First we see the food. This stimulates our brain to ready our stomach to receive food, by increasing gastric secretions. Then, we eat the food. Amylase is a digestive enzyme in our saliva that helps to break down carbohydrates. Mechanical manipulation by chewing breaks the food into smaller pieces which provide more surface area. This increased surface area helps the enzymes in the small intestine absorb the nutrients in our food better. This is why it is important to chew your food well. Next, we swallow and food arrives at the stomach.
The stomach is responsible for further mechanical breakdown of food and some chemical breakdown. Proteins are broken down by pepsinogen into peptide chains and fat is broken by gastric lipase (to help you navigate through these terms, any word with “ase” at the end generally denotes an enzyme responsible for some kind of digestion). When food has been through the stomach, it becomes chyme; an acidic mixture of hydrochloric acid from our stomach, pepsinogen, lipase and amylase. The entrance to the small intestine from the stomach is controlled by the pyloric sphincter; a controlled doorway that prevents too much chyme from entering the small intestine at once.
The duodenum is the first part of the small intestine. The duodenum’s job is to neutralize the acidic chyme before allowing it to continue through the rest of the small intestine by way of bicarbonate from pancreatic juice. The food then enters the jejunum, the part of the small intestine that is responsible for the majority of nutrient absorption. Fat, peptides and carbohydrates are further broken down by enzymes into units that are small enough to be transferred in the bloodstream to the target organs. The primary fuel the body needs to run properly is glucose; so much of the food is broken down and recombined into glucose.
At this point, the food has spent between 30 minutes and 2 hours in the stomach and between 2 and 6 hours in the small intestine and 90% of the nutrients have been extracted. The left over material has lots of water and sodium left in it. The body wants to reclaim these substances before defecation; food takes 72 hours to be processed in the large intestine. The jobs of the large intestine are to reclaim the water from the food, reclaim the sodium from the food, and provide healthy bacteria to ferment fiber that has not been digested. This fermentation provides nutrients to keep the cells in the large intestines healthy. Fiber that has not been digested adds bulk to the waste products to facilitate elimination.
In summary, at the short end, digestion takes about 75 hours, but it can take up to 80 hours from the time it enters the mouth to the time it leaves the body in the form of stool.