What are calories exactly and how do they effect our body?

A calorie is a unit of energy. A small calorie (symbol: cal) is defined as the amount of energy needed to raise the temperature of one gram of water by one degree Celsius at a pressure of one atmosphere. What is widely used in nutrition is the large calorie (symbol: kcal or Cal), which equals 1000 small calories.

Calories have a direct bearing on our body weight. Our body requires a certain number of calories per day to maintain its normal functions and to provide energy for our daily activities. We acquire this energy from food, which contains calories. When the calorie intake from food exceeds calorie consumption through metabolic process and physical activities, our body weight will increase; when calorie intake is less than calorie consumption, our body weight will decrease.

The number of calories each person needs every day varies, depending on that person's age, gender, weight, height, activity level, body composition, and some other factors. The number of calories in food can be calculated by adding up calories from carbohydrates, fat and protein (1g carb = 4Cal; 1g fat = 9Cal; 1g protein = 4Cal).

Although it's very important to balance calorie input and output in order to control our body weight, the source of calories is also crucial to our overall health. For example, if one gets most of his or her calories from high-fat food and eats less healthy food, even if he or she has no calorie surplus and his or her body weight remains unchanged, he or she is likely to have a high body fat percentage and lack many nutrients. To give another example, when an athlete eats a high-calorie diet rich in protein and vitamins, even though he or she may have calorie surplus and his or her body weight may increase, his or her body fat percentage remains low, because what contributed to the weight gain is muscle mass rather than fat. In addition, it should be pointed out that although calorie deficit is the only way to achieve weight loss, it must be done in a proper way; otherwise insufficient calorie intake may lead to loss of muscle mass and cause vital organs to stop functioning properly.