22.11: Absorption moves nutrients from your gut to your cells.

Chewing, tearing, churning, acidifying, dismantling. You put a lot of energy into reducing food to its simplest component molecules. Eventually, though, there is a payoff. That payoff is absorption, the process by which the energy-rich food particles are taken from the digestive tract into the bloodstream and then into cells throughout the body, where they can be used for energy and building materials. Absorption occurs mainly in the small intestine; however, a few molecules are absorbed in the stomach, including aspirin and alcohol, which explains why drinking alcohol on an empty stomach can lead to unexpectedly rapid inebriation.

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The secret to effective absorption is simple: surface area. For a molecule such as a sugar or an amino acid to be absorbed by the body, the molecule must be in direct contact with the cell membrane of the cell that is going to absorb it. The greater the number of cells that can come in contact with the chyme passing through the small intestine, the greater the amount of nutrients that can be absorbed. For this reason, the tremendous surface area of the small intestine, as well as other structural characteristics, allows very efficient absorption of nutrients (FIGURE 22-21).

Figure 22.21: The small intestine has a very large area for absorbing nutrients.
Figure 22.22: Absorption moves nutrients into the bloodstream.

The molecules that can be absorbed in the small intestine include simple sugars, individual amino acids (and, occasionally, short chains of two or three amino acids), fatty acids, vitamins, and minerals. When a nutrient molecule is small enough to be absorbed, it gravitates toward the villi lining the interior of the small intestine. The tiny particles become trapped in the microvilli and are drawn across the cell membranes into the cells (FIGURE 22-22). The nutrients then diffuse out of the cells and into the interstitial fluid bathing the cells. Finally, from here, the nutrients are picked up by capillaries and thus move into the bloodstream, where they can be delivered to the organs and tissues that need them.

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There is a food myth that if certain foods are combined, digestion is impaired. This is not true. The myth ignores a couple of facts: the body is able to produce its digestive enzymes for fats, proteins, and carbohydrates simultaneously, and it can absorb nutrients regardless of which other nutrients are also in the digestive tract. In fact, the contrary is often true. Many foods enhance the absorption of nutrient molecules in other foods. Vitamin C in citrus fruits, for example, increases the efficiency of iron absorption from a meal of beef or beans.

Q

Question 22.9

Is it true that certain food combinations should not be eaten together?

TAKE-HOME MESSAGE 22.11

Absorption is the process by which energy-rich food particles are taken up from the digestive tract into the cells of the body, where they can be used for energy and building materials. Absorption takes place primarily in the small intestine.

Most nutrient absorption occurs in the small intestine. Describe the key features of the small intestine that allow it to efficiently absorb nutrients.