For more than 1 billion Muslims around the world, Thursday marks the beginning of Ramadan. It’s the beginning of a month of obligatory fasting; of abstention from food and drink between sunrise and sunset; of purification and self-discipline.

For over 100 soccer players from six predominantly Muslim nations and several others, however, it will also be a month of preparation for the biggest days of their professional lives. One day before the holy month ends, the 2018 World Cup will begin.

Ramadan, therefore, presents a dilemma.

“It’s not easy, for sure, to train and play while I’m fasting,” says Omar Gaber, an Egyptian defender named to his country’s provisional roster for the upcoming tournament in Russia. “But I have to do it. Some players make up [the fast] later. But for me, it’s only 30 days in the whole year. And for us, in our religion, it’s a very good thing to be near to God.”

Other Muslim players will choose differently. Many will take advantage of exemptions, reserved for those who are ill, traveling, or carrying out physically strenuous tasks. Germany’s Mesut Ozil, for example, did not fast when Ramadan fell during the 2014 World Cup knockout stages because, as he said at the time, he was “working.”

View photos Germany’s Mesut Ozil and Sami Khedira will be two of many Muslim stars at the 2018 World Cup. (Getty) More

And there is good reason to delay the fast until a later date. Bodies need fuel. “By the end of Ramadan, there is an increased risk of nutritional and energy deficiency in active athletes,” a document outlining upcoming FIFA-commissioned research on the topic states. “The post-Ramadan effects have been given little attention and exactly how long the effects, if any, will last remains unclear.”

But for professionals wholeheartedly committed to both their faith and their football, the decision can be agonizing. It has been the source of friction between managers and players. A majority of Muslim players featured in a study at the 2012 Olympics said their coaches did not want them to fast. In 2014, France manager Didier Deschamps called it a “touchy subject.” This time around, a French team spokesman told Yahoo Sports that the question, as it pertains to Muslim stars such as Paul Pogba and N’Golo Kante, was a “private matter.”

Fortunately, though, it’s a matter that has been the subject of extensive medical research. And it’s a topic on which experts and team doctors are far more knowledgeable now than they were in the past.

The effects of fasting on performance

Fifteen years ago, according to Dr. Yacine Zerguini, a member of FIFA’s medical committee, “the available literature contained no scientific studies analyzing the interaction of football and Ramadan.”

Since, there have been many. There have been international conferences. And they’ve yielded a few tentative conclusions: that eating only two meals a day – one early in the morning, one late at night – can lead to nutritional deficiencies; that dehydration can be a problem; that sleep loss can lead to daytime fatigue; and that changes to training schedules to accommodate Ramadan can come with their own side-effects.

But none of the studies have been both conclusive and comprehensive. Some have been conflicting. Some have shown no detrimental effects. Others have shown that overuse injuries increase if typical training regimens are maintained. Others have shown that changes to training regimens lead to decreases in physical fitness.

Some players, on the other hand, have said there are benefits. “You clean your body … and you feel even stronger after Ramadan,” Ivorian defender Kolo Toure said in 2013.

Zak Abdel, formerly Egypt’s goalkeeping coach, has a similar anecdote: “When we had [legendary Egyptian midfielder Mohamed] Aboutrika, he was so strong mentally that he played his best when he was fasting.”