Energy Metabolism Explained
'How your body uses, stores and burns energy'.


Dutch translation by Niels Hatzmann (

This site hopes to explain how your body uses energy. A secondary goal is that it may help you lose weight. Before we start off, the basics must be clear. After explaining where energy comes from we move on to how fuel is transported and stored in your body. Then we treat how it is converted into useable energy, and how this energy is expended. Finally, there is a page with some rambling on how I think a lot of people, including me, get fat and how they can lose weight now that they understand how their metabolism works.

As this is a complicated subject, not everything is treated. This is mostly about energy. It is not about healthy living and vitamins, sorry. Maybe later. 1500 page books are written coverything just parts of the human metabolism. When cramming everything in a few pages, something has to go.

All information found here can be checked with medical textbooks. Most of it can be found in 'Advanced Nutritition and Human Metabolism' by Groff & Gropper. You will also see a lot of tiny numbers, these are references to pages in books or websites which back up the preceding paragraph.

An analogy

Your body is like a machine, it requires energy to run. But food is not like electricity in that it can be used to power you directly. It is far more involved than that. These complications explain how it is possible to be hungry while being overweight.


Food contains ingredients which your body can use as fuel. But even fuel is not yet energy! If I give you fully charged battery, try using it to bake an egg. Having an energy containing fuel does not mean that it is ready to use.

It all starts with food, and its metabolically active ingredients.

Metabolically active ingredients

You can see these ingredients mentioned on the packaging of most foods. With the exception of fibers, all these carry energy. Fat contains 9 kcalories per gram, carbohydrates and proteins in the order of 4 1

While fibers do not directly yield energy they are very important in making digested food leave your body again and are generally useful to have in your colon. There are indications that they may power intestinal bacteria which help you digest other food. 2a


Your body can extract at least the following three kinds of fuel from what you ingest: Fatty acids are made from fat. Longer carbohydrates are chopped up into smaller ones, leading to glucose, which is stored as glycogen. Proteins get converted to amino acids. So the conversion of ingredients to fuels is: By now you may be confused about the relation between carbohydrates, glucose, glycogen & sugar. If you want to know the details, see here.

Next: Transport and storage.