By Michael Ormsbee, Ph.D., Florida State University
Edited by Kate Findley, The Great Courses Daily
It’s a new year, and like many other people, you may be starting a new diet plan in order to get in better shape. But how much do you really understand about how your food is processed when it’s inside your body? What exactly happens to the food that you eat? Michael Ormsbee, Ph.D. of Florida State University spoke with The Great Courses at length about the subject.
Purpose of Digestion and Absorption
Digestion and absorption prepare the food you eat for storage or to be burned as energy. First, digestion takes place as your body breaks down the food into individual nutrients. Next, absorption occurs as the body absorbs the individual nutrients to either use them or store them for later.
Once you take a bite of food, the first section of the gastrointestinal (GI) tract that it hits is the mouth. The food then travels down the esophagus and enters into the stomach to be chemically and mechanically broken down into smaller fragments.
Next, these food fragments enter the small intestine followed by the large intestine. Whatever nutrients and materials the body does not use up along that process are excreted through the rectum and the anal canal.
Along the route, your pancreas, gallbladder, and liver all help digest and absorb the food by producing various enzymes. These are specialty proteins that help reactions occur in your body, and the enzyme solutions they make also help with the breakdown and transport of foods.
Food Path from Mouth to Stomach
When food enters your mouth, water in your saliva—along with two major digestive enzymes called alpha-amylase and lingual lipase—begin to dissolve and chemically digest your food.
Once the food is mixed with saliva, it’s now referred to as a bolus. The bolus leaves your mouth and travels through the pharynx and into your esophagus.
The esophagus has a sphincter at the top called the esophageal sphincter, and this separates the oral cavity from your pharynx. Then, a wavelike neuromuscular movement, known as peristalsis, occurs to progressively move the bolus of food down your esophagus.
Next, your food heads down to your stomach through another sphincter, called the gastroesophageal sphincter. This entire esophageal process—which involves the bolus of food moving from the top to the bottom of your esophagus—usually takes about 10 seconds.
The bolus then enters into your stomach.
Stomach’s Role in Digestion Process
The stomach is the major site of chemical digestion, where any larger food items remaining are broken down into individual nutrients. For example, large carbohydrates are broken into individual sugars, and large proteins are broken down into individual amino acids.
This process has to occur because whole carbohydrates, fats, and proteins are too big to enter your body from your GI tract. So, it is the smaller building blocks of the larger macronutrients that are needed to actually enter your body from the GI tract.
While the mouth and the stomach are both referred to as digestive organs, they do things a bit differently.
The mouth mainly takes care of the physical breakdown of food through chewing and the stomach takes care of the chemical breakdown of food using very specific enzymes:
- Pepsin breaks down large proteins into smaller amino acids.
- Alpha-amylase breaks starches into simple carbohydrates.
- Gastric lipase breaks down some fats into smaller components that are easy to absorb.
Along with these three main enzymes, several other hormones and secretions play a role in the chemical digestion activities of the stomach. So, the stomach is where partially digested food is mixed with stomach juices and enzymes to create a substance called chyme.
The Not-So-Small Intestine
Once food is fully mixed and the chyme is ready to be emptied out of the stomach, it moves through yet another sphincter, called the pyloric sphincter, and enters into your small intestine.
The small intestine is the main site for completing the digestion process. It is also the main site that absorbs all of the nutrients that you’ve just broken down.
Contrary to its name, this organ is far from small in both length and in importance. For example, if you were to unravel the small intestine into a straight line, it would stretch out to be about 20 feet long. Also, the surface area spreads out to be about 300 m2.
Picture a three-foot-wide walkway that is more than three football fields in length. This is the amount of space that your small intestines would take up.
Having such a huge surface area is a unique aspect of the small intestine, thus, allowing the body to maximize digestion and absorption.
How Long Does Absorption Take?
Once all the nutrients are broken down into their simplest forms, they are taken up into the cells of the GI tract to be transported throughout the entire body. This is where the process of absorption really occurs.
Most of the carbohydrates, fats, and proteins that you consume are generally absorbed within about 30 minutes after reaching the small intestines.
Any material that is not absorbed by the time it reaches the end of the ileum, or the last portion of the small intestine, passes through the ileocecal valve and then into the large intestine or the colon.
Overall, it can take anywhere from 12–70 hours for the food remnants to pass all the way through to the colon.
“Overall, your body composition is highly influenced by the digestion and absorption of the nutrients from the food you eat,” said Dr. Michael Ormsbee, Associate Professor in the Department of Nutrition, Food, and Exercise Sciences at Florida State University. “Having a better understanding of these processes will help you to tune in more effectively to what your body is telling you.”
This article was edited by Kate Findley, Writer for The Great Courses Daily, and proofread by Angela Shoemaker, Proofreader and Copy Editor for The Great Courses Daily.
Michael Ormsbee is an Associate Professor in the Department of Nutrition, Food, and Exercise Sciences and Interim Director of the Institute of Sports Sciences and Medicine in the College of Human Sciences at Florida State University. He received his MS in Exercise Physiology from South Dakota State University and his PhD in Bioenergetics from East Carolina University.