Dieting results in long term changes to hormones and muscle fibers.

[eridani]

274. Energy input required for burning protein is lower that required for burning fat

http://www.bodypeptalk.com/2012/02/18/what-happens-when-you-dont-get-enough-calories/

When the energy output is greater than the energy input, there is a negative energy balance. This means that the body has to find ways to obtain extra energy to match the expenditure and balance the equation. This energy comes from body storages, mainly glycogen[2] and fat. Don’t get too excited, though, because burning fat as fuel is not as easy as it seems. Before mobilizing fat, the body uses glucose stored in the liver because this is the fastest way to obtain energy. However, the glycogen storage is very limited, and if the energy shortage persists, the body has to come up with a different solution.

You must be thinking, “Now we’ll be using fat, right?” Wrong! The problem is that fat can only be used for energy in a few body tissues; our nervous system, for instance, is not one of them. The brain can only use glucose as fuel. Since fat cannot be converted into glucose, the body starts breaking down protein for energy, protein that comes from your hard-working muscles and other lean tissues. In addition, as you consume your own body protein, large amounts of water are lost in the process. That’s how people obtain rapid weight-loss through very low calorie diets. In reality, they are not burning fat at all – just muscle and water.

If the low calorie intake persists, the body’s last alternative is a process called ketosis. Ketosis is the partial breakdown of fat which creates compounds known as ketone bodies. The upside is that half of the brain cells can use these compounds for energy, momentarily sparing muscles and other lean tissues from being consumed. However, ketosis upsets the acid-base balance of the blood which can harm the body. In addition, the body ultimately shifts into a conservation mode, slowing down the metabolism in order to save energy. This makes you feel sluggish and fatigued.

Admittedly, this oversimplifies variations among individuals, who vary in their metabolic preferences for breaking down fat as opposed to lean tissue for energy. Mice and rats have been bred who lose really large percentages of lean tissue for energy rather than relying on fat.