Does Cardio support or hinder muscle gain

There are 2 answers to this:

Yes: Cardio exercise (running, rowing etc) improves the efficiency of your heart and cardiovascular system. Cardio forces blood around the body increasing the size of blood vessels and capilleries that are the transport routes for nutrients to muscle. A stronger cardiovascular system aids muscle growth by supplying vital nutrients when needed.

No: Protein is broken down during intense cardio (from muscle) and converted to energy. This does waste muscle (leg muscle while running etc).

Although muscle is broken down during cardio it can take a while. If muscle building is the priority keep the cardio moderate for under 45 minutes, or do intense intervals for less than 30 minutes. It is a bad idea to avoid cardio to keep muscle gains. Aslong as you are fuelling yourself properly with well timed carbs and consistent protein, you should only be losing the unwanted belly fat.