If the Doctors believe that fasting may be dangerous and life threatening for this person then he should not fast and cover his missed fasts after Ramadan when he recover. I recommend he takes a second opinion from a Muslim doctor,

my uncle is diabetic and is on insulin…his doctor has asked him not to fast since his sugar level fluctuates…what should he do?

Fasting | June 22, 2015 4:58 pm
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What is the ruling on fasting but thought of breaking the fast due to hungry/thirst but realised shaytaan playing with his head, is this person fast valid?
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I have a question concerning the the time when suhoor time ends. Allah says in the quran “and eat and drink until the white thread (light) of dawn appears to you distinct from the black thread (darkness of night)” [al-Baqarah 2:187]. Practically we are unable to say when this dawn breaks. is it the time of fajr? does fajr starts with this dawn?