When to Take L Carnitine for Optimal Training Benefits Transparent Labs L carnitine's acts as a fat burning supplement. During bulking periods, it can help limit fat gains and make a cleaner bulk. If you're cutting, it can help transport the fat you have into L carnitine's acts as a fat burning supplement. During bulking periods, it can help limit fat gains and make a cleaner bulk. If you're cutting, it can help transport the fat you have into L Carnitine for Cutting vs Bulking: How It Works in Both Phases