A calorie is a unit in energy used to describe energy consumption and usage. It refers to both the energy gained by consuming food and the energy spent by physical exercises and survival. There are two units of calories: cal and kcal.
A calorie is the energy needed to raise the temperature of certain amounts of water by 1 degree Celsius. For cal, the amount is 1 gram of water, and for kcal, the amount is 1 kilogram. Calories are essential for survival as they provide the cells with the energy required to live.
The human body needs a certain amount of calories to stay healthy. Whereas the exact amount changes from one person to another based on height, level of physical activity, gender, weight and other factors, there are rough estimates that apply to the average person. These are 2,700 for men and 2,200 for women.
The three main components of food translate to different amounts of calories. One gram of carbohydrates and protein contains 4 calories, and 1 gram of fat contains 9 calories. There are also empty calories that, despite providing energy, contain little or no nutritional value. Such calories contain little or no amino acids, fiber, minerals, antioxidants or vitamins.
Consuming more calories than the body needs results in weight gain, whereas consuming less than the required amount leads to weight loss. The required amount increases with physical activity and decreases with a sedentary lifestyle.