Molly Roemer

Proper digestion is essential to a healthy body. Both chemical and mechanical aspects are extensive to describe. However, simply put, digestion begins in the mouth and ends with the large intestine. The three macronutrients, fat, protein, and carbohydrates will be discussed.

In your mouth, your saliva has enzymes to begin the breakdown of starches and fats. Your food then passes down your esophagus using what is termed, peristalsis. Peristalsis is how your food is propelled using a kind of squishing motion. Your food then enters the stomach. Contrary to popular belief, little to no actual absorption of nutrients occurs in the stomach, but the stomach acts as a powerful mixer and hydrochloric acid (HCL) breaks down and emulsifies fat globules. There may also be a small amount of protein digestion happening.

Your food then travels into the small intestine, which plays the leading role in digestion. The small intestine is about 20 feet long and has a lot of surface area due to folding and projections called villi. On top of those villi are even smaller villi called, microvilli. The small intestine is divided into three sections based on function. The duodenum attaches to the stomach, the jejunum is in the middle, and the ileum is attached to the large intestine. The duodenum is where the pancreas and gall bladder attach and secrete enzymes and bile to help further digest your food. Most of the absorption of nutrients occurs in the jejunum. Here is a breakdown of how each macronutrient is digested in the small intestine:

Fat: Upon entering the duodenum of the small intestine, your gallbladder secretes bile (which was made in the liver) to emulsify the fat. Then the pancreas secretes enzymes to further emulsify and breakdown the fat. Fats are molecularly split. The villi of the small intestine absorb short-chain fatty acids, which then go into the bloodstream to be taken to the liver (your body’s unsung operative). Long-chain fatty acids (found in most fats and oils we eat) are built into triglycerides (glycerol backbone with three long fatty acid chains), given a protein coating, and become a chylomicron ball. This chylomicron then goes into the lymphatic system, into the bloodstream, and finally makes its way to the liver.

Carbohydrates: The pancreas also secretes enzymes in the small intestine to breakdown starches into disaccharides (maltose, sucrose, lactose) and then within the walls of the small intestine they are broken down into monosaccharides (fructose, glucose, and galactose). These monosaccharides go into the bloodstream, and fructose and galactose are chemically altered into glucose because that is the only form of carbohydrate the body can use. Glucose is then used for energy to fuel your body’s cells and the excess is then taken back to the liver to be packed into long chains (glycogen) for energy storage within the liver and muscles, or converted into fat cells for long-term energy storage.

Protein: Within the small intestine, protein gets broken down into peptides and amino acids. Those amino acids are then absorbed into the bloodstream through the villi of the small intestine and go to the liver to be used. The remaining go back into the bloodstream to be circulated and sent to your cells to build, repair, and maintain.

The large intestine is split into four sections. The ascending colon, which includes the appendix, the transverse colon, the descending colon, and finally the sigmoid colon which leads to the rectum and anus (all forming a square-like form). The large intestine mainly absorbs minerals and water and its job is to form your waste so that it can be stored in the rectum and then excreted through the anus.

It can be kind of discouraging to think of your food as merely going through a long tunnel in your body and then excreted. Some may even find it depressing to think of food as a tool to fuel your body instead of something to be enjoyed. However, food plays a vital role in physical health and wellbeing through many different aspects. Life is about finding balance, using moderation, and living with variety—especially when it comes to food. Happy digesting!

Molly Roemer graduated with a degree in Dietetics from BYU and currently resides in Alamo. She enjoys food and family and seeks to enrich the lives of others through both. Email questions or comments to