Supplying enough energy to support the many functions of the
body at work and play is one of the chief functions of food.
This energy comes from the fats, carbohydrates, and proteins
in the food you eat. Of the three, fat is the most concentrated
source of energy because it furnishes more than twice as much
energy for a given weight as protein or carbohydrate.
Energy requirements are ordinarily expressed in terms of calories.
The calorie used in nutritional discussions is actually the "large
calorie" - Calorie. This is really a kilocalorie - the amount
of heat energy required to raise the temperature of one kilogram
(about 1 quart) of water one degree Celsius.
The energy requirement for a person is divided into two parts:
basal metabolic requirements and energy required for activity.
Basal metabolic rate (BMR) is the heat eliminated from the body
at rest when temperature is normal. An average person requires
2000-2400 Calories per day while a large man doing heavy work
may require up to 6000 Calories per day.
The three main classes of food provide the following amount
of energy: carbohydrates and proteins provide about 4 Calories
per gram; fats provide about 9 Calories per gram. When you choose
foods that furnish more energy, or Calories than you need, the
excess energy is stored as fat in the body -- a gain in weight
may result. If too little food is eaten to meet energy demands,
the body's stored fat serves as an energy source -- a weight
loss will result. Your weight stays about the same if the energy
from food matches the energy requirements of the body.