A weak person should tell himself: “I am strong!” It is specifically at a time of weakness that the person’s true character is apparent — it can prove that he loves Hashem with all his heart and that he will not abandon the service of Hashem for any reason but that he will push himself to do the will of his Creator. Our Sages commented on the verse, This is the teaching regarding a man who would die in a tent (Bamidbar 19:14): “Even in the hour of death one should be engaged in the study of the Torah” (Shabbos 83b). The greater the hardship, the greater the reward.
However, a person who is physically weak is obligated to put in the effort to have himself treated medically, and he must eat and drink more than he could normally afford. This is considered a mitzvah and Hashem will repay him for it. This can be seen from the teaching of the Sages, who said: “If one makes light of the meal after bleeding [i.e., the meal they used to eat after a blood-letting in order to regain their strength] his food will be made light of by Heaven, for they say; He has no compassion for his own life, shall we have compassion upon him!?” (ibid. 129a).
There is a time for everything. There is a time for fasting and for withholding luxuries, but there is also a time to do one’s best to heal one’s body. Our Sages admonished that one should always be careful to eat the bread of breakfast, and they enumerated several benefits of eating bread for breakfast. It is therefore proper to eat immediately after the morning prayers, without fail.