"What happens on a fast if nutrients are introduced"?
"When in ketosis say after day 4 of a Water Fast is there a sudden reversal of the body's metabolic state when a small amount of food is introduced"?
In answer to the second part of your question, there is no sudden reversal of metabolism if you break the fast once it has been established, and lowered after 4 to 6 days of between one-fourth to two-fifths. It will take at least 4 weeks for the metabolic rate to normalize after refeeding.
However, if nutrients are introduced at any stage of the fast, then this will inform the body not to rely on its own internal reserves exclusively, and therefore prevent the conservation of nutrients; it is at this point that the body will rely partially on external nutrients. If this continues, and at various intervals, then what Shelton referred to as a "lop-sided diet" will take place, and the the very real risk of malnutrition and its associated diseases will occur eventually, and sooner rather than later.
Water only will conserve nutrients to the max' in the the bodys attempts to prevent ultimate starvation (the feeding of the body on its vital tissues), whereas a diet, even a low calorie diet that is not varied, will bring about malnutrition also.
The awful condition for sufferers known as "anorexia-nervosa" will bring on malnutrition eventually, because of a severely restricted diet. Reducing diets for weightloss allow the dieter to remain in partial ketosis because of calorific restriction, but unless their diet is "nutrient-dense", then the overall risk is one of malnutrition as well.
We should also bear in mind that today's food supply is mostly nutrient-depleted because of commercial/intensive farming methods, in comparison to the food supply of yesteryear; so all the more reason to eat organic foods of high nutrient-density before the commencement of a fast.
Dr Al Sears MD has made a scientific/analysis/comparison between the nutrition of an apple grown over 20 years ago, with those grown today: you would have to eat 20 apples grown today, for the same nutrition of just one of those apples grown at that time.
Personally, I supplement my diet for that very reason, and the difference in health is so very markedly noticeable, even though I live mostly on an organic Nutritarian diet.
I hope that has helped, and if you have any other queries feel free to ask.