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The calorie is generally defined as the metric unit of measurement for energy. In most fields, the calorie has been superseded by the joule, the SI unit (French for Système International d'Unités, [International System of Units]) for energy. However, the calorie (cal) remains in common usage by nutritionists and others in the food industry as the unit of measurement for heat energy obtained from the digestion of foods. It is also used to measure the amount of heat energy expended by the human body when performing various activities.

The food calorie—sometimes also called the nutritional or dietary calorie—s usually stated simply as the calorie. It is equivalent to the kilogram calorie (kcal), or kilocalorie. The kilogram calorie, or the large calorie, is defined as the heat energy necessary to increase the temperature of one kilogram of water by one degree within the Celsius temperature scale. It is equivalent to about 4.185 kilojoules (where one kilojoule is equal to 1,000 joules). Further, the gram calorie, or the small calorie, is the amount of heat energy necessary to raise the temperature of one gram of water by 1° Celsius. It is equivalent to about 4.185 joules. Thus, one food calorie is equal to one kilogram calorie, which is equal to 1,000 gram calories. When a label on food reads 4.5 calories, these 4.5 food calories are the same as 4.5 kilogram calories, or 4,500 gram calories.

Many industrialized countries worldwide require their food manufacturers to label the caloric content of their products. Since 1994, the United States Food and Drug Administration (FDA)—through the Nutrition Labeling and Education Act (NLEA) of 1990—has required that most packaged food products contain a Nutritional Facts label. The FDA mandates that food manufacturers abide by these NLEA-stated guidelines with respect to the presentation of these labels, being regulated primarily by the type of food and its package size. Some of the information found on these labels includes total calories, calories from fat, total fat (and saturated fat), cholesterol, sodium, total carbohydrates, dietary fiber, sugars, protein, vitamin A, vitamin C, calcium, and iron. Thus, the calories listed on these Nutritional Facts food labels are the amount of energy that these particular foods provide the body minus the energy that is expended during digestion.

However, food calories are not technically measured as amounts of heat energy but, rather, as approximate comparisons of the energy density (in units of kilocalories per gram) for a large number of different food samples. Generally, a food product is first analyzed by separating it into its constituent parts (carbohydrates, proteins, fats, sugars, etc.). The results are then compared to standardized chemical tests in order to estimate the product's digestible constituents. The results are converted into an equivalent energy value based on an internationally approved conversion table for energy densities of predetermined food samples. For instance, fats have an energy density of 9 kilo-calories per gram (kcal/g), which is an especially high value. Proteins and carbohydrates have a value of 4 kcal/g. Special metabolic equipment can then be used to evaluate the caloric output (consumption) that occurs when performing specific physical activity such as running and walking.

Although the calorie is used as the unit for comparing foods eaten, foods contain other substances that are needed in the human body. Some nutrients are needed because the body does not produce them or produces them in amounts that are too minute. These so-called essential nutrients are obtained from food sources such as carbohydrates, fats, minerals, proteins, vitamins, and water. Nonessential nutrients are those nutrients that are already manufactured in the human body and, thus, are not necessary to be acquired from the intake of food. One such nonessential nutrient is cholesterol.

Each person requires various amounts of essential nutrients depending on such factors as age, gender, health, and certain other conditions. Specific health conditions such as illness, pregnancy, and breastfeeding often increase the need for certain essential nutrients. Dietary guidelines, which take many of these conditions into account, provide general information for daily nutritional needs. The U.S. Department of Health and Human Services (HHS) and the Department of Agriculture (USDA) jointly publish Dietary Guidelines for Americans every five years. These guidelines, provided since 1980, help to communicate good dietary habits in order to promote health and well-being and reduce diseases and illnesses. The recommended daily energy intake values for adult men in the United States is 2,500 kcal (with a range, depending on size and age, of about 1,700 to 2,800 kcal) and 2,000 kcal (with a range of about 1,300 to 2,100 kcal) for women.

When the human body eats food, a certain amount of energy is received by the body from the food. This amount of energy (number of calories consumed by humans) is called caloric intake. The particular amount of energy taken in depends on the specific foods, with some foods providing more energy than other foods.

The average number of food calories for a few commonly eaten foods and drinks include:

  • coffee (one coffee cup, 0.46 pint; 220 ml): 15.4 calories
  • apple (4 oz; 112 g): 53 calories
  • brown bread (one medium slice): 74 calories
  • orange juice (one drinking glass, 0.42 pint; 200 ml): 88 calories
  • American cheese (1.5 oz; 42.5 g): 110 calories
  • doughnut (1.7 oz; 49 g): 140 calories
  • almonds (1 oz; 28 g): 171 calories
  • chicken breast (7.1 oz; 200 g): 342 calories
  • chocolate (3.5 oz; 100 g): 530 calories

The amount of energy that the human body uses up during a specific activity is called caloric output. The size and weight of a person and the amount of effort and time needed for a particular activity affects the specific amount of caloric output. For the most part, more energy is required as the intensity and strenuousness of the sport increases. The average food caloric output for a 150-lb (68-kg) person in various sports activities is the following:

  • golf: 270 calories per hour
  • ice skating (leisurely): 300 calories per hour
  • walking on a level surface: 360 calories per hour
  • water skiing: 390 calories per hour
  • bicycling at 10 mph (16 km/h): 420 calories per hour
  • racquetball: 540 calories per hour
  • swimming (recreationally): 600 calories per hour
  • running at 7.5 mph (12 km/h): 750 calories per hour
  • walking upstairs: 1,050 calories per hour

In any sport, food calories are an important part of how well an athlete trains and ultimately competes. Eating a balanced diet is critical to sports nutrition. The correct combination of fuel (calories) from carbohydrates, proteins, and fats gives a person plenty of energy for top performance.

Carbohydrates are considered the most important source of fuel (calories) for the human body. They come in the form of such foods as breads, cereals, fruits, honey, pastas, rice, and vegetables. For a balanced diet, proper health, and peak performance, carbohydrates should provide at least half (usually about 60% to 70%) of a person's daily calories. In addition, about 12% to 15% of a person's daily caloric requirements should come from proteins. They come in such foods as beans, dairy products, eggs, and nuts. Proteins provide the human body with power to build new tissues along with other necessary functions. Proteins are not stored in the body but burned immediately for energy or converted to fat. Only small amounts of fats are needed. They come in the form of saturated fats from such products as cheese, eggs, meats, and milk, and as unsaturated fats from such products as corn oil, palm oil, and sunflower seed oil. Fats are used as an energy source primarily when resting or performing low to medium intensities of exercise.

Whether planning for an endurance-type sporting event or exercising on a daily basis, the sports-minded and health-minded individual should consistently eat foods that provide the proper amount and types of calories. On the day of an extended sporting event or activity, a person is likely to perform better by following tips that include:

  • Eat a good meal in the morning in order to provide necessary nutrients and hydration throughout the day.
  • Eat meals high in carbohydrates.
  • Choose easily digestible foods and avoid high-fat and high-protein foods.
  • Eat three to four hours before the event, and drink liquids two to three hours before.
  • Avoid sugary foods and drinks within one hour of an event.
  • Drink sufficient fluids to ensure hydration—that is, about 20 oz (0.60 l) of water one to two hours before exercise, and an additional 10 to 15 oz (0.30 to 0.45 l) within 15 to 30 minutes of the event.
  • Consume sports drinks that contain a large amount of carbohydrates in order to provide extra energy; the most effective sports drinks contain over 0.53 oz (15 g) of carbohydrate in every 8 oz (0.24 l) of fluid.
  • Replace fluids lost to perspiration (generally drink 3 oz to 6 oz [0.9 to 1.8 deciliters]) of water or dilute sports drink every 10 to 20 minutes throughout the sports activity.

Every person has different caloric requirements depending on such attributes as age, body size, health, and activity level. If a person is within his or her ideal weight range, then that person is probably getting the correct amount of calories. However, the correct type of calories must be consciously monitored by each person in order to maximize the good calories (those from healthy foods containing a high density of nutrients) and minimize the bad calories (those from unhealthy foods containing few nutrients).

see also Caloric intake; Carbohydrates; Energy drinks; Exercise and fluid replacement; Fat intake; Nutrition and athletic performance; Protein ingestion and recovery from exercise.