Why your body needs food?

Your body is made up of skin, muscle, bone, blood, and many other materials. Where do these materials come from? How does your body repair broken bones, or replace blood when you bleed? As you grow, your body makes new skin and muscle, as well as bone and other type:; of material. Where does this new material come from? Just as a car needs fuel to run, your body needs energy to power all its activities. Where does your body get its energy?
The answer to all of these questions is, "from the food you eat". Your food contains many different substances. Some of these sub­stances provide you with energy as well as with the materials your body needs to grow and repair itself. In some very real ways, you are what you eat.
Your body - What is it made of?
Just like your table or your pen, your body is made of matter. The smallest particles of all matter are atoms. Two or more atoms joined together form larger particles called molecules.
There are many different kinds of atoms. An element is a sub­stance that contains only one type of atom. Although there are over­100 different elements, only some of them are found in your body. Ie fact, the human body is made up mostly of only four elements. Hydrogen, oxygen, carbon, and nitrogen make up about 96 percent of your total mass.
Other elements make up the remaining portion of a human's mao­For example, there are several grams of phosphorus and more thac 1000 g of calcium in the body of a typical adult. Elements such as iron and iodine occur in even smaller amounts. Although these and othe: elements are present in very small amounts, they are still importan­Without them, your body cannot work properly.
Most of the elements found in the human body are not present in pure form, but are joined together with other elements to form molecul called compounds. For example, most of the oxygen atoms and hydroge­atoms in your body are joined together to form molecules of water. Water is a compound made up of these two elements.
Some of the elements and compounds found in your body are used to build or repair your body cells. Other elements and compounds sur­ply energy to the cells in your body. Cells are the tiny subunits tha:: make up all living things. Inside your cells, many chemical reactions occur. It is these chemical reactions that keep your body alive and active. For the reactions to occur, your cells need energy. This energy as well as the substances your cells need for growth and repair, come from your food.

NUTRITION 
Nutrients in the food you eat contain the many different elements and compounds that your body needs. A nutrient is any material that can be taken into your body cells and that is useful to your body. The nutri­ents in food supply the energy and the materials your body needs to grow and survive.
Nutrition is the study of the nutrients in food and the effect of these :nutrients on your health. Sometimes people use "nutrition" and related words to refer to healthful foods or good eating habits. For example, a meal that contains a good balance of the nutrients your body needs is often called "nutritious." If a person practices good nutrition, she or he selects nutritious foods and eats just enough of them to supply the day's needs.
Good nutrition is especially important for you as a teenager because if the many changes taking place in your body. Adolescence is a time of :apid growth and development. Many of your bones grow longer, causing your height and overall size to increase. Your muscles also grow larger, increasing your strength. Your body may produce more fat. Such body growth requires a lot of energy and large amounts of certain ele­:nents and compounds. Teenagers need different amounts of nutrients than either children or adults. In addition, no two teenagers have exactly the same nutritional needs. Your own needs depend on many factors, including your mass, age, and level of activity.
There is also a difference between the nutritional needs of girls md boys. At the age of 10 or 11, girls usually begin a growth spurt, a time of rapid growth. Their growth spurt is often greatest at age 12 and ends by age 15. In boys, this spurt does not usually begin until 12 or 13 years of age. It is greatest at about age 14 and is usually over by age 19. A rapidly growing, active boy of 14 needs to supply his body ith more nutrients than a girl of the same age whose growth spurt as already stopped.