Wednesday, January 30, 2008

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.

Sunday, January 27, 2008

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.)

Can you trust recent studies related to nutrition?

You open your morning newspaper or turn on the evening news and read or hear that a group of researchers at an impeccably prestigious scientific organization has published a study showing that yet another thing you’ve always taken for granted is hazardous to your health. For example, the study says drinking coffee stresses your heart, adding salt to food raises blood pressure, or fatty foods increase your risk of cancer or heart disease. So you throw out the offending food or drink or rearrange your daily routine to avoid the once-acceptable, now-dangerous food, beverage, or additive. And then what happens? Two weeks, two months, or two years down the road, a second, equally prestigious group of scientists publishes a study conclusively proving that the first group got it wrong: In fact, this study shows coffee has no effect on the risk of heart disease — and may even improve athletic performance; salt does not cause hypertension except in certain sensitive individuals; only some fatty foods are risky.

Who’s right? Nobody seems to know. That leaves you, a layperson, on your own to come up with the answer. Never fear — you may not be a nutritionist, but that doesn’t mean you can’t apply a few common-sense rules to any study you read about, rules that say: “Yes, this may be true,” or “No, this may not be.”

Nutritional people

The people who make nutrition news may be scientists, reporters, or simply someone who wandered in with a new theory (Artichokes prevent cancer! Never eat cherries and cheese at the same meal! Vitamin C gives you hives!), the more bizarre the better. But several groups of people are most likely to give you news you can use with confidence. For example:
  • Nutrition scientists: These are people with graduate degrees (usually in chemistry, biology, biochemistry, or physics) engaged in research dealing primarily with the effects of food on animals and human beings.
  • Nutrition researchers: Researchers may be either nutrition scientists or professionals in another field, such as medicine or sociology, whose research (study or studies) concentrates on the effects of food.
  • Nutritionists: These are people who concentrate on the study of nutrition. In some states, a person who uses the title “nutritionist” must have a graduate degree in basic science courses related to nutrition.
  • Dietitians: These people have undergraduate degrees in food and nutrition science or the management of food programs. A person with the letters R.D. after his or her name has completed a dietetic internship and passed an American Dietetic Association licensing exam.
  • Nutrition reporters and writers: These are people who specialize in giving you information about the medical and/or scientific aspects of food. Like reporters who concentrate on politics or sports, nutrition reporters gain their expertise through years of covering their beat.
Most have the science background required to translate technical information into language nonscientists can understand; some have been trained as dietitians, nutritionists, or nutrition scientists. Consumer alert: Regardless of the source, nutrition news should always pass what you may call The Reasonableness Test. In other words, if a story or report or study sounds ridiculous, it probably is.

Nutrition Equals Life

Technically speaking, nutrition is the science of how the body uses food. In fact, nutrition is life. All living things, including you, need food and water to live. Beyond that, you need good food, meaning food with the proper nutrients, to live well. If you don’t eat and drink, you’ll die. Period. If you don’t eat and drink nutritious food and beverages:
  • Your bones may bend or break (not enough calcium).
  • Your gums may bleed (not enough vitamin C).
  • Your blood may not carry oxygen to every cell (not enough iron). And on, and on, and on.
Understanding how good nutrition protects you against these dire consequences requires a familiarity with the language and concepts of nutrition. Knowing some basic chemistry is helpful (don’t panic:Chemistry can be a cinch when you read about it in plain English). A smattering of sociology and psychology is also useful, because although nutrition is mostly about how food revs up and sustains your body, it’s also about the cultural traditions and individual differences that explain how you choose your favorite foods.
To sum it up: Nutrition is about why you eat what you eat and how the food you get affects your body and your health.

Specific nutrients for Animal and Plants

Vitamin C isn’t the only nutrient that’s essential for one species but not for others. Many organic compounds (substances similar to vitamins) and elements (minerals) are essential for your green or furry friends but not for you, either because you can synthesize them from the food you eat or because they’re so widely available in the human diet and you require such small amounts that you can get what you need without hardly trying. Two good examples are the organic compounds choline and myoinositol. Choline is an essential nutrient for several species of animals, including dogs, cats, rats, and guinea pigs. Although choline has now been declared essential for human beings, human bodies produce choline on their own, and you can get choline from eggs, liver, soybeans, cauliflower, and lettuce. Myoinositol is an essential nutrient for gerbils and rats, but human beings synthesize it naturally and use it in many body processes, such as transmitting signals between cells.
Here’s a handy list of nutrients that are essential for animals and/or plants but not for you:
Organic Compounds


Energy and Nutrients from food

Energy is the ability to do work. Virtually every bite of food gives you energy, even when it doesn’t give you nutrients. The amount of energy in food is measured in calories, the amount of heat produced when food is burned (metabolized) in your body cells. You can read all about calories in Chapter 3. But right now, all you need to know is that food is the fuel on which your body runs. Without enough food, you don’t have enough energy.

Nutrients in food
Nutrients are chemical substances your body uses to build, maintain, and repair tissues. They also empower cells to send messages back and forth to conduct essential chemical reactions, such as the ones that make it possible for you to

* Breathe
* See
* Move
* Hear
* Eliminate waste
* Smell
* Think
* Taste

. . . and do everything else natural to a living body.
Food provides two distinct groups of nutrients:

* Macronutrients (macro = big): Protein, fat, carbohydrates, and water
* Micronutrients (micro = small): Vitamins and minerals

What’s the difference between these two groups? The amount you need each day. Your daily requirements for macronutrients generally exceed 1 gram. (For comparison’s sake, 28 grams are in an ounce.) For example, a man needs about 63 grams of protein a day (slightly more than two ounces), and a woman needs 50 grams (slightly less than two ounces). Your daily requirements for micronutrients are much smaller. For example, the Recommended Dietary Allowance (RDA) for vitamin C is measured in milligrams (1⁄1,000 of a gram), while the RDAs for vitamin D, vitamin B12, and folate are even smaller and are measured in micrograms (1⁄1,000,000 of a gram).

What’s an essential nutrient?

A reasonable person may assume that an essential nutrient is one you need to sustain a healthy body. But who says a reasonable person thinks like a nutritionist? In nutrition speak, an essential nutrient is a very special thing:
  • * An essential nutrient cannot be manufactured in the body. You have to get essential nutrients from food or from a nutritional supplement.
  • * An essential nutrient is linked to a specific deficiency disease. For example, people who go without protein for extended periods of time develop the protein-deficiency disease kwashiorkor.
People who don’t get enough vitamin C develop the vitamin C–deficiency disease scurvy. A diet rich in the essential nutrient cures the deficiency disease, but you need the proper nutrient. In other words, you can’t cure a protein deficiency with extra amounts of vitamin C.
Not all nutrients are essential for all species of animals. For example, vitamin C is an essential nutrient for human beings but not for dogs. A dog’s body makes the vitamin C it needs. Check out the list of nutrients on a can or bag of dog food. See? No C. The dog already has the C it — sorry, he or she —requires.
Essential nutrients for human beings include many well-known vitamins and minerals, several amino acids (the so-called building blocks of proteins),

Protecting the nutrients in your food

Identifying nutrients is one thing. Making sure you get them into your body is another. Here, the essential idea is to keep nutritious food nutritious by preserving and protecting its components.
Some people see the term food processing as a nutritional dirty word. Or words. They’re wrong. Without food processing and preservatives, you and I would still be forced to gather (or kill) our food each morning and down it fast before it spoiled. For more about which processing and preservative techniques produce the safest, most nutritious — and yes, delicious —dinners. Considering how vital food preservation can be, you may want to think about when you last heard a rousing cheer for the anonymous cook who first noticed that salting or pickling food could extend food’s shelf life. Or for the guys who invented the refrigeration and freezing techniques that slow food’s natural tendency to degrade (translation: spoil). Or for Louis Pasteur, the man who made it ab-so-lute-ly clear that heating food to boiling kills bugs that might otherwise cause food poisoning. Hardly ever, that’s when. So give them a hand, right here. Cool.

Other interesting substances in food

The latest flash in the nutrition sky is caused by phytochemicals. Phyto is the Greek word for plants, so phytochemicals are simply — yes, you’ve got it —chemicals from plants. Although the 13-letter group name may be new to you, you’re already familiar with some phytochemicals. Vitamins are phytochemicals. Pigments such as beta carotene, the deep yellow coloring in fruits and vegetables that your body can convert to a form of vitamin A, are phytochemicals.
And then there are phytoestrogens, hormone-like chemicals that grabbed the spotlight when it was suggested that a diet high in phytoestrogens, such as the isoflavones found in soybeans, may lower the risk of heart disease and reduce the incidence of reproductive cancers (cancers of the breast, ovary, uterus, and prostate). More recent studies suggest that phytoestrogens may have some problems of their own.

You are what you eat

Oh boy, I bet you’ve heard this one before. But it bears repeating, because the human body really is built from the nutrients it gets from food: water, protein, fat, carbohydrates, vitamins, and minerals. On average, when you step on the scale
  • About 60 percent of your weight is water.
  • About 20 percent of your weight is fat.
  • About 20 percent of your weight is a combination of mostly protein (especially in your muscles) plus carbohydrates, minerals, and vitamins.
An easy way to remember your body’s percentage of water, fat, and protein and other nutrients is to think of it as the “60-20-20 Rule.”

What’s a body made of?

Sugar and spice and everything nice . . . Oops. What I meant to say was the human body is made of water and fat and protein and carbohydrates and vitamins and minerals.
On average, when you step on the scale, approximately 60 percent of your weight is water, 20 percent is body fat (slightly less for a man), and 20 percent is a combination of mostly protein, plus carbohydrates, minerals, vitamins, and other naturally occurring biochemicals.
Based on these percentages, you can reasonably expect that an average 140-pound person’s body weight consists of about
  • 84 pounds of water
  • 28 pounds of body fat
  • 28 pounds of a combination of protein (up to 25 pounds), minerals (up to 7 pounds), carbohydrates (up to 1.4 pounds), and vitamins (a trace).
Yep, you’re right: Those last figures do total more than 28 pounds. That’s because “up to” (as in “up to 25 pounds of protein”) means that the amounts may vary from person to person. For example, a young person’s body has proportionately more muscle and less fat than an older person’s, while a woman’s body has proportionately less muscle and more fat than a man’s. As a result, more of a man’s weight comes from protein and calcium, while more of a woman’s body comes from fat. Protein-packed muscles and mineral-packed bones are denser tissue than fat. Weigh a man and a woman of roughly the same height and size, and he’s likely to tip the scale higher every time.
The National Research Council, Recommended Dietary Allowances (Washington D.C.: National Academy Press, 1989); Eleanor Noss Whitney, Corinne Balog Cataldo, and Sharon Rady Rolfes, Understanding Normal and Clinical Nutrition (Minneapolis/St. Paul: West Publishing Company, 1994)

Your nutritional status

Nutritional status is a phrase that describes the state of your health as related to your diet. For example, people who are starving do not get the nutrients or calories they need for optimum health. These people are said to be malnourished (mal = bad), which means their nutritional status is, to put it gently, definitely not good. Malnutrition may arise from
  • A diet that doesn’t provide enough food. This situation can occur in times of famine or through voluntary starvation because of an eating disorder or because something in your life disturbs your appetite. For example, older people may be at risk of malnutrition because of tooth loss or age-related loss of appetite or because they live alone and sometimes just forget to eat.
  • A diet that, while otherwise adequate, is deficient in a specific nutrient. This kind of nutritional inadequacy can lead to — surprise! — a deficiency disease, such as beriberi, the disease caused by a lack of vitamin B1 (thiamine).
  • A metabolic disorder or medical condition that prevents your body from absorbing specific nutrients, such as carbohydrates or protein.
One common example is diabetes, the inability to produce enough insulin, the hormone your body uses to metabolize (digest) carbohydrates. Another is celiac disease, a condition that makes it impossible for the body to digest gluten, a protein in wheat. Need more info on either diabetes or celiac disease?
Doctors and registered dieticians have many tools with which to rate your nutritional status. For example, they can
  • Review your medical history to see whether you have any conditions (such as dentures) that may make eating certain foods difficult or that interfere with your ability to absorb nutrients.
  • Perform a physical examination to look for obvious signs of nutritional deficiency, such as dull hair and eyes (a lack of vitamins?), poor posture (not enough calcium to protect the spinal bones?), or extreme thinness (not enough food? An underlying disease?).
  • Order laboratory blood and urine tests that may identify early signs of malnutrition, such as the lack of red blood cells that characterizes anemia caused by an iron deficiency at every stage

Fitting food into the medicine chest

Food is medicine for the body and the soul. Good meals make good friends, and modern research validates the virtues of not only Granny’s chicken soup but also heart-healthy sulfur compounds in garlic and onions, anticholesterol dietary fiber in grains and beans, bone-building calcium in milk and greens, and mood elevators in coffee, tea, and chocolate.
Of course, foods pose some risks as well: food allergies, food intolerances, food and drug interactions, and the occasional harmful substances such as the dreaded saturated fats and trans . In other words, constructing a healthful diet can mean tailoring food choices to your own special body.