Humans need to eat to survive. So why would someone go voluntarily without food - or even water? Many of the major religions observe periods of fasting for spiritual reasons. Charlotte McDonald speaks to those who have fasted to find out why they do it. Also, is there a place in modern society for fasting for non-religious reasons? She speaks to Michael Mosley about the science behind fasting and why some people are choosing it as a lifestyle choice.(Image: Indian Muslims offer prayers prior to breaking their fast on the first day of the holy fasting month of Ramadan. Credit: AFP/Getty Images)