Fats (also called lipids) are a large family of compounds that do not mix with water.  Lard, butter, margarine, shortening, and cooking oil are almost pure fat; meat, dairy products, chocolate, cakes and cookies, nuts, and a few fruits and veg¬etables contain significant amounts of fat. Fats are important sources of energy in the diet.

Fatty acids are the major components of fats. They come in three basic types: saturated fatty acids, monounsaturated fatty acids, and polyunsaturated fatty acids. Saturated fatty acids are found mostly in animal fats—lard, butter and other dairy products and meat, for example—whereas monoun¬saturated and polyunsaturated fatty acids come mostly from vegetable sources.

Cholesterol is another member of the lipid family. It is a structural component of cell membranes. Some hormones and vitamin D can be formed from cholesterol. The body can make sufficient cholesterol to meet its needs. The main di¬etary sources of cholesterol are egg yolks, meat, poultry, shell¬fish, and whole-milk dairy products. In fact, cholesterol is found only in food of animal origin.

Since cholesterol is a fat-soluble compound, it does not float freely in the blood stream, which is mostly water. In¬stead, cholesterol travels through the blood stream in gigantic molecules made of fat and protein and called lipoproteins. Most of the blood cholesterol is carried in low-density lipo¬proteins (LDL). Cholesterol is also carried in high-density lipo¬proteins (HDL).

Cholesterol in LDL and HDL is called, respec¬tively, LDL-cholesterol and HDL-cholesterol. The term "total serum cholesterol" refers to the sum of cholesterol in all the lipo¬proteins. Medical experts strongly recommend that total serum cholesterol be below 200 milligrams per deciliter (mg/dl).

Protein is the major structural material in almost all living tissue except bones. Hair, skin, nails, and muscles are mostly protein. There are thousands of different proteins in the human body, each with a unique function, but they are all made from smaller units called amino acids. The body breaks down dietary protein into amino acids and then uses the amino acids in proteins. All told, there are 20 common amino acids in proteins.

The body can manufacture 11 of them, but it must obtain the other 9, the so-called essential amino acids, from food. Animal proteins, except for gelatin, and soy proteins contain all the essential amino acids in sufficient quantities and are known as complete proteins.

Most plant proteins are low in one or more of the essential amino acids, and so it is necessary to combine different protein sources to make up for these shortages. For example, peanut butter and bread com¬bined are a complete protein source. So, too, are rice and beans when eaten in the same meal. Such combinations are called complementary proteins.

Carbohydrates are the body's best source of energy, and, in fact, they are the most important source of calories for much of the world's population because of their relatively low cost and wide availability.

For our purposes, there are three types of carbohydrates: simple carbohydrates, digestible com¬plex carbohydrates, and indigestible complex carbohydrates. Simple carbohydrates, such as glucose, fructose, sucrose, and lactose, are also called sugars. Some sugars taste sweet—such as those in table sugar, honey, fruits, molasses, and maple syrup, whereas others, such as those in milk and malt, have little taste at all. The body readily digests and metabolizes simple carbohydrates.

Complex carbohydrates, or polysaccharides, are large molecules made from hundreds of sugar molecules hooked together. In essence, sugar molecules are the building blocks of carbohydrates in the same way that amino acids are the building blocks of proteins. Starches are the most abundant polysaccharides in the diet and occur in many foods, includ¬ing cereals, breads, dry beans, peas, and potatoes. The body digests polysaccharides into sugars.

ndigestible complex carbohydrates, also called fiber or roughage, are large molecules as well, but the sugar building blocks are linked together in such a way that the body cannot break them apart. Because of this, fiber does not supply energy or nutrients to the body, but it does aid in digestion and elimination. Cellulose and pectin, the two most important indigestible complex carbohydrates, are plen¬tiful in bran, whole-grain cereals and breads, fruits, and vegetables.

Vitamins fall into two families, fat-soluble vitamins and water-soluble vitamins. Fat-soluble vitamins are often found together with fats in food. Vitamins A, D, E, and K are fat-soluble vitamins. Because water-soluble vitamins mix readily with water, excess water-soluble vitamins are not stored in the body but are washed out in urine. Vitamins C (ascorbic acid), Ba (thiamin), B2 (riboflavin), niacin, B6 (pyridoxine), pan¬tothenic acid, biotin, folacin, and B12 (cobalamin) are water-soluble vitamins.

Minerals, or mineral salts, have a variety of uses through¬out the body and are involved in almost every aspect of its functioning. The minerals calcium, phosphorus, and magne¬sium are required in relatively large amounts. Calcium, for example, is the most abundant mineral in the body and ac¬counts for nearly 2 percent of body weight. More than 99 percent of the body's calcium is in the bones and teeth, but calcium is also essential for nerves and muscles to work properly.

The body also requires smaller, or trace, amounts of at least 10 other minerals. These trace elements include chro¬mium, cobalt, copper, fluoride, iodine, iron, manganese, molybdenum, selenium, sulfur, and zinc. All trace ele¬ments are toxic if too much is consumed over long periods of time.

Three other minerals, known as electrolytes, are im¬portant components of all body fluids. These minerals are sodium, potassium, and chloride. Table salt is the compound sodium chloride. Meat, peanuts, potatoes, and many fruits and vegetables are rich in potassium.