Chapter 2

Digestion: The 24/7 Food Factory

IN THIS CHAPTER

Describing the two ways you process food

Extracting nutrients for your body from what you eat and drink

When you see (or smell) something appetizing, your digestive organs leap into action. Your mouth waters. Your stomach contracts. Intestinal glands begin to secrete the chemicals that turn food into the nutrients that build new tissues and provide the energy you need for work, pleasure, and everyday life. This chapter provides a basic primer on the digestive system from start to finish with a few stops along the way to explain how you metabolize everything from apples to zucchini.

Introducing the Digestive System

Your digestive system is a collection of organs specifically designed to turn complex substances (food) into basic components (nutrients). These organs form one long, exceedingly well-organized tube that starts at your mouth, continues down through your throat to your stomach, and then goes on to your small and large intestines to end at your anus.

In between, with the help of the liver, pancreas, and gallbladder, the usable (digestible) parts of everything that you eat are converted to simple compounds that your body can easily absorb to burn for energy or to build new tissue. The indigestible residue is bundled off and eliminated as waste.

Figure 2-1 shows the body parts and organs that comprise your digestive system.

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© John Wiley & Sons, Inc.

FIGURE 2-1: Your digestive system in all its glory.

The digestive process run by these organs works in two simple ways, one mechanical and one chemical.

The rest of this chapter explains exactly what occurs and where along the digestive tract.

Digestion: One Step at a Time

Each organ in the digestive system plays a specific role in the digestive drama. But the first act occurs in three places rarely listed as part of the digestive tract: your brain, your eyes, and your nose.

The next acts take place in your mouth, your stomach, and your small and large intestines.

Your brain, eyes, and nose

When you see appetizing food, you experience a conditioned response. (For the lowdown on how your digestive system can be conditioned to respond to food, see Chapter 14; for information on your food preferences, see Chapter 15.) In other words, your thoughts — “Wow! That looks good!” — stimulate your brain to tell your digestive organs to get ready for action.

What happens in your nose is purely physical. The tantalizing aroma of good food is transmitted by molecules that fly from the surface of the food to settle on the membrane lining of your nostrils; these molecules stimulate the receptor cells on the olfactory nerve fibers that stretch from your nose back to your brain. When the receptor cells communicate with your brain, your brain sends encouraging messages to your mouth and digestive tract as the sight and scent of food make your mouth water and your stomach contract in anticipatory hunger pangs.

What if you hate what you see or smell? For some people, even the thought of liver is enough to make them want to leave the room. At that point, your body takes up arms to protect you: You experience a rejection reaction. Your mouth purses, and your nose wrinkles as if to keep the food (and its odor) as far away as possible. Your throat tightens, and your stomach turns as muscles contract, not in anticipatory pangs but in movements preparatory for vomiting up the unwanted food. Not a pleasant moment.

But assume that you like what’s on your plate. Go ahead. Take a bite.

Your mouth

Lift your fork to your mouth, and your teeth and salivary glands swing into action. Your teeth chew, grinding and breaking food into small, manageable pieces. As a result,

  • You can swallow easily.
  • You break down the indigestible wrapper of fibers surrounding the edible parts of some foods (fruits, vegetables, whole grains) so that your digestive enzymes can get to the nutrients inside.

At the same time, salivary glands under your tongue and in the back of your mouth secrete the watery liquid called saliva, which performs two important functions:

  • It moistens and compacts food so your tongue can push it to the back of your mouth and you can swallow, sending the food down your esophagus into your stomach.
  • It provides amylases, enzymes that start the digestion of complex carbohydrates (starches), breaking the starch molecules into simple sugars. (Check out Chapter 8 for more on carbs.)

No protein digestion occurs in your mouth, and although saliva does contain very small amounts of lingual lipases — fat-busting enzymes secreted by cells at the base of the tongue — the amount is so small that the fat digestion here is insignificant.

Your stomach

If you were to lay your digestive tract out on a table, most of it would look like a simple, rather narrow, tube. The exception is your stomach, a pouchlike structure just below your esophagus.

Like most of the digestive tube, your stomach is circled with strong muscles whose rhythmic peristaltic contractions turn your stomach into a sort of food processor that mechanically breaks pieces of food into ever smaller particles. While this is going on, glands in the stomach wall are secreting stomach juices — a potent blend of enzymes, hydrochloric acid, and mucus.

One stomach enzyme — gastric alcohol dehydrogenase — digests small amounts of alcohol, an unusual nutrient that can be absorbed directly into your bloodstream even before it’s been digested. Other enzymes, plus stomach juices, begin the digestion of proteins and fats, separating them into their basic components, amino acids and fatty acids.

If the words amino acids and fatty acids are completely new to you and if you’re suddenly consumed by the desire to know more about them this instant, stick a pencil in the book to hold your place and flip to Chapters 6 and 7 for the details.

For the most part, digestion of carbohydrates comes to a temporary halt in the stomach. Stomach acids can break some carb bonds, but overall the liquids here are so acidic that they deactivate amylases, the enzymes that break complex carbohydrates apart into simple sugars. Eventually, your churning stomach blends its contents into a thick soupy mass called chyme (from cheymos, the Greek word for “juice”). When a small amount of chyme spills past the stomach into the small intestine, the digestion of carbohydrates resumes in earnest, and your body begins to extract nutrients from food.

Your small intestine

Open your hand and put it flat against your belly button, with your thumb pointing up to your waist and your little finger pointing down.

Your hand is now covering most of the relatively small space into which your 20-foot-long small intestine is neatly coiled. When the partially digested chyme spills from your stomach into this part of the digestive tube, a whole new set of gastric juices are released:

  • Pancreatic and intestinal enzymes finish the digestion of proteins into amino acids.
  • Bile, a greenish liquid (made in the liver and stored in the gallbladder), enables fats to mix with water (emulsification like an oil and vinegar dressing).
  • Alkaline pancreatic juices make the chyme less acidic so that amylases can go back to work separating complex carbohydrates into simple sugars.
  • Intestinal alcohol dehydrogenase digests alcohol not previously absorbed into your bloodstream.

While these chemicals work, contractions of the small intestine continue to move the food mass down through the tube so your body can absorb sugars, amino acids, fatty acids, vitamins, and minerals into cells in the intestinal wall.

technicalstuff The lining of the small intestine is a series of folds covered with projections that have been described as “fingerlike” or “small nipples.” The scientific name for these small structures is villus (singular) and villi (plural). 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, as follows:

  • Carbohydrates — which separate quickly into single sugar units — are absorbed first.
  • Proteins (as amino acids) go next.
  • Fats — which take the 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, this is what happens:

  • Amino acids, sugars, vitamin C, the B vitamins, iron, calcium, and magnesium are carried through the bloodstream to your liver, where they’re 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 — 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. For information about metabolizing proteins, turn to Chapter 6; for fats, Chapter 7; for carbohydrates, Chapter 8; for alcohol, Chapter 9; for vitamins, Chapter 10; for minerals, Chapter 11; and for water, Chapter 12.

Your large intestine

When every useful, digestible ingredient other than water has been wrung out of your food, the rest — indigestible waste such as fiber — moves into the top of your large intestine, the area known as your colon. The colon’s primary job is to absorb water from this mixture and then to squeeze the remaining matter into the compact bundle known as feces.

Feces (whose brown color comes from leftover bile pigments) are made of indigestible material from food, plus cells that have sloughed off the intestinal lining and the bodies of bacteria, members of the microbiome (see the nearby sidebar “Probiotics: From yogurt to … yogurt”). In fact, about 30 percent of the entire weight of the feces comprises the bodies of these microorganisms, which live in permanent colonies in your colon, where they

  • Manufacture vitamin B12, which is absorbed through the colon wall
  • Produce vitamin K, also absorbed through the colon wall
  • Break down amino acids and produce nitrogen (which gives feces a characteristic odor)
  • Feast on indigestible complex carbohydrates (fiber), excreting the gas that sometimes makes you physically uncomfortable — or a social pariah

When the bacteria have finished and their bodies have been incorporated into the waste, the feces — small remains of yesterday’s copious feast — pass down through your rectum and out through your anus. But not necessarily right away: Digestion of any one meal may take longer than a day to complete.