I think the statement “the human digestive system is very efficient at squeezing everything it can out of any food?" refers to the way the stomach is good at absorbing the wanted nutrients and keeping them in the body, while disposing of the unwanted waste and toxins. The biomolecules are broken down to preserve only certain nutrients within the molecules.
The statement means that the human digestive system can adapt to extreme diets in order to survive. For example, carbohydrates can be broken down into sugars and used for energy. But when carbohydrates are not available in the required amount, it can also break down fats to provide the body with the required amount of energy. This explains why Deb has enough energy to keep going, even on her extremely limited diet.
