A desirable body weight depends on balancing energy intake – determined by food choices – with energy output – determined by one’s basal metabolism (energy needs of the body at rest) plus one’s level of physical activity. The number of calories an individual will need every day depends on age, sex, size, and level of physical activity.
All living beings need energy in order to function. Carbohydrates, fats, and proteins, in any reasonable combination, are the sources of energy, or fuel, for the human body.
Despite their importance to the body as the dominant source of energy, foods rich in carbohydrates are regarded by many people in our society as inferior. Yet they are enjoyable to eat and are the most economical of all nutrients to produce, to store, and to buy. With the high cost of meat and other protein foods, money-saving carbohydrate foods will probably continue to gain in popularity. Most nutrition scientists agree that the 12 percent of daily calories obtained by Americans from protein is about the right amount but that the 42 percent obtained from fat should perhaps drop toward 30 to 35 percent, and the 46 percent obtained from carbohydrates should increase. They suggest including more complex carbohydrates in the diet while reducing refined sugar intake.
The life cycle of plants helps to explain the chemistry of carbohydrates. All forms of life on this planet get their energy, directly or indirectly, from the sun. Through photosynthesis, plants are able to use the energy of sunlight to combine water and carbon dioxide from the air into a molecule that stores energy – a carbohydrate. When plants are eaten and digested, this molecule is broken down and its energy is released.