What is the opinion of Islam on weight loss, and is it forbidden?

A Muslim is required to prepare their body for the performance of religious and worldly duties, in accordance with the Almighty's saying: "And prepare against them whatever you are able of power." This includes physical strength. Excess weight is an impediment to performing duties, a cause of laziness and inability, and a likely source of various diseases. It also involves casting oneself into destruction, which is forbidden by the Almighty's saying: "And do not throw [yourselves] with your [own] hands into destruction." Therefore, losing excess weight is a legitimate objective according to Islamic law, and it may become obligatory if it causes harm, provided that it is done through lawful means.