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 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 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).

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 nutritionspeak, 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.

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.

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.

"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.”

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.

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 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.