The human body is a living machine , and , like all machines , it needs “fuel” (燃料) to supply it with energy . This is provided by the food we eat . But how much do we need to stay healthy ?
The energy value of food is usually measured in calories . A calorie is the amount of heat required to raise the temperature of 1 kg of water by 1℃. The number of calories people need per day is different , as the pictures show . Also , the number of calories you use at any one moment normally depends on the activity you are in . For example , you need more calories for standing than for sitting , more for running than for walking , and so on .
The energy in food is in the form of three kinds of chemical materials-carbohydrate （碳水化合物）, protein（蛋白质）and fat . Carbohydrate provides 3.8 Cal/gm , protein4.0 Cal/gm , and fat 8.8 Cal/gm . Each food contains different amount of these materials , as the round pictures show .
