Within three or four hours after you have eaten a meal, your body must find a way to absorb millions of molecules one by one. The absorptive system is ingeniously designed to accomplish this task.
Small Intestine Absorption
Most absorption takes place in the small intestine. The small intestine is a tube about 10 feet long and about an inch across, yet it provides a surface comparable in area to a tennis court. When nutrient molecules make contact with this surface, they are absorbed and carried off to the liver and other parts of the body.
Villi and Microvilli
How does the intestine manage to provide such a large absorptive surface area? Its inner surface looks smooth but viewed through a microscope, it turns out to be wrinkled into hundreds of folds. Each fold is covered with thousands of fingerlike projections called villi. The villi are as numerous as the fibers on velvet fabric. A single villus, magnified still more, turns out to be composed of several hundred cells, each covered with microscopic hairs called microvilli.
The villi are in constant motion. A thin sheet of muscle lines each villus so that it can wave, squirm, and wiggle like a sea anemone’s tentacles. Any nutrient molecule small enough to be absorbed is trapped among the microvilli and drawn into a cell beneath them. Some partially digested nutrients are caught in the microvilli, digested further by enzymes there, and then absorbed into the cells.
Specialization in the Intestinal Tract
As you can see, the intestinal tract is beautifully designed to perform its functions. A further refinement of the system is that the cells of successive portions of the tract are specialized to absorb different nutrients. The nutrients that are ready for absorption early are absorbed near the top of the tract; those that take longer to be digested are absorbed farther down. The rate at which the nutrients travel through the GI tract is finely adjusted to maximize their availability to the appropriate absorptive segment of the tract when they are ready. The lowly “gut” turns out to be one of the most elegantly designed organ systems in the body.
The Myth of “Food Combining”
Some popular fad diets advocate the idea that people should not eat certain food combinations (for example, fruit and meat) at the same meal, because the digestive system cannot handle more than one task at a time. This is a myth. The art of “food combining” (which actually emphasizes “food separating”) is based on this idea, and it represents faulty logic and a gross underestimation of the body’s capabilities.
In fact, the opposite is often true: foods eaten together can enhance each other’s use by the body. For example, vitamin C in a pineapple or citrus fruit can enhance the absorption of iron from a meal of chicken and rice or other iron-containing foods.
Absorption of Nutrients
Once a molecule has entered a cell in a villus, the next step is to transmit it to a destination elsewhere in the body by way of the body’s two transport systems—the bloodstream and the lymphatic system. Both systems supply vessels to each villus. Through these vessels, the nutrients leave the cell and enter either the lymph or the blood. In either case, the nutrients end up in the blood, at least for a while.
The water-soluble nutrients (and the smaller products of fat digestion) are released directly into the bloodstream by way of the capillaries, but the larger fats and the fat-soluble vitamins find direct access into the capillaries impossible because these nutrients are insoluble in water (and blood is mostly water). They require some packaging before they are released.
The intestinal cells assemble the products of fat digestion into larger molecules called triglycerides. These triglycerides, fat-soluble vitamins (when present), and other large lipids (cholesterol and phospholipids) are then packaged for transport. They cluster together with special proteins to form chylomicrons, one kind of lipoproteins. Finally, the cells release the chylomicrons into the lymphatic system. They can then glide through the lymph vessels until they arrive at a point of entry into the bloodstream near the heart.
Learn more about how nutrients are transported around the body.
The many folds and villi of the small intestine dramatically increase its surface area, facilitating nutrient absorption.
Nutrients pass through the cells of the villi and enter either the blood (if they are water-soluble or small fat fragments) or the lymph (if they are fat-soluble).