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 1kg of water by 1°C. The number of calories people need per day is different, as the table below shows. 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. CALORIES NEEDED PER DAY Baby 750 Office worker 2700 Child aged 8 2100 Woman feeding baby 2700 Man over 70 2100 Boy aged 16 3000 woman 2600 farmer 3600 The energy in food is in the form of three kinds of chemical material-carbohydrate (碳水化合物 ), protein ( 蛋白质 ) and fat. Carbohydrate provides 3.8 Cal/gm, protein 4.0 Cal/gm, and fat 8.8 Cal/gm. Each food contains different amount of these materials, as the following form shows. Amount Milk Rice Peanuts P 30% 10% 30% F 30% 5% 60% C 40% 85% 10%