The term we use to describe the energy derived from foods is Calorie. In other words, the terms energy and Calorie, when applied to foods, are synonymous. One calorie is defined as the quantity of heat necessary to raise one kg (1 liter) of water 1°C. What we call a calorie, therefore, is actually a kilogram calorie or kilocalorie, which is abbreviated kcal. If a food contained 100 kcal, then the energy the food contained would increase the temperature of 100 liters of water by 1°C. A capital C is used here, in the word Calorie, to indicate the kilocalorie since one calorie would actually be the amount of heat needed to raise the temperature of 1 gram of water by 1°C. How do really know how many calories food contains? How is this measured in a lab?
The Calories, or energy, from foods is measured using direct calorimetry. The instrument used is called a bomb calorimeter. This is a metal container inside which the food sample is burned in a sealed and pressurized pure oxygen environment. The food is burned inside the reaction chamber which is ignited by an electrified fuse running through the top of the chamber. This ignites the oxygen and food inside the chamber, which, as it burns, gives off heat which is absorbed into the surrounding tank. This chamber is surrounded by a water bath that is well-insulated from any changes in temperature due to the outside environment. Therefore, the heat from the food in the reaction chamber also raises the temperature of the water to some extent, which is recorded by a highly accurate thermometer. The change in temperature of the water, once the food is combusted, is the heat of combustion or the thermal energy of the food, hence the caloric content. The average caloric content of a wide variety of foods has been determined by this method. This information can be used to derive an average caloric value for types of foods, and for macronutrients.
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However, the thermal energy, or gross energy value of the food, does not represent the actual net energy that a human being can derive from the food. One reason is that digestion is not one hundred percent efficient. Since one hundred percent of all foods cannot be digested and absorbed, not all the energy can be extracted from the food nutrients through metabolism. The efficiency of digestion for different foods is called the coefficient of digestibility. This is usually given as a percentage value. For example, meats and fish, on average, have 97% digestibility.
In addition to energy lost through digestion, some amount of energy is also lost during the process of metabolism. This is especially true of protein since the body cannot oxidize the nitrogen component. The nitrogen must be combined with hydrogen to form urea, which is excreted in the urine. This loss of hydrogen, to deal with the nitrogen, represents a loss of energy, which averages about 19% of a protein molecule’s energy lost.
Because of this energy loss, the average caloric content as determined through direct calorimetry and the net energy a human derives will differ.
Lipid (Fat) Calories per Gram
Not all lipids (fats) contain the same amount of energy, as determined through calorimetry. One gram of beef or pork fat yields about 9.5kcal and this is the average for one gram lipids from meat, fish, or eggs. One gram of butterfat yields about 9.27, and one gram of dairy fat gives about 9.25. Lipids from vegetables and fruits average 9.30 kcal.
The average heat of combustion (bomb calorimeter) for lipid is generally given as 9.4 kcal per gram. The net energy (average calories) for humans is the same, at 9.4 kcal per gram of fat. This is usually rounded to 9 calories per gram.
Carbohydrate Calories
Gross energy from glucose is 3.74 kcal per gram, and 4.20 for starch. The average for carbohydrate is given as 4.2 kcal per gram gross energy, and the net energy for humans is the same. There is no way to give the exact calories per gram of carbohydrate, though. The actual amount of energy from any carbohydrate varies depending on the shape of the molecule. Therefore, the calories from carbohydrates are usually rounded to 4 calories per gram.
Protein Calories
Energy from protein can depend on the amount of nitrogen the protein contains, as well as the digestibility of the food. The higher the nitrogen content, the lower the amount of energy that can be derived through human metabolism. Proteins from meat, eggs, beans, and corn have about 16% nitrogen. Protein from nuts and seeds, and most grains (cereals) have a higher nitrogen content of around 18.9%. Protein from milk has a lower nitrogen content of about 15.7%.
In a bomb calorimeter, the average number of calories in one gram of protein is 5.65 kcal per gram. The average net energy for humans is generally given as 4.2 kcal per gram, the same as for carbohydrates. This, again, is usually rounded to 4 calories per gram.
Alcohol Calories
Alcohol (ethanol) yields around 7 kcal per gram through direct calorimetry. The net energy for humans is usually the same. If, however, a large amount of alcohol is consumed, it is possible for less energy to be available to the body, which may be due to damage to the mitochondria in the liver cells. 1Dunford, Marie, and J. Andrew Doyle. Nutrition for Sport and Exercise. Belmont, CA: Wadsworth, Cengage Learning, 2012. 45-48.,2McArdle, William D., Frank I. Katch, and Victor L. Katch. Exercise Physiology: Energy, Nutrition, and Human Performance. Baltimore: Williams & Wilkins, 1996. 83-86.
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