This is a summary/extract from The Ketogenic Diet by Lyle McDonald.
During the initial weeks of starvation, there is an excretion of 12 grams of nitrogen per day. Since approximately 16% of protein is nitrogen, this represents the breakdown of roughly 75 grams of body protein to produce 75 grams of glucose. If this rate of protein breakdown were to continue unchecked, the body’s protein stores would be depleted in a matter of weeks, causing death.
After even 1 week of starvation, blood alanine levels begin to drop and uptake by the kidneys decreases, indicating that the body is already trying to spare protein losses. During longer periods of starvation, blood levels of alanine and glutamine continue to decrease, as does glucose production by the liver. As glucose production in the liver is decreasing, there is increased glucose production in the kidney.
Because of these adaptations, nitrogen losses decrease to 3-4 grams per day by the third week of starvation, indicating the breakdown of approximately 20 grams of body protein. With extremely long term starvation, nitrogen losses may drop to 1 gram per day, indicating the breakdown of only 6 grams of body protein. However at no time does protein breakdown decrease to zero, as there is always a small requirement for glucose.
The implication of the adaptations discussed above is that the body does not require dietary carbohydrates for survival [...]. That is, there is no such thing as an essential dietary carbohydrate as the body can produce what little glucose it needs from other sources.
Of course, the price paid is the loss of body protein, which will ultimately cause death if continued for long periods of time. This loss of body protein during total starvation is unacceptable but the above discussion only serves to show that the body goes through a series of adaptations to conserve its protein. [...] the addition of dietary protein will maintain ketosis, while preventing the breakdown of bodily protein. In brief, rather than break down bodily protein to produce glucose, the body will use some of the incoming dietary protein for glucose production. This should allow maximal fat utilization while sparing protein losses.
Although the exact mechanisms behind the ‘protein sparing’ effect of ketosis are not entirely established, there are at least four possible mechanisms by which ketogenic diets may spare protein. These include decreased glucose requirements, decreased excretion of ketones from the kidneys, a possible direct effect of ketones on protein synthesis, and the drop in thyroid levels seen during starvation.