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


Expenditure of energy

Now that we know that the body actually runs on ATP and how it can be formed, it is time to address some big questions. A very interesting question which shall guide us on this page is 'How come I go hungry when I'm sure I am overweight'.

But first some information about where all this energy is going.

What does the energy do

Energy is clearly spent when you do bodily work but as it turns out, the vast majority of work is latent - it happens anyhow without you noticing it. This is called the Basic Metabolic Rate (BMR) and it is a lot.

The body spends energy on pumping your blood around, for example. Every hour, about 80 liters of blood pass through your kidneys which is a lot by any standard. Furthermore, the body needs to do work to keep you warm enough. Then there is the brain, which always requires a lot of enery, even when not thinking.

Actually doing things requires surprisingly little energy compared to the BMR. However, you can easily raise your BMR by exercising - while the exercise itself does not really cost a lot of energy, the bodily infrastructure created for it will. For 24 hours a day onwards.


The body does not always expend energy the same way and in the same amounts. This has all to do with strategy. This strategy appears to be a lot like a modern laptop: to go on as long as possible on the available fuel. Such a strategy boils down to spending a lot of energy when it is abundant and powering down in its (perceived) absence. It also means whining for more energy even when a lot is available - like a laptop claiming that its battery has run out when in fact is has hours left.

The body appears to have quite an attitide. As mentioned before, most healthy adults carry enough energy with them to survive at least a month. Most of us carry around more than that. While 'surviving' sounds big, it could probably be said that not eating for a whole week would not even tax the prowess of our metabolism.

Yet after skipping lunch, you will probably feel starved by the time you eat dinner. How come? Well, it's strategy. The body appears very attached to its energy stores. So attached that many of us grow very overweight - and still we feel hungry after missing a meal!

Strategy in detail

The strategy employed governs how energy is spent and how it is ingested. The latter part is easy. Whenever the body runs short of one kind of fuel, it sends unmistakeable signals that you should eat, even though it could also deal with the situation by conversion.

Regarding energy expenditure, the human body is generally described to be in any of three states: fed, fasting or starved. 'Fed' starts some time after eating and continues for a number of hours, after which the metabolism is said to be 'fasting'. When awaking, the body is fasting. Which is why we take 'break-fast'.

Starvation only occurs when eating does not occur for an extended period of time.

Now, when food has not arrived for a while, the body starts conserving energy. There are a lot of ways to do so - lowering the body temperature a bit, neglecting body maintenance, slowing down the brain etc.

Yet the well-fed body can really be starved

Sometimes it doesn't work. In order for conversion to work, a lot of conditions must be met. Body temperature is one - there are people with consistently low temperatures, which makes a lot of chemical reactions work slower or even the wrong way.

As said before, the process of converting stored fat into sugar is a veritable biochemical circus. A lot of things must go right and it is easy to make them go wrong.

Some theory about your blood

The blood serves as a medium for distributing fuel but it also works the other way, in taking away stuff cells have discarded. If left uncleaned the bloodstream would quickly become polluted. So there are at least three organs working on cleaning it up: two kidneys and the liver.

The kidneys function primarily as sophisticated filters. As long as things are kept wet enough, they are able to remove waste from the blood by osmosis. This waste may be treated further and is sent to the bladder.

The liver is a lot more complex and actually converts a lot of waste products into usable substances again. It can also break down molecules which cannot be filtered by the kidneys. Almost all energy conversions taking place in your body are centered around the liver.

The role of water

Update: the following to paragraphs probably simply things too much

As mentioned, the kidneys need water to function, they need to be wetter than your blood. If they cease to be so, they become unable to filter the blood, leaving (part of) this job to the liver. The liver then becomes occupied doing that and has less time or capacity left for conversions.

When this occurs the body may really become starved. The liver is unable to furnish the brain with sugar and no other energy sources are available. This quickly leads to a dizzy feeling and general incompetence.

Luckily emergency energy is available through the burning of ketones (which can be formed out of fat), so people don't actually lose consciousness in this case, but it sure does feel bad.

After all these fascinating facts, read on for some amateur advice.