Acts of worship are performed solely to attain the pleasure of Allah. When considerations other than seeking Allah’s pleasure are introduced, the sincerity (ikhlāṣ) of the act is compromised, and the act of worship stands on the verge of losing its devotional character. Therefore, all actions performed with the purpose of servitude—foremost among them acts of worship—should be grounded purely in the pursuit of Allah’s pleasure.
For this reason, adopting the approach of “while I am dieting to lose weight, let me fast and thereby clothe it with the form of worship” is not religiously sound, as it undermines sincerity. This is because fasting is, in principle, undertaken with the primary intention of worship. That said, if weight loss occurs as a natural consequence of fasting, there is no objection to this outcome.