Functional ketosis is a natural metabolic state in which your body is using fats and ketones as fuel, instead of glucose. Functional ketosis happens when carbohydrate consumption goes low enough that a person’s liver can convert adipose tissue and dietary fats into a fatty acid known as ketones, then burn the ketones for energy.
Carbohydrates is an energy source, but they’re not the only one your body can use. When there’s very little carbohydrate in your diet (like starvation) your body has a natural and readily available option – ketosis. It will then resort to fat instead of carbohydrates.
Where does the fat come from? Fat you eat or stored fat in your body.
The best examples is a person on a hunger strike who can easily live for 30-40 days or someone in a small lost boat at sea, surviving for 30 days without food.