These are the Rebbe's letters to individuals about their own situations. Throughout, the Rebbe instructed people to consult an expert doctor and follow his guidance; anyone facing a medical question should do the same.
Many wrote to the Rebbe wanting to fast or afflict themselves, whether as teshuvah or out of piety. His guidance is consistent:
In our generation, serve Hashem through a healthy body and joy, not through self-affliction. Preserving one's health is itself serving Hashem, and self-affliction and neglect of the body are not the path for teshuvah today (IK 1151, 1673, 1683, 1755, 1897, 2780, 3153, 4614). Chabad tradition discourages frequent fasting and all-night vigils; keep a healthy routine centered on Torah study (IK 2583). Self-imposed fasts and skipped meals end up harming davening and learning (IK 3399, 5489, 6244).
What to do instead. Replace voluntary fasts with tzedakah, with added Torah study, especially Chassidus, and with communal prayer (IK 199, 4151, 4287, 6003, 7344, 8680). Real refinement comes from inner discipline: guarding speech, sight and thought and controlling one's desires, rather than physical deprivation (IK 4196, 4720). Fasting, where it is done, should be a means to spiritual awakening and not an end in itself (IK 3194); where fasting is called for, it may be done in halves (IK 8525, 9028).
Required fasts and health. Yom Kippur and Tisha B'Av are obligatory, but one should minimize exertion and ask the doctor how to manage the fast (IK 6951). Unnecessary stringency against medical advice is not the Torah's way (IK 3761).