Let's look at your internal system of what you eat.
You are not just feeding yourself, you are feeding your microbiome of bacteria that is mainly responsible for digestion. If you don't have a diversity of bacteria some of the bacteria will eat your own mucus and digestive tract to survive which is not good. 10 calories for me is not 10 calories for another person. It is the good bacteria that does the digestion for you to absorb. A rural person has a more diverse microbiome than a processed food industrialized society. High fiber makes a slower healthier digestion. Sugar foods make for a fast manic digestion. For those who are microbiome-deprived a fecal transplant may be required but most people are not that far gone. gut trailer