What Is a Calorie?

In embarking on an entire book about calories, we have to begin at the beginning —what to call them. Calories are units of work or heat, but what they are called depends on who is doing the calling. We think the name inconsistencies can be so confusing that we summarize them in table 1. We believe there is an easier, commonsense way to think about the definition, as we will explain. To get to that point, let's begin with the official definition used by chemists:

One calorie is the amount of heat energy needed to raise the temperature of one gram of water by one degree centigrade, from 14.5° to 15.5°, at one unit of atmospheric pressure.

As a first step toward arriving at a more intuitive definition, let's ignore the ambient temperature and atmospheric pressure, as they usually do not make much difference except to scientists. But pay close attention to the weight units. In table 1, we emphasize gram in this definition to distinguish it from definitions based on heating an amount of water 1,000 times greater—a kilogram. A gram is about one quarter of a teaspoon. A kilogram is 2.2 pounds.

Gram units are inconvenient for most Americans, and so are so-called small calories, based on a gram standard. If food labels listed calories by the chemists' definition, your daily calorie requirements might be about 2,500,000 a day rather than 2,500, and a carrot would provide 25,000 calories instead of 25. That is why nutritionists much prefer to use kilocalories (kcal), units that are 1,000 times larger and based on the heat required to raise the temperature of a kilogram of water by one degree centigrade.

Here is where things get tricky. Because it feels awkward to say kilocalories, nutritionists shorten it to Calories, spelled with a capital C. Food labels list Calories, but they really mean kilocalories (also known as large calories or kilogram-calories). When spoken, Calories (capital C) and calories (small c) sound the same. As a result, when discussing the energy value of food, the words kilocalories (kcal), Calories (Cal), and calories (cal) have come to mean exactly the same thing: 1,000 chemists' calories. Hence: confusion.

The peculiar result of all this is that the word calories can mean two things at the same time: chemists' calories and nutritionists' Calories, which are 1,000 times greater. In common practice, most people use calories, capitalized or not, to mean kilocalories or Calories, despite the confusion this causes.

James Hargrove, who has written a fine history of the evolution of the terms, argues that "it is untenable to continue to use the same word for different thermal units (gram-calorie and kilogram-calorie) and to use different words for the same unit (Calorie and kilocalorie). The only valid use of the Calorie is in common speech and public nutrition education."

We, as it happens, are in the business of public nutrition education, and we use common speech all the time. We do not view this situation as untenable, just confusing. The various definitions lead to problems that can be amusing or annoying, depending on how you look at them. For example, Hargrove points to the inconsistent use of the terms calories and kilocalories—in this case meaning the same thing—in the U.S. Code of Federal Regulations rules for food labels: "A normal serving of the food contains at least 40 kilocalories (that is, 2 percent of a daily intake of 2,000 kilocalories).... The food contains all of the following nutrients per 100 calories based on 2,000 calorie total intake as a daily standard."

Public nutrition educators that we are, we follow common speech. Throughout this book we use calories, spelled with a small c, to refer to kilocalories/ Calories, unless we have a specific reason to use a more precise term. Food labels do this too.

JOULE CONFUSION

With this said, we must now introduce another source of confusion. We live in the United States. Practically everywhere else in the world, people use the metric system and follow the metric-based International System of Units (Système International d'Unités). In this system, food energy is expressed in joules or in their thousandfold-larger counterpart, kilojoules. We give the conversion factors in table 1. These terms also are used imprecisely, and we hear people talking about joules when they really mean kilojoules.

The easiest way to think of the conversions is to remember that 100 calories is about 420 kilojoules (or calories times 4.2) and that 2,400 calories is equivalent to 10,000 kilojoules. If you only want estimates, you can multiply calories by 4 to get kilojoules or divide kilojoules by 4 to get calories. This is obviously imprecise but should work well enough for most purposes. As you will soon see, "works well enough for most purposes" is a constant theme in this book.

And just for the record, let's add one more term. Nutrition scientists like to use megajoule (MJ) to describe...