From what i understand breathing from your diaphragm through your nose brings more oxygen to the lower lobes of your lungs, which in the long run is more efficient. Mouth breathing tends to be shallow, more erratic, less controlled.
Interesting. I've got to wonder if this is really something that means much though. Your breath flows through the trachea to get to your lungs regardless of the entry point so I don't get how the efficiency can be that different. Your diaphragm and other abdominal muscles control your breathing regardless of nose or mouth. I suppose it could be like intake runners in a fuel injection system, depending on the length and curvature could be different efficiency for different conditions (but thats because we are talking about 1,000 cubic feet of air per minute, I dont think the rate at which we breath is going to make much difference in that regard). For me I just can't pass anywhere near the volume of air I need through my nose for most of my runs. Now on a run where I'm feeling very calm and efficient and its cool, my heart rate is lower than usual and I'm not breathing hard and jogging easy, then I can. Perhaps the trick to the nose breathing is to be a more efficient runner. What I mean is nose breathing could be a result of running more efficiently, not really a cause. My heart rate seems to be improving on some of my runs lately, and I can nose breath a little more on those. I think if I could drop 10 beats of my typical rate when I run, nose breathing would be much more possible. But I think the limit of what I can nose breath is going to always be a lot less than some people since my nasal passage ways just don't flow as much.