During the entire month of Ramadan, the holiest season of the Islamic year, Muslims fast throughout the daylight hours. The fasting, or Sawm in Arabic, is meant to purify and strengthen the individual and the community. Together with practicing charity (Zakah), fasting reminds Muslims of the poor in their midst. Sawm, Zakah, and ritual prayer (Salah) are three of the five pillars of Islamic practice.
Fasting has a long tradition in Christianity, too, although not necessarily in direct connection with concern for the hungry. Jesus Christ fasted to purify his senses, and many of his followers throughout the ages have fasted as well. Last year, the Campus Crusade for Christ, an Evangelical students' organization in the United States, organized a national day of Fasting and Prayer '95. Leaders and participants were so pleased by the results that they are organizing another one this November.
You can find links to this and other sources on religions at the McKinley Magellan Internet Guide.