Nutrients fuel your body. Your body uses nutrients for growth, energy and the maintenance of cells. After your body breaks down the nutrients, they cross from the intestine into the bloodstream. From there, the nutrients are transported to the organs that need them.
Anatomy of the Human Gut
You have about 20 feet of small intestine. It has one main job -- absorbing nutrients from food and passing them into the bloodstream. Your small intestine is not a straight tube, like a straw. Instead, it is made up of a series of three projections that increase the surface area of the intestine for better absorption. The first of these projections is called circular folds, or ridges. The next section is made up of the villi, which are finger-like projections, and the third section contains the microvilli, which extend from the villi. Together, these folds, villi and microvilli increase the surface area of your intestine by 600 times. You have blood that runs directly on the other side of the villi membrane, making it easier for nutrient absorption.
Breakdown of Nutrients
Before the villi and microvilli in your intestine can absorb the nutrients, the nutrients must be broken down and removed from the food you just ate. Digestion of the food into its individual nutrients begins with the physical tearing that occurs as you chew your food. This mechanical digestion continues as the food is squeezed and pushed along your intestine. Food is further broken down by chemicals -- some of which occurs in your stomach -- through carbohydrate and protein digestion. Most of the food will be broken down in your small intestine.
Intestine to Bloodstream
Once the food has been broken down into individual nutrients, the particles are small enough to be absorbed by the villi. This absorption occurs either with or without the use of energy. Some nutrients simply move through the membrane from your intestine into the bloodstream because there is less of the nutrient in the bloodstream and your body is trying to equalize the amount of nutrients in the intestine and the bloodstream. This can occur in three ways: by itself, just passing through the villi into the bloodstream; through a special channel for that specific nutrient; or by the villi cell "eating" the nutrient. When there is more of the nutrient already in the bloodstream than in the intestine, your body requires energy to move the nutrient.
Sometimes, your body can't absorb nutrients the way that it's supposed to. It could be hindered by prolonged starvation, which makes it more difficult to absorb nutrients when you begin to eat normally again. Other times the intestine has to be shortened, usually due to illness or injury. This requires your body to adapt to absorb enough nutrients, leading to the villi becoming longer and more numerous. Illness can also lead to hindered nutrient absorption. The most common disease seen today is Celiac disease -- the inability to digest the protein gluten. The disease usually damages the small intestine, hindering absorption. Your body is able to heal once you adopt a gluten-free diet; the damage is often reversed.
- M. Liebman, Ph.D.; Professor of Human Nutrition; Laramie, Wyoming
- Nutritional Sciences; Michelle McGuire and Kathy Beerman
- SIU School of Medicine: Nutrient Absorption
- Baseline of Health: Physiology of the Small Intestine
Lisa Coffman is a food and nutrition specialist based in Iowa. She has spent time working with the elderly in long term care, nutrition education for low income families and food intolerances and food allergies. She is currently finishing her Bachelor of Science in human food and nutrition from the University of Wyoming.