Dear Claire:

It is an excellent thing to "awaken" to your faith and turning your life around. Praise God.

As for fasting we must begin with what is required. The Church requires a Fast only two days per year -- on Ash Wednesday and Good Friday; plus one hour before receiving our Lord in the Eucharist. While "devotional" fasts can be great religious exercises in mortifying the body, they are not required.

See How to Think Properly About Fasting for more information.

As to your specific question, it is not a problem to have thoughts in the back of your head that the fast could help your diet -- as long as it stays in the back of your head.

St. Paul tells us in 2 Corinthians 10:5: "...take every thought captive to obey Christ"

Whenever any improper thought comes to mind, rebuke that thought asking God to take it away, and then focus your mind on something else, like the Rosary or some other prayer.

There is nothing wrong with losing weight as a result of one's devotional fasts, per se, but it is true that the fast needs to be for the love of God and not for vanity sake.

Besides, a usual fast is not really going to do much to make you lose weight, especially if you fast according to the Church's definition of one full meal, plus two other meals that together fall just short of the equivalent of one fill meal. In otherwords, you can have 1 7/8 meals during the day.

If you desire to make devotional fasts do not fast more than one or two days a week. If you really have a problem of considering the fast a diet, then do your devotional fast only once a week and up to the limit of food per the Church's definition. This should preclude the idea that the fast is a weight-reduction diet since fasting like this will really not cause weight reduction.

Do not fret, though. It takes time and practice to "take captive every thought". Keep doing your devotions and ignore the back of your mind :)

God Bless,
Bro. Ignatius Mary