Most people don't. They were on a 400-600 calorie diet and burned all of those off plus more a day. Watered down soup with little in it(Sometimes they had bits of human in it), sometimes stale bread and very special occasions, a slice of salami and cheese. Once they were liberated the troops gave them food and a lot of it, at least for them. They would try to eat as much of it as they could and ended up eating too much.
They show this briefly on Band of Brothers. Not very much though. I wish they had shown more of the work camps and death camps.