The term calorie refers to a unit of energy. It can be further divided into two terms—small calorie and large calorie—depending on how it is used and in what region of the world. The general term for a “small calorie” (or the gram calorie [cal]) is defined as a unit of energy that is equivalent to the amount of heat needed to increase the temperature of one gram (g) of water (H2O) by one degree of Celsius (C); that is, from 14.5°C to 15.5°C. Two other definitions for the small calorie are (1) the unit of heat energy that is equal to 4.1855 joules (J) and (2) the unit of heat energy that equals 1/860 international watt-hour (W-h).

As a unit of food energy, the “large calorie” (or kilogram calorie [kilocalorie or Cal]) is the amount of energy needed to increase the temperature of one kilogram (kg) of water by one degree Celsius. Thus, one large calorie equals 1,000 small calories. One kilocalorie (sometimes also called a dietary calorie or food calorie) is also defined as the amount of energy that food has the potential to produce when consumed and digested in the body.

The kilocalorie is often called a “calorie” when referring to the amount of energy produced by foods, such as when calculating the number of food calories consumed on a daily basis. Though confusing, when a person in the United States is referring to “calories” in a diet or with food, one is really talking about “kilocalories.”


A woman uses a calorie-counting app on her phone to track how many calories she consumes in a day. These food calories are converted into physical energy or are stored in the body as fat.

A woman uses a calorie-counting app on her phone to track how many calories she consumes in a day. These food calories are converted into physical energy or are stored in the body as fat.

People all consume calories as a part of their eating of various types of food each day. Some foods, because of the various essential nutrients they contain, possess more calories than do others (by mass). Generally such essential nutrients as carbohydrates, fats, fiber, and proteins all release energy to the body. However, they do so in varying amounts. For instance, fats release a large amount of energy, about 9 kilocalories per gram (38 kJ/g). (Remember that in the United States, the term kilocalorie is what people call calorie.) Carbohydrates and proteins release less, about 4 kilocalories per gram (17 kJ/g). Alcohol is also a source of calories, with about 7 kilocalories per gram (30 kJ/g). In other words, a food high in fat has more calories than another food high in protein, when both have the same mass (in grams).


In 1824, French chemist and physicist Nicolas Clément (1779–1842) became the first person known to define and utilize the calorie at a unit of heat. Clépublished his definition—that of a kilocalorie—in the journal Le Producteur. Although in modern times, the definition for kilocalorie as a unit of energy has been generally replaced with the unit of joule (J) by the International System of Units (SI), it is still used as a measure of food energy. Today, the unit of “kilocalorie” is commonly used to describe the number of food calories that one eats during a day.


Calories are burned (expended) during exercise or physical activity. Even moving a finger to change channels on the television burns a tiny bit of calories within the body. Thus, the amount of calories expended depends on the length and type of exercise, along with other related factors such as the environment and heredity. For instance, according to the National Heart, Lung, and Blood Institute, for a 175-lb. man and a 140-lb. woman performing light activity (such as daily housework, playing a round of golf, or working in an office) the number of calories expended in 1 hour is about 300 calories and 240 calories, respectively. For moderate activities such as bicycling, dancing, and walking briskly, men burn about 460 calories per hour, while women expend around 370 calories. According to the United States Department of Agriculture, avoiding oversized portions isthe bestwayto maintain balanced calorie consumption from a healthy variety of food sources.

For strenuous activities such as playing football, swimming, or jogging, men will burn about 730 calories per hour, while the woman's rate is approximately 580 calories in the same time. For very strenuous activities, such as skiing or racquetball, men expend 920 and women 740 calories per hour. However, just raking leaves or walking around a lake help to burn off calories eaten during the day.

The main point to food calories is that regardless of where the calories come from, they are either converted to physical energy (as when people perform physical exercises) or are stored in the body as fat. If exercises and physical activities do not convert all the calories consumed in one day, then the remaining calories (not consumed) are stored as fat in the body. These stored calories do not go away unless they are used up in another day with additional physical activity or a person consumes less calories daily in the future so the body uses some of its stored fat reserves for conversion into energy.

Thus, calories are the key to weight control. If more calories are consumed than are expended in any day, then a person gains weight. On the other hand, if fewer calories are consumed, then a person loses weight. And, if the same number of calories is consumed as is expended in any particular day, then a person remains at the same weight. For example, if 3,500 calories are burned in one day from playing basketball, walking, doing yard work, and all the other daily activities, and 3,000 are consumed as food in the form of calories, then 500 calories have been taken from fat and burned by the body for energy to the muscles and other life processes. If this happens for one week (seven days), then 3,500 calories of fat have been eliminated from the body and the person has lost about one pound.

To maintain a healthy lifestyle, the U.S. Department of Health and Human Services (HHS) recommends that most Americans should do a minimum of 2.5 hours per week of moderate aerobic activity (such as walking or swimming) or 1.25 hour per week of vigorous aerobic activity (such as running). In addition, the HHS recommends that strength-training exercises (anaerobic exercises such as lifting weights) should be performed at least twice per week. This amount of physical exercise will make it more likely that a person maintains a healthy weight and fit lifestyle. Thus, when it comes to weight loss and maintenance it all comes down to calories consumed (“How much one eats”) and calories expended (“How much one exercises”).


People can “watch” their calories by counting them before the calories are consumed within food. Many free online calorie counters are available on the Web for just this purpose. For instance, FitWatch. com provides the Calorie Counter & Fitness Tracker at its website ( ), and ( ) has a calorie counter program that is geared to one's own height, current weight, and goal weight. With these tools, a person can check out the particular foods they are having that day, and the counter will state how many calories are in various serving sizes. Such information will better prepare one to stay within a stated diet for a much better chance to lose weight or maintain an already desired weight.


A scale and unit of measurement for temperature, where the freezing point of water is 0°C and the boiling point of water is 100°C.
The ability of a physical system to do work on other physical systems.
A metric unit of mass.
The International System of Units (SI) unit of energy.
The International System of Units (SI) unit of mass, where one such unit is equal to 1,000 grams.

Risks of undernutrition or malnutrition is more common than overnutrition in people from very poor countries. However, the opposite is true in developed countries of the world, such as the United States and Australia, two countries with among the highest percentages of obese citizens in the world. Medical reasons may also cause such caloric-dependent conditions, such as people who lose their appetite because of having to deal with a serious medical problem. In addition, infants, children, and adolescents are often at increased risk from undernutrition because of their extra need for calories and nutrients as they grow rapidly into adulthood.

Older people are also more apt to eat less calories and nutrients than other ages of people. Statistics show that about 14% of older people in the United States live on less than 1,000 calories each day. Such a small caloric intake of food is not adequate to maintain healthy nutrient levels.


Calories are important to life. Humans need food to live and substances that contain calories (and provide energy) are in foods. Maintaining a fit and healthy lifestyle is founded on eating nutritious foods, exercising on a regular basis each week, and maintaining a stable body weight This all revolves around eating the same number of food calories each day as are expended by the body in physical activities.

The U.S. Food and Drug Administration (FDA) came out in June 2011 with its new Food Plate, which replaces the Food Pyramid. The FDA divided “My Plate” into four sections: green for vegetables, red for fruits, orange for grains, and purple for proteins. Off to the side, it has a blue section for dairy. By going to the website ( ) of the U.S. Department of Agriculture, one can learn more about the importance of calories, maintaining a healthy lifestyle, and eating nutritious foods from each of the basic food groups. The website answers such questions as: “What types of food are in the protein group?” “What does empty calories mean?” and “Why is physical activity important to me?”

  • Where can I learn more on the Web about proper nutrition?
  • Why can't I lose weight?
  • What are empty calories? Why are they bad for me?
  • What lifestyle changes should I implement to become healthier?
  • Are diets bad? Can some diets help me? What should I look for when I am comparing diets?
  • What local organizations can help me control my eating?
  • How can I know the number of calories in food when eating at restaurants?

See also Carbohydrates ; Exercise ; Obesity ; Protein ; Weight loss .



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Karasu, Sylvia R., and T. Byram Karasu. The Gravity of Weight: A Clinical Guide to Weight Loss and Maintenance. Washington, DC: American Psychiatric, 2010.

The Mayo Clinic Diet: Eat Well, Enjoy Life, Lose Weight. Intercourse: Good Books, 2012.

Roth, Ruth A. Nutrition & Diet Therapy, 11th ed. Clifton Park: Delmar Cengage Learning, 2013.

Rust, Rosanne. The Calorie Counter for Dummies. Hoboken: Wiley, 2010.

Smith, Ian K. Eat: The Effortless Weight Loss Solution. New York: St. Martin's Press, 2011.


“Calories Burned During Fitness Activities.” MedicineNet. com. February 1, 2005. (accessed January 13, 2017). U.S. Department of Agriculture. May 31, 2011. (accessed January 13, 2017).

“Counting Calories: Get Back to Weight-loss Basics.” Mayo Clinic. April 11, 2015. (accessed January 13, 2017).

Morley, John E. “Undernutrition.” Merck Manual. August 2007. (accessed January 13, 2017).


American Dietetic Organization, 7185 SE Seagate Lane, Chicago, IL, 60606, (312) 899-0040, (800) 877-1600, .

Food and Drug Administration, 10903 New Hampshire Ave., Silver Spring, MD, 20993, (888) 463-6332, .

U.S. Department of Agriculture, 1400 Independence Ave., SW, Washington, DC, 20250, (202) 720-2791, .

World Health Organization, Ave. Appia 20, Geneva, 1211 27, Switzerland, 41 22 (791) 21 11, Fax: 41 22 (791) 31 11, .

William A. Atkins, BB, BS, MBA

  This information is not a tool for self-diagnosis or a substitute for professional care.