What is the minimum amount of food required to stay alive?
Anything less than that can cause long term damage. But, for reasons of muscle and overall body maintenance you absolutely need more than 650 calories per day! You need to eat a daily caloric net of zero. For most people, this is between 1500-2200 calories per day based on size, weight, activity level, etc.
Is there a single food you can survive on?
You'll be eating your own heart, too. However, there is one food that has it all: the one that keeps babies alive. "The only food that provides all the nutrients that humans need is human milk," Hattner said. In many parts of the world, people have no choice but to eat mostly one food: often, rice.27 Jan 2012
When your calorie intake is too low, you may not get all the nutrients your body needs. Additionally, your body's natural response to a decrease in food can lead to your body's metabolism slowing so that your body can conserve energy. In the long run, this slowing metabolism can lead to weight gain.31 Jan 2018