Why is it that burning yourself is bad because it is hurting the temple (the body), but fasting which is not healthy or good for the body(temple), is good and allowed.
Who said that fasting is unhealthy? Done in moderation, it is quite healthy. The point of fasting is not to injur oneself. The point of fasting to help a be aware of how must we love the Lord.
If any penitential act, including fasting, does become injuirous to the body, it ought to be stopped immediately.
Fr. Vincent Serpa, O.P.