Blog Post

L-Carnitine – Is it Worth Taking?

L-Carnitine Feature Image

L-Carnitine is a naturally occurring amino acid that is proposed to help with a wide variety of things. Our body produces it internally, and we also get it from food.

The most touted benefit of it is potential improvements in fat loss. Outside of that though, there are claims surrounding athletic performance, brain function and fertility.

This post will give a broad overview of these topics, and what the research shows.

Fat Loss

One of the main roles of l-carnitine is to help transport fatty acids into the mitochondria.

This mechanism has obviously made people consider the possibility that it can help with fat loss overall.

This can be complex since fat transportation does not necessarily translate to fat loss.

Energy Balance Scales

Fortunately, we have a lot of research directly looking at that topic.

The research is relatively mixed.

For example, an 8-week study involving 36 overweight women found no difference in body mass or body fat in comparison to placebo.

It is worth noting that 5 women dropped out due to side effects of nausea and diarrhea though.

A systematic review from 2016 looking at all 9 of the randomized controlled trials on the topic found an average of 1.3kg lost though.

A positive interpretation of this could be that it indicates a small benefit.

A sceptical interpretation could focus on nausea and diarrhea being common side effects. Could they explain this small difference? Maybe those symptoms result in a reduction in calorie intake.

Personally, I am of the interpretation that supplementing L-Carnitine likely does not do much for fat loss, under most circumstances.

Athletic Performance

Athletic Performance - Sprinter

Theoretically, l-carnitine supplementation could help athletic performance via:

  • Improving fat transportation from the bloodstream into muscle mitochondria for use as fuel.
  • Buffering excess acetyl-CoA, which can help maintain the rate of fuel delivery from glucose and reduce lactate accumulation.
  • Playing a role as an antioxidant, which could reduce muscle protein breakdown and help recovery.

Theoretically, for l-carnitine supplements to do this, they would need to increase muscle carnitine stores noticeably. That is a debated concept, so we will look at the research.

One study on cyclists compared l-carnitine supplementation in omnivores and vegetarians. Theoretically, vegetarians will get more benefits due to lower dietary carnitine intake.

In this study, they found no increase in muscle carnitine levels in omnivores, but a 13% increase in vegetarians.

From a performance perspective though, this did not actually translate to any improvements in performance.

Looking directly at performance, a review of all the research up to 2018 found 11 clinical trials.

This review found a bunch of benefits related to fat oxidation, lactate levels, heart rate and VO2 Max. Importantly though, improvements in actual performance were not identified.

A Caveat to The Athletic Performance Discussion

Some people have highlighted that if l-carnitine is supplemented alongside carbohydrate intake, it improves absorption and uptake into the muscles.

One key study has addressed this topic. In this trial, participants supplemented 2g of L-Carnitine alongside 80g of carbohydrates, twice per day, for 24 weeks.

To make it clear, that is 160g of carbohydrates total consumed alongside l-Carnitine, before factoring in any other carbohydrates being consumed as part of the regular diet.

In this study, muscle carnitine levels increased by 21%. Participants were omnivores, so this is a significant finding in comparison to previous studies.

Increased carnitine levels in response to supplementation

When undergoing 30-minute cycling tests, it was found that participants utilised 55% less glycogen and had a significant increase in fat oxidation. Performance improvements were seen at 24 weeks, but not at 12 weeks.

On the one hand, this could be an exciting finding. From another perspective, the practical utility needs to be considered, even if this outcome was consistently repeatable.

Consuming 160g of carbohydrates alongside a supplement is a big commitment.

The benefits of the supplement were so small that no difference was found at 12 weeks. Meanwhile changing your diet significantly to time that much carbohydrate around supplementation is a major change. In most cases, it will result in people increasing their overall carbohydrate and calorie intake.

In that study, they found participants gained 1.8kg of body fat over that time frame. It is likely that overall calorie intake increased in the process of trying to consume that carbohydrate alongside l-carnitine.

Delayed Onset Muscle Soreness

There is potential for l-carnitine to help reduce delayed onset muscle soreness (DOMs).

A review on this topic specifically highlighted that l-carnitine reduces muscle protein breakdown, markers of cellular damage and free radical formation.

The increase in muscle l-carnitine content is linked with enhanced blood flow and oxygen supply to the muscle, which is one of the mechanisms linked to these improvements.

While there is potential that l-carnitine can help with DOMs, it probably is not a front-line type of option. Considering the debate around the level of absorption in the muscle, this point is even more relevant.

Brain Function

Acetyl-l-carnitine metabolism in the brain

Acetyl-L-carnitine is a form of carnitine that crosses the blood-brain barrier more efficiently than l-carnitine. This is the form that is often linked with brain function.

There are some promising studies indicating improvements in dementia.

Other studies on brain function in the elderly without dementia have also found improvements.

It is not clear cut though.

A review of 21 studies of acetyl-L-carnitine compared to placebo found improvements in those with mild cognitive impairment or dementia.

Meanwhile, a Cochrane review that contained a lot of overlapping trials with that other review found improvements at 24 weeks, but not 12 or 52. Their conclusion was that recommending acetyl-L-carnitine for this purpose was not justified.

Those reviews were from 2003, but even as recently as 2020, there still is not a not a clear consensus.


One area worth briefly touching on is that carnitine deficiency exists as well. It is generally caused by genetics or medical conditions.

Supplementing with l-carnitine is usually part of the management of this condition. If this is not done, serious problems can occur, often related to muscular issues, or heart and liver problems.

Although there are questions about how effective oral supplementation is for increasing muscle carnitine levels in the average person, it clearly is an effective treatment for carnitine deficiency.

Fertility and Polycystic Ovary Syndrome

PCOS, inositol and pregnancy

One study on women with PCOS found significant improvements following l-carnitine supplementation. It involved 170 women taking 3g/day of l-carnitine from day 3 of their menstrual cycle, until their first positive pregnancy test results.

Compared to placebo, the women in the l-carnitine group:

  • Had 64.4% higher ovulation rates.
  • Thicker endometrial tissue (10.1mm vs 6.8mm).
  • Had fewer miscarriages.

A systematic review of l-Carnitine and PCOS found consistent small improvements in lot of markers such as body mass, cholesterol, and triglycerides.

Contrary to the promise that the prior individual study found, the review did not find any significant improvements in pregnancy or ovulation rates.

In terms of male fertility, l-carnitine has been found to have some benefits for factors such as sperm motility, but it has not translated to improved pregnancy rates.

Dosage and How to Take

L-Carnitine, Now Foods

The standard dosage for l-carnitine is generally around 2g per day.

Research has found that even at dosages of 3g per day, side effects of nausea, abdominal cramps, diarrhea, and changes in smell are not uncommon.

Some studies have used split dosages of 2g in the morning and 2g in the evening to help avoid some of those side effects though.

To optimise absorption, it is best to consume l-carnitine alongside carbohydrates. The research involving taking 2g l-carnitine alongside 80g of carbs, two times per day, increasing muscle carnitine by 21% is a good example of this.

The downside of that is just that it could be impractical for a lot of people to use that strategy. For example, if somebody was taking l-carnitine for fat loss or PCOS, it would be counterproductive to implement that.

Another option that has not been researched extensively is injectable l-carnitine.

It is an option that has been explored as a performance enhancer in the real world among certain athletes. Theoretically, it gets around the issue of the difficulty in increasing muscle carnitine levels. It is a bit beyond the scope of this post though.

Practical Summary

On a tier list of supplements that have good evidence of being effective, l-carnitine would be ranked relatively low.

It shows promise in certain areas. But overall, the outcomes are mixed.

When you factor in the difficulties seen with significantly increasing muscle carnitine stores, it is easy to see why the research is so mixed too. And if improving this requires the consumption of a significant amount of carbohydrates alongside supplementation, that adds other challenges too.

By Aidan Muir

Aidan is a Brisbane based dietitian who prides himself on staying up-to-date with evidence-based approaches to dietetic intervention. He has long been interested in all things nutrition, particularly the effects of different dietary approaches on body composition and sports performance. Due to this passion, he has built up an extensive knowledge base and experience in multiple areas of nutrition and is able to help clients with a variety of conditions. One of Aidan’s main strengths is his ability to adapt plans based on the client's desires. By having such a thorough understanding of optimal nutrition for different situations he is able to develop detailed meal plans and guidance for clients that can contribute to improving the clients overall quality of life and performance. He offers services both in-person and online.