Depends. Are you a vegan? Do you live in area with lots of bugs? Time of day. More bugs at dawn and dusk.

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They constantly try to escape from the darkness outside and within
Dreaming of systems so perfect that no one will need to be good T.S. Eliot

Given that it wasn't stated in the first post I think we can all safely assume the OP isn't.....

Tony Martin's breathing mouth open position whilst tt'ing has always reminded me of a sperm whale feeding:

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Take a short break from ST and read my blog:
http://tri-banter.blogspot.com/

How do you lunch and run at the same time with your mouth shut?

I actually talk to myself, so ...

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"What's your claim?" - Ben Gravy
"Your best work is the work you're excited about" - Rick Rubin

I guess I am being cheeky. I am not vegan but where I live at the time I run I suck in a few bugs. Which I don't like. I don't see any benefit of not opening your mouth and no way I can run breathing just through nose. If you mouth is open you are breathing through both.

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They constantly try to escape from the darkness outside and within
Dreaming of systems so perfect that no one will need to be good T.S. Eliot

Only if you are Mick Jagger...

A former pro cyclist, coach and nutrition specialist explained to me that I should always be breathing through my nose unless I was trying to keep up with him on the bike. Nose breathing is much more healthier- and you can Google that for more information. I tend to breath through my nose and mouth because of physical restrictions caused by a blow to my nose as a kid. I have been doing the Buteyko Breathing Exercises to help with this, and it seems to be working.

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"Remember: a bicycle is an elegant and efficient tool designed for seeking out and defeating people who aren't as good as you."

--BikeSnobNYC

Or Steven Tyler

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"What's your claim?" - Ben Gravy
"Your best work is the work you're excited about" - Rick Rubin