that are vital for life. The proteins of muscles， the hor-<br >mones， the enzymes， the fats， and the stored forms of<br >sugar are examples of such necessary compounds. As a<br >result of these complex reactions， chemical waste prod-<br >ucts are produced as well and must be eliminated from<br >the body. And， of course， all-important energy is either<br >stored or released as these reactions are taking place.<br >What is energy? Energy is simply the ability to do work.<br >Today we talk about the energy value of foods and about<br >the number of calories that a particular food provides.<br >The calorie is a unit of heat energy; that is， it measures<br >energy as a form of heat. The energy value of individual<br >fbods depends on the amount of carbohydrates， fats， or<br >proteins present. Measured in calories per gram， carbo-<br >hydrates and proteins supply four calories per gram，<br >and fats yield nine calories per gram. One gram is often<br >described as equal to the mass of a paper clip. And here<br >comes a mini-math lesson: There are 28.3 grams in an<br >ounce and 453 grams in a pound. A milligram (mg) is<br >1./1，000 of a gram; a microgram (pg) is 1/1，000，000 of a<br >gram， or 1/1，000 of a milligram.<br >Different types of energy are interchangeable. For<br >example， the chemical energy of carbohydrates， which<br >are important body fuels， can be converted into heat<br >energy to help maintain a constant body temperature. It<br >can also be changed to kinetic energy necessary for<br >muscle action and physical activity， or it can be trapped<br >as chemical energy in other body compounds.<br > The Essential Nutrients<br > As the science of chemistry developed in the 18th and<br > 19th centuries， so did procedures to analyze what we<br > eat. Scientists soon discovered the great variety ot<br >