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.

Nutrition’s primary task is figuring out which foods and beverages (in what quantities) provide the energy and building material you need to construct and maintain every organ and system. To do this, nutrition concentrates on food’s two basic attributes: energy and nutrients.

Energy 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 later. 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), and at least two fatty acids.

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. 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 absolutely 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, so to find out more about phytochemicals, including phytoestrogens, check out later.

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.”

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.

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 of life, the aim of a good diet is to maintain a healthy nutritional status.

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, anti-cholesterol 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 fats. In other words, constructing a healthful diet can mean tailoring food choices to your own special body. Not to worry: You can do it.

Finding Nutrition Facts

Getting reliable information about nutrition can be a daunting challenge. For the most part, your nutrition information is likely to come from TV and radio talk shows or news, your daily newspaper, your favorite magazine, a variety of nutrition-oriented books, and you hear or read is really right?

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. Want some guidelines for evaluating nutrition studies? Read on.

Can you trust this study?

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.”

Does this study include human beings?

True, 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.)

Are enough people in this study?

Hey, researchers’ saying, “Well, I did give this to a couple of people,” is simply not enough. The study must include sufficient numbers and a variety of individuals, too. If you don’t have enough people in the study — several hundred to many thousand — to establish a pattern, there’s always the possibility that an effect occurred by chance.

If you don’t include different types of people, which generally means young and old men and women of different racial and ethnic groups, your results may not apply across the board. For example, the original studies linking high blood cholesterol levels to an increased risk of heart disease and linking small doses of aspirin to a reduced risk of a second heart attack involved only men.

It wasn’t until follow-up studies were conducted with women that researchers were able to say with any certainty that high cholesterol is dangerous and aspirin is protective for women as well — but not in quite the same way: In January 2006, the Journal of the American Medical Association reported that men taking low dose aspirin tend to lower their risk of heart attack. For women, the aspirin reduces the risk of stroke. Vive la difference!

Is there anything in the design or method of this study that may affect the accuracy of its conclusions?

Some testing methods are more likely to lead to biased or inaccurate conclusions. For example, a retrospective study (which asks people to tell what they did in the past) is always considered less accurate than a prospective study (one that follows people while they’re actually doing what the researchers are studying), because memory isn’t always accurate. People tend to forget details or, without meaning to, alter them to fit the researchers’ questions.

Are the 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.