Not Fasting due to Illness

It is a well-established principle of Shariah that, it is permissible not to fast or break an already kept fast due to reasons that are beyond one’s control, such as travelling, illness, pregnancy, breastfeeding, etc, although a traveller may not enter into a fast, but it will not be permissible for him to break an already kept fast. (See: Radd al-Muhtar, 2/421)

Allah Most High says:

“(Fasting) for a fixed number of days; but if any of you is ill or on a journey, the prescribed number (should be made up) from days later.” (al-Baqarah, 184)

Regarding illness, the great Hanafi jurist, Imam al-Haskafi (Allah have mercy on him) states in the chapter of “excuses that permit not to fast” in his excellent work Durr al-Mukhtar:

“(It is permissible not to fast) for an individual who is ill and fears that his/her illness will increase, or a healthy person who fears that he will become ill due to fasting.” (Radd al-Muhtar ala al-Durr al-Mukhtar, 2/422)

Thus, there is no sin for not fasting or breaking a fast that was already kept due to illness such as diabetes in your case. The compensation will be (as mentioned in the verse of the Qur’an quoted earlier) making up for the fast on another day (qadha) when you are reasonably able to do so.

And Allah Knows Best