Ramadan and Fasting
When I worked in the middle east this subject was of particular concern to me. I have experienced the overall feeling of not being quite 'up to it' when the blood sugar levels are low just through missing breakfast so the idea that a pilot would go without food all day really worried me.
I spoke with as many of the muslim pilots that I could and by far and away the majority said that they definitely could eat during the day if the nature of their work required it and flying definitely fell into this category. It was up to them if, subsequently, they arranged to fast for a number of days equal to the ones they had missed during Ramadan.
That said, I have flown with co-pilots who not only insisted on fasting but also would get out a prayer mat on the flight deck and pray!