Germany's Mesut Ozil will not observe Ramadan when it starts on Saturday 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.