Should Cyclists Supplement with Carnitine?

Carnitine is most well-known in the body-building community as a fat burner. But there are many more claims around this supplement that could be interesting to cyclists. Here is a list of the most important ones.

Carnitine is involved in the process of burning fat for energy. During fasted or low-to-mid intensity endurance rides, fat is the main source of energy. Fat molecules need to be transported to mitochondria, which is the powerhouse of the cell, where it’s transformed into usable energy. That’s where carnitine helps. The problem is that muscles have about 1,000x higher carnitine concentration than blood.

To move carnitine from the blood into the muscles, our body uses a transport protein called OCTN2. But there’s a limit to how much carnitine OCTN2 can move. Even if we take extra carnitine as a supplement, it doesn’t usually increase the amount in our muscles. If we can’t get more carnitine into our muscles with a supplement, it can’t produce the claimed effects.

Several studies have shown that oral carnitine intake doesn’t alter muscle carnitine levels. Even direct carnitine infusion failed to increase its concentration in muscles. The carnitine supplementation was unable to increase muscle carnitine concentration in these studies for more than one reason:

This would mean that carnitine can’t produce any of the claimed fat-burning benefits. However, there might be a solution for the low carnitine uptake. Some studies suggest that muscle carnitine levels can be elevated if carnitine is consumed when insulin levels are high. This strategy involves daily carnitine supplementation accompanied by high carbohydrate intake. While this method might increase muscle carnitine, it’s not very practical for weight loss. High carbohydrate intake may even lead to weight gain rather than increased fat oxidation and fat loss.

If carnitine supplementation could successfully increase carnitine levels in the muscle during exercise, then it could enhance fat oxidation during exercise, conserving muscle glycogen and delaying fatigue. This is how it could theoretically be beneficial for cyclists looking to boost endurance. An early study has shown that after 14 days of consuming 4-6 grams of carnitine daily, there was no increase in muscle carnitine levels during high-intensity sprint cycling.

A different study showed that prolonged supplementation with carnitine tartrate alongside carbohydrates did increase muscle carnitine. It also showed that glycogen was spared during low-intensity exercise, which would suggest increased fat oxidation and lactate accumulation was lower during high-intensity exercise. Overall, these changes were associated with an 11% improvement in a 30-minute work-output exercise performance trial.

Studies show that it’s really difficult to increase carnitine concentrations in muscle using supplements. The only way to reliably achieve this is long-term carnitine supplementation with a very high carbohydrate intake. This may bring a modest benefit to exercise performance. When it comes to weight management, carnitine doesn’t seem like a practical supplement because you need to be regularly loading on carbs. Overall, there isn’t a strong incentive for carnitine supplementation for cyclists.