The human（人类） body needs energy，which is provided by the food we eat. But how much do we need to stay healthy？
Calories（卡路里） usually decide the energy value（价值） of food. To raise the temperature of I g of water by 1℃ needs one calorie. The number of calories people need each day is different.
Calories Needed Each Day | |
Baby 750 | Office worker 2，700 |
Child aged 8 2，100 | Woman feeding baby 2，700 |
Man over 70 2，100 | Boy aged 16 3，000 |
Woman 2，600 | Farmer 3，600 |
Also，the number of calories you use at any moment usually 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 or three kinds of chemical materials—carbohydrate（碳水化合物），protein（蛋白质） and fat. Carbohydrate provides 3. 8 Cal/gm（卡路里/每克） of energy，protein 4. 0 Cal/gm，and fat 8. 8 Cal/gm. Each food has different amounts（量） of these materials，as the round pictures show.
（P＝protein C＝carbohydrate F＝fat）
