When you first deprive your body of carbohydrates and replace them with protein and fat, the metabolism begins to shift to accommodate this. The first stage is known as lipolysis, and is the initial burning of fat to use as energy.
Ketosis, put simply, is the second part of the process that takes place when your body's metabolism shifts from getting energy from carbohydrates to taking it from fat. When it is taking place, this is the time that you lose the most fat. The name ketosis relates to the blocks fat that are stored for release as energy, which are known as ketones.
Ketosis should not lead to a loss of muscle tissue, unless you are completely out of fat. This makes you even better at burning fat, as you retain the strength throughout your body to do so, particularly in the smaller internal muscles. When ketosis is taking place it is possible to lose up to a stone in little over a week.
The main side effects of ketosis are bad breath and sluggish mobility. Bad breath is caused due to a change in the bacteria and enzymes throughout the digestive system, and slow movement because fat is much harder to use for energy production, making you low on energy. Neither of these are likely to be harmful.
However, if you are diabetic, ketosis can lead to a condition called ketoacidosis, which results in the blood becoming more acidic. This will then cause symptoms including confusion, vomiting, and can even go as far as causing comas and death.
If you want to know if your body is carrying out ketosis, head down to your local chemist and buy ketone testing strips. These operate in a very similar way to universal indicator strips (if you can remember your old chemistry lessons) and will turn purple if ketones are contained in the urine.