MacroNutrient

The first four groups are called macronutrients, because the body needs them in large (or macro) amounts. The last two are required in only small quantities and so are known as micronutrients.

Water is needed in great amounts because the body consists largely of water. Usually, between 50 and 75 percent of a person's body weight is made up of water.

The body requires large quantities of carbohydrates, fats, and proteins because these nutrients provide energy. The energy in food is measured in units called kilocalories. A kilocalorie is equal to 1,000 calories. A calorie is the amount of energy required to raise the temperature of one gram of water one Celsius degree. However, kilocalories are often referred to as simply "calories." The "calories" mentioned in this article are actually kilocalories.

Although minerals and vitamins are needed in only small amounts, they are as vital to health as any other nutrients. Minerals and vitamins are needed for growth and to maintain tissues and regulate body functions.

Water is, perhaps, the most critical nutrient. We can live without other nutrients for several weeks, but we can go without water for only about one week. The body needs water to carry out all of its life processes. Watery solutions help dissolve other nutrients and carry them to all the tissues. The chemical reactions that turn food into energy or tissue-building materials can take place only in a watery solution. The body also needs water to carry away waste products and to cool itself. Adults should consume about 21/2 quarts (2.4 liters) of water a day in the form of beverages or water in food.

Carbohydrates include all sugars and starches. They serve as the main source of energy for living things. Each gram of carbohydrate provides about 4 calories. (A gram is about 0.035 ounce.)

There are two kinds of carbohydrates--simple and complex. Simple carbohydrates, all of which are sugars, have a simple molecular structure. Complex carbohydrates, which include starches, have a larger and more complicated molecular structure that consists of many simple carbohydrates linked together.

Most foods contain carbohydrates. The main sugar in food is sucrose, ordinary white or brown sugar. Another important sugar, lactose, is found in milk. Fructose, an extremely sweet sugar, comes from most fruits and many vegetables. Foods containing starches include beans, breads, cereals, corn, pasta (macaroni, spaghetti, and similar foods made of flour), peas, and potatoes.

Fats are a highly concentrated source of energy. Each gram of fat provides about 9 calories.

All fats are composed of an alcohol called glycerol and substances called fatty acids. A fatty acid consists of a long chain of carbon atoms, to which hydrogen atoms are attached. There are three types of fatty acids: saturated, monounsaturated, and polyunsaturated. A saturated fatty acid contains as many hydrogen atoms as its carbon chain can hold. A monounsaturated fatty acid is lacking a pair of hydrogen atoms. In a polyunsaturated fatty acid, the carbon chain contains at least four fewer hydrogen atoms than it could hold.

Certain polyunsaturated fatty acids must be included in the diet because the body cannot manufacture them. These essential fatty acids serve as building blocks for the membranes that surround every cell in the body. Polyunsaturated fatty acids are found in the oils of such plants as corn and soybeans and in such fish as salmon and mackerel. Common sources of monounsaturated fatty acids include olives and peanuts. Most saturated fatty acids are contained in foods derived from animals, such as butter, lard, dairy products, and fatty red meats.

Proteins provide energy--like carbohydrates, 4 calories per gram--but more importantly, proteins serve as one of the main building materials of the body. Muscle, skin, cartilage, and hair, for example, are made up largely of proteins. In addition, every cell contains proteins called enzymes, which speed up chemical reactions. Cells could not function without these enzymes. Proteins also serve as hormones (chemical messengers) and as antibodies (disease-fighting chemicals).

Proteins are large, complex molecules made up of smaller units called amino acids. The body must have a sufficient supply of 20 amino acids. It can manufacture enough of 11 of them. Nine others, called essential amino acids, either cannot be made by the body or cannot be manufactured in sufficient amounts. They must come from the diet.

The best sources of proteins are cheese, eggs, fish, lean meat, and milk. The proteins in these foods are called complete proteins because they contain adequate amounts of all the essential amino acids. Cereal grains, legumes (plants of the pea family), nuts, and vegetables also supply proteins. These proteins are called incomplete proteins because they lack adequate amounts of one or more of the essential amino acids. However, a combination of two incomplete proteins can provide a complete amino acid mixture. For example, beans and rice are both incomplete proteins, but eaten together they provide the correct balance of amino acids.