levothyroxine (Benvenga 2001). That is a reason for anyone with an underactive thyroid, or taking thyroid medication, to check with their clinician before taking either form.
  • Heart and TMAO: gut bacteria turn dietary L-carnitine into TMAO, a compound linked to cardiovascular risk in observational data and to atherosclerosis in mice (Koeth 2013). That research studied L-carnitine, not acetyl-L-carnitine, and does not show that supplements cause heart disease, but it is worth knowing if you have heart disease or high cardiovascular risk.
  • Blood thinners: two published case reports describe L-carnitine strengthening the effect of acenocoumarol, a warfarin-type anticoagulant (Martinez 1993; Bachmann 2004). Both involved L-carnitine, not acetyl-L-carnitine, and two cases cannot show how common this is. If you take a blood thinner or any other prescription medication, ask your prescriber or pharmacist before starting either form.
  • For acetyl-L-carnitine, the side effects our profile lists are stomach upset, a fishy body odor at higher doses, and restlessness or poor sleep if taken late in the day.
  • Pregnancy, breastfeeding, seizure disorders and kidney disease: use only with a clinician's guidance.

Which one to choose

If your interest is memory or thinking, acetyl-L-carnitine is the form with the relevant trials, but those trials were mostly in older adults with cognitive impairment, and the results were modest and mixed. Expect little from it as a focus aid in a healthy, well-rested adult. If your interest is exercise recovery, L-carnitine is the form that research used. Neither is a stimulant, and neither should be judged on how you feel the same day.

Acetyl-L-carnitine's dose range, evidence grade, cautions and references, in one place. See the full acetyl-L-carnitine profile

The bottom line

L-carnitine and acetyl-L-carnitine are two forms of one nutrient with two different research records. ALCAR is the form studied for cognition, with modest and mixed results mostly in older adults with cognitive impairment; L-carnitine is the form studied for exercise recovery. The popular claim that ALCAR reaches the brain better rests on preclinical work, not on human comparisons. Pick the form that matches the question you are asking, and check the thyroid caution first.

References

This article draws on the research below; see the linked studies for full methods and doses.

  • Montgomery SA, Thal LJ, Amrein R. "Meta-analysis of double blind randomized controlled clinical trials of acetyl-L-carnitine versus placebo in the treatment of mild cognitive impairment and mild Alzheimer's disease." International Clinical Psychopharmacology, 2003;18(2):61–71. PMID: 12598816.
  • Hudson S, Tabet N. "Acetyl-L-carnitine for dementia." Cochrane Database of Systematic Reviews, 2003;(2):CD003158. PMID: 12804452.
  • Pettegrew JW, Levine J, McClure RJ. "Acetyl-L-carnitine physical-chemical, metabolic, and therapeutic properties: relevance for its mode of action in Alzheimer's disease and geriatric depression." Molecular Psychiatry, 2000;5(6):616–632. PMID: 11126392.
  • Nałecz KA, Miecz D, Berezowski V, Cecchelli R. "Carnitine: transport and physiological functions in the brain." Molecular Aspects of Medicine, 2004;25(5–6):551–567. PMID: 15363641.
  • Fielding R, Riede L, Lugo JP, Bellamine A. "L-Carnitine Supplementation in Recovery after Exercise." Nutrients, 2018;10(3):349. PMID: 29534031.
  • Benvenga S, Ruggeri RM, Russo A, et al. "Usefulness of L-carnitine, a naturally occurring peripheral antagonist of thyroid hormone action, in iatrogenic hyperthyroidism: a randomized, double-blind, placebo-controlled clinical trial." Journal of Clinical Endocrinology and Metabolism, 2001;86(8):3579–3594. PMID: 11502782.
  • Koeth RA, Wang Z, Levison BS, et al. "Intestinal microbiota metabolism of L-carnitine, a nutrient in red meat, promotes atherosclerosis." Nature Medicine, 2013;19(5):576–585. PMID: 23563705.
  • Martinez E, Domingo P, Roca-Cusachs A. "Potentiation of acenocoumarol action by L-carnitine." Journal of Internal Medicine, 1993;233(1):94. PMID: 8429297.
  • Bachmann HU, Hoffmann A. "Interaction of food supplement L-carnitine with oral anticoagulant acenocoumarol." Swiss Medical Weekly, 2004;134(25–26):385. PMID: 15340883.