We are what we digest - not what we eat! Every morsel of food we eat is broken down into nutrients that can be absorbed by the body - hence the reason it takes hours to fully digest our food. Protein must be broken down into amino acids, starches into simple sugars, and fats into fatty acids and glycerol. The water in our food and drink is also absorbed into the bloodstream to provide the body with the fluid it needs.
"Fast food" / "Junk food" takes longer for the body to process - plus there is a much lower percentage of nutritional value absorbed into the system. Poor food choices and poor eating habits ultimately lead to poor digestive function. The first step in improving the health of your digestive system is to improve the quality of the food that you put in your body. The more natural your food intake - the better it will be digested and the better your digestive system will operate.