User blog comment:B214/Question and Answers 6/@comment-90.202.240.151-20161009210010/@comment-29963364-20161010073129

It's actually a mix of both (except that it's his "life-force", not "life-span").

In vol. 17, when Issei is training with Sun Wukong, Issei is trying to learn how to bring his fluctuating aura under control. He does this by raising his power to a certain level, then punching the training dummy with the same amount of power 10 times in a row.

The effects of Issei's fluctuating power and life-force can be seen in vol.21. When Issei uses DxD, his aura is increased to "infinity", but his body couldn't handle that. So after he used it, all the organs in his body stopped working due to the sheer stress, and he had to be hooked up to life-support machines for days.

So essentially, if Issei's aura increases to "infinity", his body will be instently overworked and will shut down. If his aura is decreased to "zero", his body will just stop working because he'll have no life-force.

These are being prevented by Koneko doing regular Senjutsu sessions with Issei to regulate his aura, but if he trains his body and aura, then he can prevent this from happening on his own. At least theoretically.