Of equal concern is the disruption to available hours of sleep. While fasting, if that's what you want to do, then any eating needs to occur outside of daylight hours and is accompanied by a strict regime of prayer. So, depending on geographic location and time of year, this can mean getting up as early as 3 am for a morning meal (break fast, get it) and not being able to go to bed until 10 or 11 at night.
This inevitably has a major impact on restful sleep patterns. Combine it with flying duties and a bit of ketone on the breath then becomes a minor problem. Though not for the person sitting next to you.