The foods we eat contain thousands of different chemicals. 
 However, only a few dozen of these chemicals are absolutely essential
  to keep us healthy.  These few dozen are the nutrients--the substances 
  we must obtain from the foods we consume.  

Nutritionists classify nutrients into six main groups:
 (1) water, (2) carbohydrates, (3) fats, (4) proteins, (5) minerals, and (6) vitamins.
   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.  

Minerals are needed for the growth and maintenance of body structures. 
 They are also needed to maintain the composition of the digestive juices 
 and the fluids that are found in and around the cells. 
  People need only small amounts of minerals each day.  

Unlike vitamins, carbohydrates, fats, and proteins, minerals are inorganic compounds.
  This means that they are not created by living things.  Plants obtain minerals 
  from the water or soil, and animals get minerals by eating plants or plant-eating animals.  Unlike other nutrients, minerals are not broken down within the body.  

The required minerals include calcium, chlorine, magnesium, phosphorus, potassium, 
sodium, and sulfur.  Calcium, magnesium, and phosphorus are essential parts of the bones
 and teeth.  In addition, calcium is necessary for blood clotting. 
  Milk and milk products are the richest sources of calcium. 
   Cereals and meats provide phosphorus.  Whole-grain cereals, nuts, legumes, and
    green leafy vegetables are good sources of magnesium.  

Still other minerals are needed only in extremely tiny amounts.
  These minerals, called trace elements, include chromium, copper, fluorine, iodine,
   iron, manganese, molybdenum, selenium, and zinc.  Iron is an important part of hemoglobin,
    the oxygen-carrying molecule in red blood cells. 
	 Copper helps the body make use of iron to build hemoglobin. 
	  Manganese and zinc are required for the normal action of various enzymes. 
	   Green leafy vegetables, whole-grain breads and cereals, seafood, and liver
	    are good sources of trace elements.  

Vitamins are essential for good health.
  Small amounts of these compounds should be supplied daily in the diet. 
   Vitamins regulate chemical reactions in which the body converts food into energy and tissues.  

There are 13 vitamins: vitamin A; the vitamin B complex, which is a group of 8 vitamins;
 and vitamins C, D, E, and K. Scientists divide vitamins into two general groups,
 fat-soluble vitamins and water-soluble vitamins. 
  The fat-soluble vitamins--vitamins A, D, E, and K--dissolve in fats. 
   The vitamins of the B complex and vitamin C dissolve in water.  

Vitamin A is necessary for healthy skin and development of the bones.
  Sources of this vitamin include liver, green and yellow vegetables, and milk.  

Vitamin B-1, also called thiamine, is necessary for changing starches and sugars into energy.
  It is found in meat and whole-grain cereals.  

Vitamin B-2, also known as riboflavin, is essential for complicated chemical 
reactions that take place during the body's use of food. 
 Milk, cheese, fish, liver, and green vegetables supply vitamin B-2. 

Vitamin B-6, also called pyridoxine, and two other B vitamins known as pantothenic acid
 and biotin all play a role in chemical reactions essential for growth. 
  Liver, yeast, and many other foods contain these vitamins.  

Vitamin B-12 and folic acid, also called folate or folacin, are both needed for 
forming red blood cells and for a healthy nervous system. 
 Vitamin B-12 is found in animal products, especially liver. 
  Folic acid is present in green leafy vegetables.
    Doctors recommend that all women who are capable of becoming pregnant 
	consume small amounts of folic acid each day to reduce the risk of spina bifida, 
	which is a serious birth defect.  

Niacin is also part of the B complex. 
 Cells need niacin in order to release energy from carbohydrates.  
 Liver, yeast, lean meat, fish, nuts, and legumes contain niacin.  

Vitamin C, also called ascorbic acid, is needed for the maintenance of the ligaments,
 tendons, and other supportive tissue.  It is found in fruits and in potatoes.  

Vitamin D is necessary for the body's use of calcium.
  It is present in fish-liver oil and vitamin D-fortified milk.
    Vitamin D is also formed when the skin is exposed to sunlight.  

Vitamin E, also known as tocopherol, helps maintain cell membranes. 
 Vegetable oils and whole-grain cereals are especially rich in this vitamin. 
  It is also found in small amounts in most meats, fruits, and vegetables.  

Vitamin K is necessary for proper clotting of the blood.
  Green leafy vegetables contain vitamin K. It is also manufactured by bacteria 
  in the intestine.