Understanding the Controversy Surrounding L-Carnitine as a Sports Supplement

Sport supplements are pervasive in fitness culture, often marketed as essential for enhancing performance and recovery. Among these, L-carnitine has garnered considerable attention due to its perceived benefits and associated risks. Originally discovered in 1952 and once thought to be a vitamin (termed vitamin BT), L-carnitine is now classified as a quasi-vitamin because the human body can synthesize sufficient amounts in the kidneys, liver, and brain. Nonetheless, it is commonly sold as a dietary supplement and added to energy drinks and protein powders to boost their nutritional value.

L-carnitine naturally occurs in foods, particularly in meat and in smaller amounts in dairy products. Since it was first identified in meat over a century ago, the name 'carnitine' derives from the Latin word 'carnis,' meaning 'flesh.' While dietary intake from these sources is generally sufficient, some athletes and fitness enthusiasts turn to supplements, which raises questions about the potential benefits and dangers.

Proponents suggest that L-carnitine supports muscle recovery and enhances exercise performance. Indeed, some research indicates it may improve muscle function post-exercise. However, the evidence remains mixed and controversial. Critical concerns have emerged related to its metabolic effects, especially regarding its interaction with gut microbes. Less than 20% of supplemental L-carnitine is absorbed by the body; the rest proceeds to the colon, where gut bacteria metabolize it into trimethylamine (TMA). TMA is then absorbed and converted in the liver into trimethylamine N-oxide (TMAO), a compound linked to increased risk of cardiovascular diseases.

Substantial research shows that elevated TMAO levels in the bloodstream can contribute to atherosclerosis and other heart conditions. Studies, including those at the Cleveland Clinic, have demonstrated that nutrients similar to L-carnitine can promote blood clots and other health risks when converted into TMAO. Therefore, while L-carnitine itself offers some health benefits, its supplementation carries potential adverse effects mediated by gut microbiota.

To counteract these risks, recent laboratory studies suggest that dietary interventions might reduce TMA production from L-carnitine. For instance, supplementing with pomegranate extracts rich in polyphenols—plant compounds known for their antioxidant and antimicrobial properties—has shown promise. Ellagitannins, a key polyphenol in pomegranates, can reach the colon mostly intact, interacting with gut microbes to reduce TMA formation.

Our experiments indicate that consuming polyphenol-rich foods like pomegranates, berries, or walnuts may mitigate some of the harmful cardiovascular effects associated with L-carnitine supplements. Ongoing research plans to assess in human trials whether combining L-carnitine with pomegranate extract can effectively lower TMAO levels, potentially making the supplement safer.

In summary, while L-carnitine can support muscle health, its supplementation must be approached with caution due to its complex interactions with gut microbes and the risk of cardiovascular harm. Incorporating polyphenol-rich foods into diets may offer a natural strategy to reduce these risks, emphasizing the importance of balanced nutrition over reliance on supplements alone.