For the first time since 1986, the World Cup and Ramadan will overlap. Ramadan's a month in the Islamic calendar—a lunar calendar—in which Muslims, among other things, abstain from food and water during the day, breaking fast at sundown. The fasting will start Saturday. There's a substantial number of Muslim athletes still in the World Cup. How will this affect them?
First off, let's make it clear that not every Muslim is required to fast during Ramadan. There are exceptions, like children, the elderly, pregnant women, and others with conditions that require food or water. If you're traveling or sick, you don't need to fast. You're only expected to fast if you're healthy.
The athletes at the World Cup can technically say they're traveling, but they're also all healthy. Muslims in the knockout round include France's Karim Benzema, Germany's Mezut Özil, Switzerland's Xherdan Shaqiri, and various players from Nigeria and Algeria. In the hot, sticky temperatures of Brazil, fasting could significantly affect their performance, if not done properly. One imam in 2011 said that athletes can make up their days of fasting at a later date for a special circumstance—like, say, the World Cup, which only happens every four years. So, it'll mainly depend on their devotion.
There aren't many scientific studies regarding the effect of Ramadan on sports, but there's a bit of anecdotal evidence. Rockets center Hakeem Olajuwon fasted during his NBA career, even during one NBA Finals, and claimed that his play improved. Former Vikings safety Husain Abdullah didn't drink water during workouts, and he was fine. Ron Maughan, a professor of sports nutrition at England's Loughborough University, helmed a study on how Ramadan affected athletes at the 2012 Olympics in London, but he told the Times there are too many variables in soccer to show a tangible effect.
My personal experience of fasting for Ramadan doesn't really involve sports, but here's how it usually goes: The first week (or so) is tough. The abrupt halt definitely affects your body, and you're a little weaker throughout the day. After that first week, though, it gets... easier? Your body gradually accepts the routine, and while you'll still be a little slower during daylight, fasting becomes less of a struggle.
There's a "trick," too, if you can call it that. Since Ramadan involves fasting only during daylight, some Muslims set alarms before dawn to consume water and food. That way, they get to make the most out of their period of breaking fast each day.
The last time I dealt with Ramadan overlapping with athletics was in high school, which was a little easier in the fall, when days are shorter. (Since Ramadan's on a lunar calendar, its beginning and end change every year.) My parents didn't force me to fast; I chose to start when I was 13. I wouldn't eat during school hours, but before soccer games, I'd have water and a sandwich so I could have at least a little energy. When Ramadan overlapped with my wrestling practices in November, I could stick through the entire day without eating or having water after starting with a few half-days.
There's no concrete proof that Ramadan will make athletes perform better or worse. But the Muslims that do choose to fast during the World Cup will—along with their training staffs—do everything in their power to make sure that when they're on the field, no one can even tell that they're fasting.