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Calories, which measure the amount of energy contained in food, are necessary fuel for the functioning human body.

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What Is a Calorie?

A calorie is a unit of energy that measures the amount of heat it takes to raise the temperature of water by one degree. Calories in food are a source of fuel, and the number of calories assigned to particular foods—like carbs, fats, and proteins—represents the amount of potential energy it can give you.

Nutritionists recommend that adult men consume approximately 2,500 calories per day and that adult women consume approximately 2,000 calories per day for weight maintenance. Some people choose to consume fewer calories than they burn in a day, creating a calorie deficit that may achieve weight loss.

Where Do Calories Come From?

There are calories in everything you eat, which your body needs for energy and nutrition. Different kinds of foods net different amounts of calories, such as:

  • Fats: One gram of fat contains nine calories, making high-fat foods like cheese, avocados, and oils among those with the largest calorie count.
  • Carbohydrates: One gram of carbohydrates contains four calories. At least half of your daily calories should come from carbohydrates. Carbs like rice, grains, and starches are a quick source of energy that breaks down easily.
  • Proteins: One gram of protein also contains four calories. Approximately less than a third of your caloric intake should come from proteins like meat, fish, and nuts.

How Many Calories Should You Consume?

The recommended number of calories you should consume in a day is different for every person. According to the US Office of Disease Prevention and Health Promotion, adult men should consume around 2,000–3,000 calories per day, and that adult women should consume around 1,600–2,400 calories per day. The specific amount of calories each person needs also depends on age, genetics, metabolism, physical activity level, and current weight.

How Are Calories and Kilocalories Different?

There are two kinds of food calories, a small calorie and a large Calorie (or kilocalorie). Here is how the two are different.

  • Small calorie: A small calorie, spelled with a lowercase “c,” refers to the amount of heat needed to raise the temperature of one gram of water by one degree Celsius. This measurement is rarely used outside of the physical sciences and chemistry research.
  • Large Calorie: A large Calorie, spelled with an uppercase “C,” refers to the amount of energy needed to raise the temperature of one kilogram (or 1,000 grams) of water by one degree Celsius. Large Calories are alternatively referred to as kilocalories (or kcal) to mitigate confusion with small calories. Calorie contents listed on food labels refer to large Calories, or kcals. When nutritionists refer to “calories,” they are referring to large Calories.