(Copied reply I made to another comment) Your body is designed to breathe primarily through the nose, and you have more efficient uptake of oxygen by breathing through the nose here's an article that goes over some other benefits. If you are running near VO2 max, you do need the additional volume from breathing through your entire airway, but if you are doing a normal workout (moderate effort, below lactic threshold), you can and should actually breathe through your nose. The reason I think many people take issue with this is because they might breathe out of their nose in general, which again is suboptimal. It's not like your whole life is ruined if you choose not to or can't breathe through your nose, but you should try to the extent that you can.