Best Time to Take L Carnitine: A Complete Guide The Science of Good Health When to Take L Carnitine for Optimal Training Benefits Transparent Labs L Carnitine for Cutting vs Bulking: How It Works in Both Phases L carnitine is involved in transporting fatty acids into your cells' mitochondria to be burned for energy. During a bulk, the idea is to gain muscle, ideally with minirmal fat gain. Here's how,