This is a summary/extract from The Ketogenic Diet by Lyle McDonald.
In one sense, the ketogenic diet is identical to starvation, except that food is being consumed. That is, the metabolic effects which occur and the adaptations which are seen during starvation are roughly identical to what is seen during a ketogenic diet. The primary difference is that the protein and fat intake of a ketogenic diet will replace some of the protein and fat which would otherwise be used for fuel during starvation.
During the first 3 days of fasting, blood glucose drops from normal levels of 80-120 mg/dl to roughly 65-75 mg/dl. Insulin drops from 40-50 µU/ml to 7-10 µU/ml. Both remain constant for the duration of the fast. One thing to note is that the body strives to maintain near normal blood glucose levels even under conditions of total fasting. The popularly held belief that ketosis will not occur until blood glucose falls to 50 mg/dl is incorrect.
Additionally, the popular belief that there is no insulin present on a ketogenic diet is incorrect. One difference between fasting and a ketogenic diet is that the slight insulin response to dietary protein will cause blood glucose to be maintained at a slightly higher level, approximately 80-85 mg/dl. This most likely occurs due to the conversion of dietary protein to glucose in the liver.
Although the liver is producing ketones at its maximum rate by day three, blood ketone levels will continue to increase finally reaching a plateau by three weeks. The decrease in blood glucose and subsequent increase in FFA and ketones appear to be the signal for the adaptations which are seen [...]. Measurements of fuel use show that approximately 90% of the body’s total fuel requirements are being met by FFA and ketones by the third day. After three weeks of starvation, the body may derive 93% of its fuel from FFA.
Most tissues except the brain, stop using ketones for fuel after the third week of ketosis. This is especially true for skeletal muscle. While muscle initially derives up to 50% of its energy requirements from ketones, this drops to 4-6% by the third week of ketosis.