L-carnitine is a compound the body makes and that also comes from foods like red meat and dairy. It shuttles fatty acids into mitochondria so cells can turn them into energy.
Many people take it to improve energy, exercise performance, and post-workout recovery. Others use it to support heart function, particularly when carnitine levels are low. Evidence for routine heart health use outside of deficiency is thinner.
Typical doses land between 500 and 2000 mg per day, usually in capsule or powder form. Taking it with a meal boosts absorption. Different forms exist. L-carnitine tartrate and acetyl-L-carnitine among them. And they can differ in how the body takes them up.
Since kidneys clear L-carnitine from the body, anyone with kidney disease needs medical guidance before using it. Severe impairment can cause buildup. Some notice mild stomach upset at high doses.