If a person is certain or confident that fasting will cause him significant harm or he deems this probable, and the probability creates fear in him, in the event that his deeming it probable would be considered by rational people to be reasonable, it is not obligatory for him to fast.
In fact, if that harm would result in him dying or losing a limb, fasting is unlawful; otherwise, if he fasts with the intention of rajāʾ and afterwards he realises that it did not cause him any significant harm, his fast is valid.