How Do Nutrition Labels Determine How Much Energy (Calories) Foods Contain?

Let’s not think of food energy as energy to be used only for exercising and running around.

Our bodies utilize the energy we get from food not only to move, but to digest and metabolize the food itself, to repair the continuous day-today wear and tear on our cells, to build new growth, and to fuel the thousands of incredibly complex chemical reactions that keep everything balanced and working right.

As evidenced by the multi-billion dollar weight-loss and diet industry, different individuals make use of their food’s Calories in vastly different ways.

A Calorie, as the term is used by nutritionists, is the amount of energy it takes to raise the temperature of one thousand grams (a kilogram) of water by one degree Celsius. The chemist’s calorie (small c) is one thousandth of the nutritionist’s Calorie (capital C), but you generally won’t see the capital C being used.

People say that exercise “burns Calories.” That’s a very loose statement, of course. Energy doesn’t burn; you can’t set fire to it. But as any novice cook quickly learns, you can burn food. The energy in a food is released when the food burns, just as the energy in coal is released when we burn it. And that’s how they determine the Calorie content of a food: They actually burn it and measure how many Calories of heat are released.

When we burn coal, the coal plus oxygen produces energy and carbon dioxide. Similarly, our bodies burn food, we call it metabolism, although much more slowly, and mercifully without the flames. (Heartburn doesn’t count.) But the overall results are the same: Food plus oxygen produces energy and carbon dioxide. Remarkably, the amount of energy we get out of our food by metabolism is exactly the same as if we had burned it in a fire.

Nutritional technicians put a known amount of the dried food into a steel chamber full of high-pressure oxygen, immerse the whole thing in water, ignite the contents electrically, and measure the rise in the water’s temperature. From that they can calculate the number of Calories that were released: For each kilogram of water, each degree (Celsius) of temperature rise means that one Calorie of heat was released.

After setting fire to every food in sight, people eventually began to realize that every gram of protein gave off just about the same number of Calories, pretty much regardless* of which protein it was or what particular food it was a part of. And the same went for fats and carbohydrates. They found that proteins and carbohydrates contain four potential Calories of energy in every gram, while fats contain nine Calories per gram. So these days, nobody bothers to burn the foods. Chemists analyze them for the number of grams of protein, fat, and carbohydrate, and then they calculate the total number of Calories from that.

Of course, everybody still burns marshmallows.

It’s really rather surprising that when food and oxygen are converted into energy and carbon dioxide, it doesn’t matter how that conversion is accomplished, whether by slow metabolism in a human being or by a blazing explosion inside a steel container in a laboratory. The energy released, the number of Calories, is the same, either way.

It’s a general principle of chemistry: In any chemical process, if you start with chemicals in condition A and wind up with chemicals in condition B, the overall change in chemical energy will be the same, no matter how you got from A to B. We can liken amounts of energy to amounts of altitude: higher energy, higher altitude.

If you start out on a hill of altitude A and hike to a hill of altitude B, you’ve changed your altitude (your content of potential energy) by the amount B minus A, regardless of how roundabout your route may have been from A to B.