What is the ruling on a weight-loss competition where each participant pays a sum of money, and the one who loses the most weight wins the entire sum? And what is the permissible way according to Sharia?

This act is considered prohibited gambling (maysir (definition)) in Islamic law, which is any transaction oscillating between gain and loss. It is not permissible to take a prize in competitions except for horses, camels, and arrows, as the Prophet, peace and blessings be upon him, said: "There is no prize except in archery, or hoof (horses), or camel." This is because these are instruments of war that we are commanded to learn. One should focus on religious matters and Islamic sciences. There is no harm in losing weight if it is excessive, for the sake of maintaining health, but without exaggeration or violating Islamic rulings.