I might think that one who gave it (second-tithe) as a gift to his neighbor (outside of Jerusalem, and he ate it there), he (the giver) was liable; it is, therefore, written "You shall not be able to eat it in your gates" — The eater is liable, but not the giver as a gift.
R. Yossi says: I might think that there was liability only for (eating) tevel (untithed produce) from which nothing had been tithed. Whence is it derived (that there is liability) if terumah had been taken from it, but not first-tithe; if first-tithe had been taken, but not second-tithe; if second-tithe had been taken, but not poor-tithe? From (Ibid. 17) "You shall not be able to eat in your gates," and, elsewhere, (Ibid. 26:12) "And they shall eat it in your gates and be sated." Just as there, poor-tithe (is being referred to), so, here.