Ramadan has coincided with the Olympics this cycle and there has been a real discussion about its effects on the athletes who are observing it. Ramadan is the 9th month on the Muslim calendar during which adherents abstain from food and water during the hours the sun is up. This generally involves waking before dawn for a light meal and prayers, then fasting throughout the day until sunset.
Given that a large population has practiced a form of intermittent fasting regularly for over 1000 years for 1 month at a time, preliminary conclusions can be drawn from it.
Ati et al 1995 demonstrated that while bodyweight and BMI do not change, blood lipid profiles improve dramatically. It was found that animal protein and fat sources increases during this time period and that carbohydrate sources are cut drastically.
The Al-Hourani and Atoum 2007 study demonstrated some different data. 57 women from a university in Jordan were studied. During the fast, total caloric intake remained constant but as seen before, the percentage by carbohydrate dropped. In this case, protein remained constant and fat alone made up for the shortfall from the carb cut. BMI and fat mass both showed minor drops during the fast.
The Memari et al 2011 study looked at a female taekwondo team. In 2009 in Tehran, the fast was Aug 22-Sept 20 with an average fast time of 14 hours/day. By the second week of Ramadan, all athletes had lost an average of 1kg and had corresponding BMI and % body fat adjustments. They also performed a battery of tests. They demonstrated that while vertical jump was not affected, agility and balance were slightly impaired by the second week. This is likely due to the fact that a vertical jump relies on the ATP-PC system and would not be directly affected by a fast while completing an agility course or a balance assessment over time would begin to enter other substrates. The variations were minor and within the margin of error but were all in the same direction and should be noted.
Thus, elite athletes who are fasting will find they will lose some weight ad may in fact experience minor performance deficits and one should consider their position.