I see prohibitions in the Bible against taking the Lord’s name in vain, and against gluttony.
However, I don’t see any prohibition being fat, unless you automatically associate being fat with gluttony (it is interesting to not that gluttony is nearly always associated with drunkenness in the Bible). Nor do I see one against smoking.
‘Your body is the temple of the Holy Spirit.’
I believe it's clear that a Christian doing anything to harm oneself body which is the Temple of God is a sin...