Counting calories is one way to monitor weight loss. A calorie is a unit of energy. Historically, scientists have defined "calorie" to mean a unit of energy or heat that could come from a variety of sources, such as coal or gas. In a nutritional sense, all types of food — whether they are fats, proteins, carbohydrates or sugars — are important sources of calories, which people need to live and function.
So for one, we want to nourish our body right and our brain right. If we don’t get enough of those nutrients, there are negative consequences, whether its losing lean muscle mass, not being able to concentrate or not having the energy we need on a day-to-day basis."
The physics of calories
A calorie was defined as the amount of heat needed to raise the temperature of 1 kilogram of water from 0 to 1 degree Celsius. In 1925, calories became scientifically defined in terms of joules, which are units typically used by physicists to describe the amount of work needed to force one newton through one meter. This is why you sometimes see calories being called kilojoules, especially in Europe and Australia. One calorie equals 4.18 joules; 1 joule equals 0.000239006 of a calorie.
The amount of heat needed to make a calorie differs at different temperatures. Different temperatures yield different types of calories, such as the small calorie, also called the gram calorie or the 15-degree calorie. This calorie refers to the amount of heat needed to raise the temperature of 1 gram of water from 14.5 to 15.5 degrees Celsius.
A calorie in nutrition is actually 1,000 of these small calories.
Different types of macronutrients have standard amounts of calories. One gram of protein has 4 calories. One gram of carbohydrates has 4 calories. One gram of fat has 9 calories.