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What is a Calorie?
Chemistry and Nutrition Definitions

What is a calorie? If you ask chemists and dietitians to define "calorie", they will give you two different answers. Let's look at the calorie definition from a chemist's point of view first.

Chemistry calories

To chemists, a calorie is the amount of heat required to raise the temperature of one gram of water one degree Celsius.

To be more precise, chemists usually mean the energy needed to heat the water from 14.5 degrees C to 15.5 degrees C.

They also assume that the reaction occurs at a pressure of one atmosphere. 

A calorie is a very small amount of heat, since one gram of water is a tiny amount: only 1/28th of an ounce. Today, chemists seldom use calories to measure energy.

Instead, they usually measure it in joules. Joules are even smaller than calories. One calorie = 4.184 joules. Dietitians never use chemistry calories or joules when they talk about food.

Food calories

You might expect the energy in a food calorie to be similar to that in a chemistry calorie. After all, one calorie worth of food seems like a small amount, doesn't it? It is not! It is 1000 times the energy in a chemistry calorie. That is why nutrition books call food calories kcals, and food joules kilojoules or KJ. One calorie (kcal) = 4.182 kilojoules (KJ).

How do we know the number of calories in a portion of food? Food scientists could dry the food and then burn it in a bomb calorimeter, but the heat givien off would not equal the calories we absorb. The fiber would burn and give off heat, but our bodies don't absorb many of the calories from fiber. Most of it just passes through our digestive tract and out.

Instead of using a bomb calorimeter, food scientists use more accurate chemical tests that ignore the fiber. They measure how much fat, protein, and carbohydrate are in the food. Most of our calories come from one of the these three macronutrients:

Macronutrient Calories (kcal)
 per gram
kilojoules (KJ) per gram
Fat 9 38
Protein 4 17
Carbohydrate 4 17

As you can see in the above table, fat has more than twice the calories per gram of protein and carbohydrates. This is a big reason we limit our fat calories to lose weight. People who drink alcohol also get calories from ethanol, at 7 calories (30 KJ) per gram. Those are calories we don't need.

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