Calories, that is, calories and kilocalories (kilocalories) are units of energy. 1 calorie (cal) is the energy required to raise 1 gram of water by 1 degree. Food energy is usually calculated in kilocalories, also known as calories.

In the international unit, energy should be expressed in joules. 1 joule =4. 182 calories, then 1 kcal =4. 182 kj. This unit is sometimes used to represent the energy in food. Many times, people use the unit of kilocalories to calculate the energy in food because it is more convenient. 1 g starch or sugar contains 4 kilocalories; 1g fat contains 9 kcal, 1g protein contains 4 kcal. If expressed in joules, there are many odds and ends, which is not convenient for calculation. For example, if you eat 10 grams of fat, it will bring 90 kilocalories to your body. If you eat 50 grams of popcorn, you will increase the energy supply by about 170 kcal. The energy a person needs every day is also calculated in kilocalories. If you eat a medium-sized apple (about 90 kcal) every day without doing too much exercise, you will gain 10g of fat every day, which is equivalent to gaining 3.6kg of weight every year.