Germany's Mesut Ozil will not observe Ramadan when it starts today but many World Cup players who do follow the Muslim fasting month will be under strict medical surveillance.
While Islamic Iran and Bosnia, which has a sizeable Muslim population, have dropped out 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.
