In the continuation of the previous post, I am going to try to explain how energy is obtained from foods, and how it is measured. As stated, in part I, energy is stored in foods that we eat in the bonds between atoms (mainly between carbon atoms). When these bonds are broken, energy is released and then trapped by the body. Let’s take dietary fats, for example. Here is an example of the chemical structure of fats:
C – carbon
H – hydrogen
O – oxygen
This is just a 2 dimensional drawing. Each line between the atoms represents a bond. It takes energy for this bond to be constructed, and vice-versa, energy is released as the bond is broken.
These bonds are necessary because they keep the atoms glued together and give the structure as a whole the properties that it naturally has.
So, let’s say that you eat this one fatty acid (they are called acids because the –COOH group on the left is acidic). Essentially, all of those bonds between carbons will be broken, and the energy that it took to create the bonds will be released. Then, through the magic of biochemistry (basically creating the same type of bonds in other molecules), the human body would gather up that energy and move it to wherever it is needed.
In the United States, energy in foods is measured in calories. The technical definition of a calorie is “the amount of energy it takes to raise one gram of water by 1° Celsius.” In Canada, the amount of energy in foods is measured in joules (1 calorie = 4.184 joules).
However, the “calorie” on a food label is actually a KILOcalorie (Kcal). The pre-fix kilo means 1000. So a kilocalorie is actually 1000 calories. This is very confusing, and I am really not sure why it is done this way. Therefore, when you see that a banana has 100 “calories”, know that it should technically be 100 kilocalories (4184 joules).
Also, when you use a metabolic calculator to figure out the amount of energy you need, it should say 2000 KILOcalories rather than 2000 calories.
Different macronutrients have different amounts of energy stored in their bonds. A gram of carbohydrate has 4 kcal, a gram of protein has 4 kcal, and a gram of fat has 9 kcal. Notice that the measurements given are kcal/gram, which is also called energy density. Since fat yields more energy for the same weight than do carbohydrates or proteins, it is said to be more energy dense.
In the part III, we will talk about how energy is stored in the human body.