Answered by Shaykh Faraz Rabbani
If I fast with the intention to purify my soul and lose weight for the purpose of regaining my energy level for being able to pray without sluggishness ,will it be blameworthy?
Answer: Walaikum assalam,
I pray this finds you in the best of health and spirits.
There is nothing wrong with fasting sincerely for the sake of Allah, and making a secondary intention of physical benefit.
Secondary intentions do not go against sincerely performing an act for the sake of Allah, as they are understood to be (1) means of seeking Allah’s pleasures and (2) considered expressions of how one is seeking to make one’s action pleasing to Allah.
And Allah is the giver of success and facilitation.