What is the ruling on making fasting obligatory upon oneself when one is weak?

Moderation is commendable in all matters, while excessiveness in abstaining from food is blameworthy and can lead to weakness and perdition. The Prophet, peace and blessings be upon him, used to break his fast and forbade continuous fasting (wisal). He also partook in the pre-dawn meal (sahur) and encouraged his nation to do so.