There is no one perfect food. We need an assortment of nutrients that can be obtained only by eating a wide variety of foods. What is it that our bodies need?

Scientists have identified more than 40 different nutrients in food. These substances are essential for growth and for the chemical reactions and processes that keep us alive and functioning (metabolism).

Except for an extremely small number of foods that consist almost entirely of one nutrient, the vast majority of the foods we eat are mixtures of many nutrients. Nevertheless, each group of foods included in the Food Guide Pyramid (grains, fruits and vegetables, milk products, and meats) is unique in the types of nutrients it contributes to our diets.

For example, fruits and vegetables are the main source of many vitamins, minerals, and complex carbohydrates in our diets, and the meat group (including dry beans and legumes, eggs, poultry, and fish) is the main source of protein for most people.

It can be difficult to understand the difference between the nutrients themselves and the foods that contain them. For example, when you hear nutrition experts talk about the need to get more complex carbohydrates, what do they mean and what foods contain those nutrients?

In this article, we focus on the nutrients themselves—how they are digested, what happens to them in the body, and what they do for you. We also say a little about the best food sources of each nutrient, because, after all, when you go to the supermarket, you don’t look for protein, starch, fiber, and antioxidants, you look for chicken, rice, raisin bran, and orange juice.

Nutrients are sorted into categories on the basis of their chemical structures and functions. Carbohydrates, proteins, and fats contained in foods are known as the macronutrients, because they are required in the largest quantities. In addition to their other functions, macronutrients provide energy in the form of calories.

Vitamins and minerals are known as the micronutrients. They are required by your body in much smaller quantities. Although the micronutrients help your body use the energy in macronutrients, they provide no energy (calories) themselves. Water is also an essential, calorie- free nutrient.

The work our bodies do each day causes us to deplete some of our stores of these essential nutrients. Only by maintaining a diet that is rich in various nutrient-containing foods can we replace those lost nutrients.

In addition to the known nutrients, substances in foods of plant origin, called phytochemicals or phytonutrients (phyto is the Greek word for plant), have been identified in recent studies. These phytochemicals may promote health and help prevent certain diseases.

Hundreds of such compounds are being identified in the fruits, vegetables, nuts, beans, and grains we eat, although only a few have been thoroughly studied. How these various phytochemicals influence our health is a promising new area of research for nutrition experts.

Macro Nutrients

Each of the macronutrients—carbohydrates, proteins, and fats—plays various roles in the function of our bodies. In addition to their unique functions, all of the macronutrients supply calories. When we eat more protein, carbohydrate, or fat than we need to replenish what we have used, the excess is converted to and stored as fat.

Calories are used to support all muscular activity, to carry out the metabolic reactions that sustain the body, to maintain body temperature, and to support growth. But when we consistently take in more calories than we use, we gain weight. Weight is maintained when energy (calorie) intake balances energy output.

Water

As an essential nutrient, water is the most often overlooked and taken for granted. Yet 75 percent of our body weight is water. Water contributes to nearly every major process in our bodies.

It keeps our body temperature stable, maintains body chemicals at their proper concentrations, carries nutrients and oxygen to cells, and removes waste products. Water also cushions joints and protects organs and tissues.

An insufficient intake of water or excessive loss of water can result in dehydration and heat exhaustion, a condition characterized by dizziness, vomiting, muscle cramps, fatigue, and confusion. Fortunately, under most circumstances our bodies are good at telling us when we are nearing dehydration.

When we feel thirsty, our bodies need water. However, as we get older, our bodies’ ability to sense dehydration decreases, and older adults often lose the sense of thirst. Our need for water increases with exercise; exposure to hot or even warm, dry, or extremely cold conditions; pregnancy and breastfeeding; and the use of some medications.

Nutritionists recommend that we drink 8 or more 8-ounce glasses of water daily. Many fruits and vegetables are 80 to 90 percent water. Therefore, in addition to the vitamins and minerals they supply, fruits and vegetables also contribute to our total water intake.

Phytochemicals

Carotenoids. Isoflavones. Capsaicins. You may have heard these words in radio advertisements for the latest supplement or seen them in last week’s newspaper.

But what are they? All of them fall into the category of substances called phytochemicals or phytonutrients. A phytochemical is, literally, any chemical found in a plant (phyto is the Greek word for plant).

Armed with the knowledge that people whose diets are rich in foods of plant origin are at lower risk for many serious diseases, nutritional scientists have begun to try to isolate the actual chemicals in foods that may be responsible for promoting health and preventing disease.

Nutritionists have adopted the term “phytochemical” or “phytonutrient” to refer to any one of a growing list of substances they have isolated that appear to prevent disease in laboratory animals.

The phytochemicals identified so far are known to have various roles in the plants from which they originate, including capturing the energy from sunlight and conferring resistance to infection by fungi, bacteria, and viruses.

How they function in our bodies, and how they may be responsible (along with the known vitamins, minerals, and fiber found in plant foods) for the health-promoting effects, is just beginning to be understood.

The antioxidant beta-carotene is one of a group of phytochemicals known as carotenoids. Beta-carotene, the substance that gives carrots their orange color and their name, is converted to vitamin A (retinol) in our body.

Other carotenoids include lutein and zeaxanthin (from green vegetables) and lycopene (from tomatoes). Diets rich in foods containing carotenoids have been associated with a reduced risk of cancer and heart disease.

Researchers are also investigating whether lutein-rich diets may be linked to a lower risk for macular degeneration, a disease of the retina that may lead to blindness. The isoflavones found in soybeans are associated with lower blood cholesterol and a decreased risk for coronary artery disease.

In addition, isoflavones, which are also referred to as phytoestrogens (estrogen-like molecules isolated from plants), appear to reduce some of the symptoms of menopause and may confer a lower risk for breast and other cancers. For health benefits, 20 to 25 grams of soy protein per day is recommended. For some of the other phytochemicals that have been identified.

Although some phytochemicals are now available in pill form, we do not yet know enough about how they function to assume that any one, on its own, will promote health and prevent disease without the presence of the vitamins, minerals, fiber, and other, as yet unidentified, substances in plant foods.

Scientists are making progress in determining how phytochemicals work, but the best way to ensure an adequate intake of all potentially healthpromoting substances in foods of plant origin is to eat the foods themselves—fruits, vegetables, beans, nuts, seeds, and whole grains.