Supercharge Your Diet Plan—Understand How Your Body Processes Carbs, Fats and Proteins

The body metabolizes carbs, fats, and proteins at different speeds

By Michael Ormsbee, Ph.D.Florida State University
Edited by Kate Findley and proofread by Angela Shoemaker, Wondrium Daily

Every year, many of us decide to change our diet to feel healthier and look good. To benefit the most from any diet, it’s important to understand the science behind how our body processes the foods we eat. Michael Ormsbee, Ph.D., of Florida State University spoke to The Great Courses at length about macronutrients and how they affect our bodies.

Close up of man cutting up chicken breast with vegetables on wooden table
Knowing how your body metabolizes the three macronutrients of carbohydrates, fats, and proteins helps you to make better eating choices. Photo by KucherAV / Shutterstock

Easiest Macronutrient to Metabolize: Carbohydrates

The three macronutrients of carbohydrates, fats, and proteins are used by the body’s cells to create energy. It’s important to understand where these nutrients go after they are taken up into the small intestine’s cells.

Through digestion, complex carbohydrates are eventually broken down into individual units known as monosaccharides, with the most common being glucose. During their travel through the length of the small intestine, most nutrients are absorbed.

Carbohydrates have very efficient absorption and are usually cleared of the small intestine more quickly than fats or proteins. For glucose, it is absorbed in the intestinal cells by a protein complex called the sodium-glucose transporter. During this active process, energy, or adenosine triphosphate (ATP), is required to make the process happen.

After glucose is absorbed into the intestinal cells, it is then transported out and into the bloodstream. Once glucose enters the blood, it goes directly to the liver.

The liver is the main site of glucose metabolism; it gets first choice on what to do with the incoming glucose—it can use it, store it, or simply let it move on. 

Glucose that is not metabolized or used by the liver passes through to systemic or whole body circulation. From there, glucose travels to various other tissues, where it can be taken up through a facilitated transport process using other transport proteins before it is used within the tissue.

Breaking Down Fats

Digestion of fat begins in the mouth by the action of the enzyme called lingual lipase, and only very small amounts are broken down in the stomach. The bulk of triglyceride—or fat—digestion occurs in the small intestine. 

Before fats are broken down into their component parts, they first must be emulsified by bile. Bile is produced in the liver but stored in the gallbladder; and essentially, fats have to enter the aqueous environment of the small intestine. 

Fats are hydrophobic, or water fearing, so emulsification by bile helps fat break apart into smaller lipid droplets, which can exist in the small intestine before being broken down. 

Unlike carbohydrates, fats are a little tricky when it comes to their absorption and transportation. They are naturally insoluble in water; and because of this, they cannot move through the brush border of the small intestine into the blood as an individual lipid unit.

To solve this problem, broken down fat particles mix with bile salts and form a water-soluble substance called a micelle. Fat particles move out of the micelle and into the intestinal cell, and the water-soluble bile salts continue down through the small intestine until they are reabsorbed and sent back to the liver.

Processing Fats

Next, the fat particles actually recombine so they can be transported. Fats are not only broken down differently than carbohydrates, but they also travel differently. Fats must travel through the lymph system instead of the blood. 

The lymphatic system is a network of vessels that transports nutrients of the cells and collects cell waste products. The only way fats can travel out of the intestine and into the lymph is by combining with fat-soluble vitamins like vitamin A, D, E, or K, and a protein layer into particles known as chylomicrons. 

The chylomicron serves as the carrier for all digested fats that come from the food you eat. Through a slow process, these fats move into the lymphatic vessels and eventually into your blood where they freely travel to tissues to deliver fats for use or for storage. Some fat will be used by the muscle cells to produce energy and some will be deposited into the fat cells. 

Several factors are involved in how fats are used and stored, including when you last ate, your overall calorie intake for the day, your exercise habits, and how physically fit you are. 

Because this transportation process for fats is relatively slow and somewhat individualized, it could take anywhere from 30 minutes to three hours after you eat a high-fat meal for your blood lipids to reach the highest level. After about five to six hours from your meal, however, the chylomicrons will have delivered the fats to the designated tissues, and your blood levels should return back to normal.

Processing Protein

The structure of protein is very complex, which means that the digestion of protein makes more demands on your body’s metabolism. Proteins stay fully intact until they get into your stomach, and large proteins are broken down into individual amino acids in the stomach by the very acidic stomach juice. 

Proteins that are still intact after leaving the stomach are broken down further in the top portion of the small intestine. The end products of protein digestion are free amino acids, which are absorbed into your intestinal cells. 

Proteins are absorbed in a process more similar to carbohydrates than that of fats, and this process occurs along the entire length of the small intestine. Amino acids, just like glucose, require sodium-dependent carriers to get them into your intestinal cells.

There are several uses for particular amino acids within the intestinal cell; however, amino acids that are not used within the intestinal cells themselves are transported out of those cells and into the blood and make their first stop at your liver. About 50 percent to 65 percent of all of the ingested amino acids are metabolized in your liver. 

“The rest of the amino acids are used in other tissues, particularly the skeletal muscles, kidneys, and brain,” said Dr. Michael Ormsbee, Associate Professor in the Department of Nutrition, Food, and Exercise Sciences at Florida State University. “In fact, these amino acids are used by skeletal muscle to directly repair muscle, maintain muscle mass, or even help with growth of muscle, especially if it’s combined with the proper exercise routine. It is this skeletal muscle mass that has a huge influence on your muscle mass and your body fat ratio.”

The entire process of digestion and absorption of macronutrients is important to understand because it can determine how the food you eat makes you feel, and whether the food you eat is ultimately stored as body fat or glycogen or used as energy.

This article was edited by Kate Findley, Writer for Wondrium Daily, and proofread by Angela Shoemaker, Proofreader and Copy Editor for Wondrium Daily.
Dr. 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.

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.