Energy—the capacity to do work—comes in different forms, including electric, heat, chemical, and nuclear energy. As discussed in Chapter 8, a calorie (note the small c) is a unit of heat energy; specifically, it is the amount of heat necessary to raise the temperature of 1 gram of water 1˚C. Because this is such a tiny amount of energy, physiologists commonly use the kilocalorie (kcal) as a unit of measure (1 kcal = 1,000 calories). Nutritionists also use the kilocalorie as a standard unit of energy, but they traditionally refer to it as the Calorie (Cal), which is capitalized to distinguish it from the single calorie.

Just about any food container you pick up in the United States carries the label “Nutrition Facts,” which includes the item “Calories.” How do Calories (or kcal) relate to the discussion in Chapter 9 about how energy in the chemical bonds of food molecules is transferred to the high-energy phosphate bonds of adenosine triphosphate (ATP) and is then used to do cellular work? And why do we use heat energy as a measure of nutrition?

The reason is found in the *laws of thermodynamics, which tell us that energy cannot be created or destroyed, but can be converted from one form to another.

However, every energy conversion is inefficient; a large portion of the original energy always ends up as heat. Whether we are using the energy in glucose to make ATP or are using that ATP to power muscle contraction or ion transport, most of the available chemical energy is lost as heat. The bottom line is that if an animal is not growing, not doing any external work, and not changing its body temperature, the heat it loses to the environment is a measure of its total energy expenditure, or metabolism.

An animal’s energy needs must be met by the ingestion, digestion, and assimilation of food. The basal energy expenditure (basal metabolic rate, BMR) of a human is 1,300–1,500 Cal/day for an adult female and 1,600–1,800 Cal/day for an adult male. Any physical activity adds to the BMR. For a person doing sedentary work, about 30 percent of the total Calories expended are used for skeletal muscle activity; for a person doing heavy physical labor, more than 95 percent of caloric expenditure is for skeletal muscle activity.

The components of food that provide energy are fats, carbohydrates, and proteins. Fats yield 9.5 Cal/gram, carbohydrates 4.2 Cal/gram, and proteins about 4.1 Cal/gram. Figure 50.2 shows some equivalencies of food, energy, and energy consumption.

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Even though the units calorie, kilocalorie, and Calorie remain in popular use, most scientists now use the International System of Units (ISU). In this system the basic unit of energy is the joule: 1 joule = 0.239 calories, and the measure of energy use is 1 joule/second = 1 watt. You are familiar with light bulb ratings, so think about that when you convert kcal/day into watts. The 1,700 Cal/day energy expenditure of the average man converts to 82 watts (note that a watt includes the time dimension, so it is a rate of energy use).

Thus it is possible to quantify the caloric value of any food an animal eats. It is also possible to quantify the caloric expenditure of any activity or behavior an animal performs. By comparing calories consumed with calories expended, we can construct energy budgets that allow ecologists and evolutionary biologists to apply a *cost–benefit analysis to feeding behavior.