Hey 
Xなければ is negative ば form (negative conditional), meaning “if (someone does) not (do) X then”.
It is used in Xなければいけない(ならない、etc), the “must” construction, but if we were to translate it a bit more literally, it means "if not X then it will be not good (いけない - “not good” in this case).
So なければ can be either contraction of the must construction, or simple negative ば form itself. It all depends on the context.
In this case, we don’t have any context so we have to use common sense:
Even ice cream can do you good. That is if you eat too much.
doesn’t make much of sense, so alternative:
Even ice cream can do you good. That is if you don’t eat too much.
is the correct translation.
I hope it helps,
Cheers 



