L-Carnitine is arguably one of the oldest and most popular single-ingredient supplements in the sport of bodybuilding, having been around for some 3 decades. In contrast, its acetylated form, namely, acetyl l-carnitine has only risen to prominence in the last decade or so. With the two compounds differing only in the acetyl moiety, one of the common questions that pop up is ‘which is the better form: l-carnitine or acetyl l-carnitine’?
While there are plenty of supplement experts who may be able to rattle off a prepared answer at the drop of a hat; when it comes to the hard evidence, there’s very little research in humans to fall back on. The only really solid research comparing l-carnitine with acetyl l-carnitine has been conducted in animal models, which makes a straight forward answer not so straight forward. In attempting to answer the question of which form of carnitine is better, this article will provide a brief overview of how the two forms of carnitine are absorbed and metabolised and their relative effects on the levels of different forms of carnitine in the body.
Carnitine Distribution in the Body
Answering the question of which type of carnitine is better requires some basic understanding of how carnitine is distributed in the body and the different forms in which it exists. In terms of its distribution, 90% of the total body stores of carnitine are found in muscle, with the remainder distributed between plasma, kidney and liver. A point in note is that stores of carnitine within muscle are turned over at a much slower rate than those of plasma, kidney and liver. Carnitine exists in the body in both free form and what are called ester forms or carnitine esters. Examples of popular forms of carnitine esters are acetyl l-carnitine, propionyl-l-carnitine and palmitoyl-l-carnitine. When it comes to the relative amounts of the different forms of carnitine in the human body, free carnitine is the most predominant followed by acetyl l-carnitine, which is probably the underlying reason why l-carnitine was the first of the two forms of carnitine to be researched as an oral supplement. Other forms of carnitine such as propionyl-l-carnitine and palmitoylcarnitine occur at much lower levels (i.e. several orders of magnitude).
Absorption of L-Carnitine vs Acetyl L-Carnitine
One of the most popular beliefs concerning the benefits of acetyl l-carnitine is that it is absorbed better than l-carnitine. Sadly though, there have been no robust experiments directly comparing the two forms in humans. The best evidence comes from a recent study in rats, where they tested the effects of oral ingestion of acetyl l-carnitine and l-carnitine on the levels of free, acyl, and total l-carnitine in plasma and brain. The results of this study indicated that l-carnitine was more effective than acetyl l-carnitine at raising plasma levels of both l-carnitine and acetyl l-carnitine as well as total carnitine levels1. But this study was in rats so it’s hard to extrapolate its findings directly to humans.
There are further studies in rats looking specifically at how different forms of carnitine are absorbed in the intestine and subsequently released into circulation. The results of these suggest that acetyl l-carnitine is released into the circulation quicker than l-carnitine in animals such as rats and guinea pigs3. These studies which were conducted in the early 90s probably fuelled the wide held belief that acetyl l-carnitine is absorbed better than l-carnitine. However, as highlighted above, this question remains to be answered in humans.
In one of the only oral bioavailability studies in elderly adults (70–86 years) with senile dementia, taking 2g of acetyl l-carnitine in three divided doses for 50 days was effective in significantly raising plasma concentrations of acetyl l-carnitine. Together with the knowledge on how levels of carnitine are regulated in the body, this study suggests that small divided doses may be the best approach to raise body carnitine levels. This is because research into carnitine metabolism has shown that the body tightly controls the flux and levels of carnitine, such that any dramatic or immediate change in carnitine intake (whether orally or intravenously) results in a significant increase in urinary excretion so as to maintain normal carnitine levels.
Benefits of Acetyl L-Carnitine
One of the distinct benefits of acetyl l-carnitine versus l-carnitine is its effects on neurological health and function. Acetyl l-carnitine is unique in that it can provide acetyl groups for acetylcholine synthesis in the brain, which can help protect the health and development of neurons4. Clinical studies in humans have demonstrated positive effects of acetyl-L-carnitine on brain function, cognition, and memory2, with some even suggesting it may slow or reverse mild cognitive impairment and the progression of dementia in Alzheimer’s disease5. For more detailed information on the benefits and functions of acetyl l-carnitine, check out our acetyl l-carnitine article.
Benefits of L-Carnitine
Compared to acetyl l-carnitine, l-carnitine has been studied much more for its possible beneficial effects on exercise. While there has been a wealth of research conducted on l-carnitine, there are some conflicting results. For the most part, scientists seem to agree that oral supplementation with l-carnitine does not raise muscle levels (i.e. the body’s largest tissue reserve of carnitine) of l-carnitine. Therefore, in the studies that have shown beneficial effects from oral carnitine supplementation are thought to have mediated their effect by changes in plasma, kidney or liver. One of the major developments in l-carnitine research has shown that oral l-carnitine supplementation can in fact lead to an increase in muscle carnitine levels when the carnitine is consumed with sufficient carbohydrate. The carbohydrate induces an increase in insulin, which is thought to be the key mediator for muscle uptake of carnitine. For more detailed information, please see our corresponding article on l-carnitine.
So What Form of Carnitine Should I Take?
So to wrap things up, oral supplementation with either l-carnitine or acetyl l-carnitine leads to an increase in all the endogenous forms of carnitine. The decision as to what form is best to supplement with really depends on ones goals and health. Elderly people looking to supplement with carnitine would be better off taking acetyl l-carnitine simply because of its proven benefits for neurological health. For younger individuals primarily interested in the purported performance benefits of carnitine, they would be best suited with l-carnitine, especially if they can supplement with it at the same time as carbohydrate intake. However, for overall health benefits, acetyl l-carnitine is probably the better of the two, because it boosts free carnitine and carnitine ester levels to a similar extent as l-carnitine, however, it also has superior antioxidant properties and benefits for neurological health.
1. Liu J, et al. Comparison of the effects of L-carnitine and acetyl-L-carnitine on carnitine levels, ambulatory activity, and oxidative stress biomarkers in the brain of old rats. Ann N Y Acad Sci. 2004;1033:117-131.
2. Rebouche CJ, et al. Kinetics, pharmacokinetics, and regulation of l-carnitine and acetyl-l-carnitine metabolism. Ann NY Acad Sci. 2004;1033:30–41.
3. Gross CJ & Savaiano DA. Effect of development and nutritional state on the uptake, metabolism, and release of free and acetyl-L-carnitine by the rodent small intestine. Biochim Biophys Acta. 1993;1170:265–274.
4. Onofrj M, et al. Acetyl-L-carnitine: from a biological curiosity to a drug for the peripheral nervous system and beyond. Expert Rev Neurother. 2013;13(8):925-36.
5. Montgomery SA, et al. 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. Clin Psychopharmacol. 2003;18:61–71.