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.
“Our brains, our muscles — every cell in our body — require energy to function in its optimal state,” said Jennifer McDaniel, a registered nutritionist dietitian in Clayton, Missouri, and spokesperson for the Academy of nutrition and dietetics “So for one, we want to nourish our body right and our brain right. If we don’t get enough of those nutrients [that calories provide], there are negative consequences, whether it’s losing lean muscle mass, not being able to concentrate, or not having the energy we need on a day-to-day basis.