Non medical reply -
The worst effects of fasting as related to flying would be low blood glucose levels. This is something which diabetes sufferers and those close to them will be familiar with. The loss of rational thought which begins as hypoglycaemie begins to take hold would render a person incapable of flying. For non diabetic people, this would only be apparent after several days of prolonged fasting, unless the person was extremely thin to begin with.
Ramadan fasting is only through the daylight hours, followers can 'tank up' all they like during the hours of darkness, and usually do. Muslim friends commonly relate to me that they put on weight during Ramadan. Thus, the most that a Ramadan observer would suffer would be about 12 hours of food deprivation and hypoglycaemia should not be a problem. A prolonged west-bound flight extending the daylight hours may be a problem. What is of concern to me is possible dehydration, the effects of which will be obvious well before the effects of loss of food intake.
Regards,
Old Smokey