Nutrition For Dummies. Carol Ann Rinzler

the effects of amylases on carbohydrates.

      1 Put a small piece of plain, unsalted cracker on your tongue.No cheese, no chopped liver — just the cracker, please.

      2 Close your mouth and let the cracker sit on your tongue for a few minutes.Do you taste a sudden, slight sweetness? That’s the salivary enzymes breaking a long, complex starch molecule into its component parts (sugars).

      3 Now swallow.The rest of the digestion of the starch takes place farther down, in your small intestine.

      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, which few non-physicians have ever seen except for those TV viewers addicted to the show My 600-lb. Life.

      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.

      

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 “Prebiotics and probiotics: The good gut twins”). 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