Vegetables, meat, eggs, rice, and more—each food has a different calorie content. Food labels also list various nutrients and calories, including protein, carbohydrates, and fat.

How are calories and calories derived from food? What do nutritionists mean by a “calorie”? How do we know how these calories in food convert into energy for the body?

How do calories in food convert into energy for the body?

On supermarket shelves, every food label tells us how many calories it contains. I know what a calorie is (it’s a quantity of energy), but how do I determine how much energy a particular food actually provides me?

Don’t think of food energy as energy for movement and running around. Our bodies use energy from food not only to move around, but also to digest and absorb food, repair daily cell damage, generate new growth, and perform thousands of incredibly complex chemical reactions.

All of this keeps everything in balance and functioning properly. As the multi-billion dollar weight loss and dieting industries demonstrate, each person uses food energy in a very different way.

Calories and Chicalories

The term used by nutritionists, a calorie, refers to the amount of energy required to raise the temperature of 1,000 grams (1 kilogram) of water by one degree Celsius. Chemists, on the other hand, refer to a chicalorie, which is one-thousandth of a nutritionist’s calorie.

People often say that exercise “burns calories.” That’s a rather imprecise statement, of course, because you can’t burn energy with fire. But as any novice cook quickly learns, you can burn food. When food burns, it releases its energy, just as when we burn coal, it releases energy. This is how they determine how much energy food contains: by actually burning it and measuring the calorie release as heat.

Metabolism

When we burn coal, the coal and oxygen produce energy and carbon dioxide. Our bodies burn food in a similar way—we call it “metabolism,” but at a much slower rate and, mercifully, without flames (a burning stomach ache doesn’t count).

But the overall result is the same: food plus oxygen produces energy and carbon dioxide. Apparently, the energy we derive from food through metabolism is precisely the same as the energy we gain from burning it.

Methods of Measuring Calories

Nutritionists place a known amount of dry food into a cylinder filled with pressurized oxygen, then immerse the cylinder in water. They then ignite the contents with electricity and measure the rise in water temperature. From this number, they can calculate how many calories were released. For every 1 degree Celsius increase in the temperature of 1 kilogram of water, 1 kcal of energy is released.

After burning every type of food, humans finally understood that every gram of protein (regardless of the type or source) releases approximately the same number of calories. The same is true for fats and carbohydrates. They found that each gram of protein and carbohydrates contains 4 kcal, while each gram of fat contains 9 kcal.

So, no one bothers burning food anymore. Chemists analyze the amount of protein, fat, and carbohydrates in a food and calculate the total number of calories.

Of course, we still bake fudge.

Why do metabolism and burning food yield the same energy?

This is truly astonishing. Regardless of how food and oxygen are converted into energy and carbon dioxide—whether by the slow metabolism of the human body or the blazing fire in a laboratory cylinder—the same amount of energy (calories) is released.

A general principle of chemistry is that if a chemical begins in state A and ends in state B during a chemical process, the total change in chemical energy is the same, regardless of how it got from A to B. We can compare energy to altitude: the higher the energy, the higher the altitude. If you start hiking from hill A to hill B, no matter what route you take from A to B, the change in your altitude (potential energy) is B minus A.