The small intestine

Open your hand and put it flat against your belly button, with your thumb pointing up to your waist and your pinkie pointing down. Your hand is now covering most of the relatively small space into which your 20-foot-long small (20 feet? small?) intestine is neatly coiled. When the soupy, partially-digested chyme spills from your stomach into this part of the digestive tube, a whole new set of gastric juices are released. These include:

• Pancreatic and intestinal enzymes that finish the digestion of proteins into amino acids
• Bile, a greenish liquid (made in the liver and stored in the gallbladder) that enables fats to mix with water
• Alkaline pancreatic juices that make the chyme less acidic so that amylases (the enzymes that break down carbohydrates) can go back to work separating complex carbohydrates into simple sugars
• Intestinal alcohol dehydrogenase, which digests alcohol not previously absorbed into your bloodstream While these chemicals are working, contractions of the small intestine continue to move the food mass down through the tube so that your body can absorb sugars, amino acids, fatty acids, vitamins, and minerals into cells in the intestinal wall.

The lining of the small intestine is a series of folds covered with projections that have been described as “finger-like” or “small nipples.” The technical name for these small fingers/nipples is villi. Each villus is covered with smaller projections called microvilli, and every villus and microvillus is programmed to accept a specific nutrient — and no other. Nutrients are absorbed not in their order of arrival in the intestine but according to how fast they’re broken down into their basic parts:

• Carbohydrates — which separate quickly into single sugar units — are absorbed first.
• Proteins (as amino acids) go next.
• Fats — which take longest to break apart into their constituent fatty acids — are last. That’s why a high-fat meal keeps you feeling fuller longer than a meal such as chow mein or plain tossed salad, which are mostly low-fat carbohydrates.
• Vitamins that dissolve in water are absorbed earlier than vitamins that dissolve in fat.

After you’ve digested your food and absorbed its nutrients through your small intestine:

• Amino acids, sugars, vitamin C, the B vitamins, iron, calcium, and magnesium are carried through the bloodstream to your liver, where they are processed and sent out to the rest of the body.
• Fatty acids, cholesterol, and vitamins A, D, E, and K go into the lymphatic system and then into the blood. They, too, end up in the liver, are processed, and are shipped out to other body cells. Inside the cells, nutrients are metabolized, or burned for heat and energy or used to build new tissues. The metabolic process that gives you energy is called catabolism (from katabole, the Greek word for casting down). The metabolic process that uses nutrients to build new tissues is called anabolism (from anabole, the Greek word for raising up).

How the body uses nutrients for energy and new tissues is, alas, a subject for another chapter. In fact, this subject is enough to fill seven different chapters, each devoted to a specific kind of nutrient.