NOT MEDICAL ADVICE - CONSULT A DOCTOR FOR QUESTIONS ABOUT YOUR HEALTH
But first some information about where all this energy is going.
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 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!
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.
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.
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.
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.