Hello everyone,
While raging, if the barbarian is reduced below 0 hit points, 1 hit point of lethal damage per barbarian level is converted to nonlethal damage. If the barbarian is at negative hit points due to lethal damage, she immediately stabilizes.
Now with regular attacks, one can deal lethal and nonlethal. But what if the barbarian suffers a strong energy damage attack?
Does part of that turn into non-lethal energy, all of a sudden?
Guarded life is for lethal damage the barbarian has already taken. Once damage has been taken, PF does not track whether it came from fire, frost, slashing, piercing, or what-have-you.
But there are ways to deal nonlethal energy damage anyway. A merciful flaming weapon does. So does a flaming scimitar wielded by someone with Sarenrae's Divine Fighting Technique.
All in all, it's not something to worry about.