Germany's Mesut Ozil will not observe Ramadan when it starts on Sunday but many World Cup players who do follow the Muslim fasting month will be under strict medical surveillance.
While Islamic Iran and Bosnia, which have a sizeable Muslim population, have been knocked out of the touranment of the tournament, Algeria has qualified for the last 16 for the first time and they will face a quick Ramadan test on Monday against Ozil's Germany.
Religious authorities in several countries take a pragmatic attitude to football and Ramadan when eating is not allowed during the daylight hours.
In 2008, the Dar al-Ifta, Egypt's main Islamic body, allowed professional footballers to eat during Ramadan if they were bound by contracts to play during the holy month and they felt that fasting will impact their performance.
Other workers involved in "hard labour" are also given a dispensation.
Ozil said he falls into this category.
"I can't take part," said Arsenal's attacking midfielder who added that the World Cup is "working". "It will be impossible for me to take part this year." The Algerian team will nearly all be fasting when they battle Germany in Porto Alegre, however.
The Algerians are using Hakim Chalabi, a sports medicine specialist at the Aspetar clinic in Doha and one of FIFA's leading experts on fasting footballers.
"It is a period when the risk of injury increases, especially in the lower back, the joints and the muscles" said Chalabi. "This is mainly because of dehydration and not the lack of eating." Players can lose up to six litres (11 pints) of fluids during a match.
The expert, a former medical chief at French football giants Paris St Germain, said the level and quality of nutrition had to be changed to cope with exercise during Ramadan.
"The players must hydrate themselves better. We also advise them to take a longer siesta during the afternoon to make up for some of the lost sleep." Muslims sleep less at night because of the meal rules.
Taking part in sport in Ramadan is a "major physiological performance penalty", Mark de Marees, director of exercise physiology at the German Sport University in Cologne, said.
"If players take no water or food during the day, they can only take part in low levels of physical stress and in significantly different climates without a serious health and performance penalty."
Claude Leroy, who has been a coach for the national teams of Ghana, Cameroon and Oman, said that players who fast would have real troubles in Brazil where a majority of the games start at 1:00pm or 5:00pm. "It is very complicated to strictly follow Ramadan," he said.
Algeria's captain Majid Bougherra confirmed that drinking enough is the most difficult part of Ramadan. "But we are okay. The climate is good. Some players can delay the fasting. In my case I am going to do it in line with my physical state. But I think I am going to do it."
Religious sensibilities raised by Ramadan also worry some coaches, especially in teams from a mixture of ethnic backgrounds.
France's coach Didier Deschamps said he would be giving no orders to Muslim players in his team, who include midfielder Paul Pogba.
"This is a sensitive and delicate topic," said Deschamps.
"I have no order to give. We respect everyone's religion. The players are used to it, we are not discovering the situation today. I have no worries." Muslims will at least have an easier time in Brazil, where it gets dark at about 5:30pm, than at the London Olympics where the Iftar meal at the end of Ramadan could not be served before 8:30pm or 9:00pm.
Chalabi said there could also be a psychological boost during the fasting month.
"Curiously there are some athletes who have better results during Ramadan because they really want to do the fast," said the doctor.