Turning starches into sugars

Salivary enzymes (like amylases) don’t lay a finger on proteins and leave fats pretty much alone, but they do begin to digest complex carbohydrates, breaking the long, chainlike molecules of starches into individual units of sugars; this simple experiment enables you to taste firsthand 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. 3Okay, you can swallow now. The rest of the digestion of the starch takes place farther down, in your small intestine.

The 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 pouchy part just below your gullet (esophagus).
Like most of the digestive tube, your stomach is circled with strong muscles whose rhythmic contractions — called peristalsis — move food smartly along and 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. For the most part, digestion of carbohydrates comes to a screeching — though temporary — halt in the stomach because the stomach juices are so acidic that they deactivate amylases, the enzymes that break complex carbohydrates apart into simple sugars. However, stomach acid can break some carbohydrate bonds, so a bit of carb digestion does take place. Back to the action. 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.

The mouth

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

• You can swallow without choking.
• 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:
• Moistening and compacting food so that your tongue can push it to the back of your mouth and you can swallow, sending the food down the slide of your gullet (esophagus) into your stomach.
• Providing amylases, enzymes that start the digestion of complex carbohydrates (starches), breaking the starch molecules into simple .

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

The eyes and nose

When you see appetizing food, you experience a conditioned response 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 — “Listen up, there’s good stuff here!” — your brain sends encouraging messages to your mouth and digestive tract.

In both cases — eyes and nose — the results are the same: “Start the saliva flowing,” they say. “Warm up the stomach glands. Alert the small intestine.” In other words, the sight and scent of food has made your mouth water and your stomach contract in anticipatory hunger pangs.
But wait! Suppose you hate what you see or smell? For some people, even the thought of liver is enough to make them want to barf — or simply leave the room. At that point, your body takes up arms to protect you: You experience a rejection reaction — a reaction similar to that exhibited by babies given something that tastes bitter or sour. 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 — muscles contracting not in anticipatory pangs but in movements preparatory for vomiting up the unwanted food. Not a pleasant moment.
But assume you like what’s on your plate. Go ahead. Take a bite.

Digestive System Introduction

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 to keep zipping through the days, months, and years.
This chapter introduces you to your digestive system and explains exactly how your body digests the many different kinds of foods you eat, all the while extracting the nutrients you need to keep on truckin’. Your digestive system may never win a Tony, Oscar, or Emmy, but it certainly deserves your applause for its ability to turn complex food into basic nutrients. Doing this requires not a cast of thousands but a group of digestive organs, each designed specifically to perform one role in the two-part process. Read on.

The digestive organs
Although exceedingly well-organized, your digestive system is basically one long tube that starts at your mouth, continues down through your throat to your stomach, and then goes on to your small and large intestines and past the rectum 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.

Digestion: A two-part process
Digestion is a two-part process — half mechanical, half chemical:

• Mechanical digestion takes place in your mouth and your stomach. Your teeth break food into small pieces that you can swallow without choking. In your stomach, a churning action continues to break food into smaller particles.
• Chemical digestion occurs at every point in the digestive tract where enzymes and other substances, such as hydrochloric acid (from stomach glands) and bile (from the liver), dissolve food, releasing the nutrients inside.

Are the nutrition study’s conclusions reasonable?

When a study comes up with a conclusion that seems illogical to you, chances are the researchers feel the same way. For example, in 1990, the long-running Nurses’ Study at the Harvard School of Public Health reported that a high-fat diet raised the risk of colon cancer. But the data showed a link only to diets high in beef. No link was found to diets high in dairy fat. In short, this study was begging for a second study to confirm (or deny) its results. And while we wait for that second and, naturally, third study, you can bet we’re keeping an open mind. The nature of life is that things do change, sometimes in surprising ways. Consider dioxin, a toxic contaminant found in some fish. Consider Olestra, the calorie-free fat substitute that makes some tummies rumble. As you read this page, dioxin’s still a bad actor, but in 2005 researchers at the University of Cincinnati and the University of Western Australia announced that eating foods containing Olestra may speed your body’s elimination of — you guessed it — dioxin. A-maz-ing.

Does nutrition study always involve human subject?

Not always, animal studies can alert researchers to potential problems, but working with animals alone cannot give you conclusive proof. Different species react differently to various chemicals and diseases. For example, although cows and horses can digest grass and hay, human being can’t. And while outright poisons such as cyanide clearly traumatize any living body, many foods or drugs that harm a laboratory rat won’t harm you. And vice versa. For example, mouse and rat embryos suffer no ill effects when their mothers are given thalidomide, the sedative that’s known to cause deformed fetal limbs when given to pregnant monkeys — and human beings — at the point in pregnancy when limbs are developing. (And here’s an astounding turn:

Modern research shows that thalidomide is beneficial for treating or preventing human skin problems related to Hansen’s disease [leprosy], cancer, and/or autoimmune conditions, such as rheumatoid arthritis, in which the body mistakenly attacks its own tissues.)