Maughan led a working group that examined the effects of fasting on athletes at the 2012 London Olympics, the last time Ramadan coincided with a major sporting event. He found that fasting had an effect on athletes competing in the events requiring the greatest strain, like the marathon, but that the impact was relatively small over all.

The problem lay in trying to measure the effect in a complicated, multifaceted sport like soccer.

“There are so many variables in football that it would be almost impossible to show an effect,” Maughan said.

He added: “In track athletics, 0.1 percent or even 0.01 percent may make the difference between winning and losing. In football, there are many factors affecting outcome, but games are often decided by the smallest of margins.

“It might be fair to say there is no large effect,” he said, suggesting that a team would not have a significantly improved chance of beating a powerhouse like Barcelona even if all the Barcelona players were fasting.

Ahead of this World Cup, FIFA commissioned several studies into the effects of fasting.

“We made an important study with the doctor of the Algerian Football Federation, and it was very positive,” said Dr. Michel D’Hooghe, the chairman of FIFA’s medical committee and a member of its executive committee. “If you do it intelligently, then you can adapt perfectly. Before the sun comes up, they have enough hydration to go on through the whole day.”

Islam provides some exemptions to fasting for Ramadan, for the sick and for those who are traveling, and the United Arab Emirates’ soccer team was granted an exemption during the 2012 Olympics by the country’s highest religious body on those grounds. Muslims are also permitted to catch up on missed fast days at a later date, although this is sometimes frowned on in the Middle East and parts of Africa.