Why the l-carnitine sport supplement is controversial

Sport supplements are hard to get away from if you like to exercise regularly. Even if you’re not interested in them, there’s a good chance your gym will have posters extolling their virtues or your sporty friends will want to talk to you about them.

It can be hard to know what supplements to take as there is a lot of mixed information out there. L-carnitine is among the more controversial supplements. While there is evidence it supports muscle recovery and enhances exercise performance, research has also shown it can contribute to cardiovascular disease.

In a new study, my colleagues and I found it may be possible to counter the negative effects of l-cartinine by eating pomegranate with it.

First, it’s important to understand what l-carnitine is. Your body produces a small amount of l-carnitine naturally. This happens in the kidneys, liver and brain.

When l-carnitine was first identified in humans in 1952, it was thought to be a vitamin and it was referred to as vitamin BT. After years of research on this compound, l-carnitine is now considered a quasi-vitamin because for most people the human body can produce enough l-carnitine itself.

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L-carnitine can be bought as a dietary supplement, but the nutrient is also added to energy drinks and some protein powders by manufacturers to try and enhance the value of their products. Manufacturers normally clearly state it on the product if it contains l-carnitine – it’s not something a company will try to hide.

Some foods naturally contain l-carnitine, such as meat and in tiny amounts in dairy products. L-carnitine is not fed to livestock but it is present in muscle tissue. L-carnitine was first found in meat in 1905. It is for this reason that the name carnitine is derived from the Latin word carnis, meaning “of the flesh”.

The harmful effects of l-carnitine supplements

It is not thought to be intrinsically harmful. Your gut microbes are to blame for the risks associated with l-carnitine.

Less than 20% of l-carnitine supplements can be taken in by the human body. The unabsorbed l-carnitine travels down the gastrointestinal tract and reaches the colon. The colon is home to trillions of microbes, including bacteria, viruses and fungi.

When the remaining 80% of the l-carnitine supplement arrives in the colon, the microbes start absorbing the nutrient and they use it to produce something else: trimethylamine (TMA). TMA is a compound the human body can efficiently absorb, and that is where the potentially harmful effects of l-carnitine supplements arise.

Once the body absorbs TMA, it goes to the liver via the blood stream. The liver converts TMA to trimethylamine N-oxide (TMAO). Research has shown that high levels of TMAO in the blood can contribute to cardiovascular disease.

For example, a research group at the Cleveland Clinic in the US gave human participants a nutrient similar to l-carnitine that is also converted into TMA by gut microbes. The researchers found that the nutrient caused an increased risk of thrombosis (blood clots) in their participants.

L-carnitine itself is a beneficial nutrient. When it is produced by our bodies, which happens in the kidneys, brain and liver, it’s not metabolised by the gut microbiota and isn’t converted to TMAO. Your body can absorb more l-carnitine from meat than from supplements, which makes it less harmful as that means less of it ends up in the colon.

Dietary intervention can reduce harmful effects

In my team’s lab at the Quadram Institute in Norwich, England, we simulated what happens when the l-carnitine supplement reaches the microbes in the colon. We fed a culture of gut microbes with l-carnitine and measured the TMA that the microbes produced.

Then, we fed a culture of gut microbes with l-carnitine together with a pomegranate extract, which is rich in polyphenols. Polyphenols are plant compounds with antioxidant, antimicrobial, and anti-inflammatory properties that may help keep you healthy and protect you against diseases.

The main polyphenols in pomegranate belong to a group called ellagitannins, a type of polyphenol that can reach the colon almost entirely intact, where they can interact with the gut microbiota. When we measured the TMA that the gut microbes produced in the second experiment, we saw much less TMA.

Our experiments in the lab show that a polyphenol-rich pomegranate extract can reduce microbial TMA production and eliminate the potentially harmful effects of l-carnitine supplements.

Our laboratory experiments showed that the pomegranate extract can reduce the production of TMA. Ellagitannins are also abundant in other fruits and nuts, such as raspberries and walnuts. So, if you take l-carnitine supplements, our research suggests that it may be a good idea to include ellagitannin-rich foods in your diet. Eating more fruits and nuts can be good for your health, so including these in your diet will probably be beneficial anyway.

Our group is now moving the science outside of the lab. We are testing in human participants how effective the pomegranate extract is at reducing TMAO production from l-carnitine supplements. This study will tell us whether taking an l-carnitine supplement along with a pomegranate extract may be better than taking the supplement on its own.

Julia Haarhuis works at the Quadram Institute and receives funding from the Wellcome Trust.