Apart from the fact, of course, that re-feeding after a fast is a very different context from adapting to a ketogenic diet from the standard dietary pattern
Refeeding after fasting
While we fast, our insulin/glucagon ratio remains low, because of the need for gluconeogenesis. Hence the metabolic situation is dominated by catabolism, and insulin remains low, as it must, so that the twin processes of gluconeogenesis and ketogenesis may proceed unhindered. The serum insulin is low in both the keto-adapted and carb-burners when they fast, because of the absence of dietary carbohydrate.
After an extended fast, such as after weeks without food in a lifeboat or months on starvation in a concentration camp, the risk of killing survivors by giving them food is very real. It is called “re-feeding” syndrome and is the result of one’s electrolytes’ getting severely out of balance, resulting in an abnormally low serum potassium (hypokalemia). If not handled correctly, hypokalemia can kill a patient, even when the doctor is aware of the problem and is proceeding cautiously.
Re-feeding syndrome can also be a problem when ending even a much shorter fast, which is a good reason not to give in to hunger when breaking a fast.
Switching from the standard American diet to a well-formulated ketogenic diet.
In the context of someone who has not been fasting, but who is now making a dietary change, the situation is completely different. There has been food intake all along, and electrolytes are highly likely to be in balance or close to it. This means that any imbalance resulting from the removal of restrictions on excreting sodium when we switch to keto is likely to manifest itself, at worst, as the “keto flu,”—which is easily remedied by eating a bit more salt. The more profound electrolyte imbalance of prolonged fasting is not going to apply in this situation, so advising eating to satiety carries with it no risk.
Under the SAD, the main reason for hunger is not lack of food intake (especially not in the case of anyone eating the recommended six meals a day plus snacks), but rather inhibition of certain hormonal pathways by excessive serum insulin. These pathways are cleared as soon as insulin drops sufficiently low, which happens fairly quickly after the restriction of dietary carbohydrate. Exactly how quickly and how low the insulin will drop will most likely depend on the person’s degree of insulin resistance, or so I would imagine.
We know the twin processes of gluconeogenesis and ketogenesis are started up in the liver shortly after excess serum glucose and excess stored glycogen are used up, because without them, the brain and the the erythrocytes will soon starve from lack of fuel. But although we generally talk as though these processes are stimulated by a drop in insulin level, they are actually initiated by a rise in serum glucagon, with insulin acting primarily as the brake to keep the processes from running away with themselves. it is the insulin/glucagon ratio which is the controlling factor, not the absolute level of either hormone. When the ratio is low, bodily processs are primarily catabolic, when high, primarily anabolic.
Eventually, however, the serum insulin level drops low enough for the hunger and satiety hormones to begin to resume their proper roles in regulating appetite. At that point, there is no reason not to rely on hunger and satiation as guides to food intake.
My experience was that it took 2 to 2-1/2 weeks of ketogenic eating before my appetite hormones re-regulated themselves. Up to that point, I was still thinking in terms of my old carb-loaded habits when judging food portions, and my appetite was still quite strong, though more easily satisfied by the quantities of fat I was eating. But one day, out of nowhere, I stopped being hungry in the middle of a meal and was forced to stop eating. I simply wanted no more food, which was weird for someone accustomed to eating to the literal bursting point while still wanting more. From that day forward, I have rarely wanted more than two meals a day, if that.
We strongly advise newcomers to eat to satiety for two reasons: first, to help overcome the deeply ingrained fear of caloric abundance; and second, to avoid the metabolic effects of restricting calories. I suppose a third reason would be to get people in the habit of trusting their bodies again. One of the most deleterious effects of our current approach to diet and nutrition (to my mind, at any rate) is the pernicious habit of thinking that our bodies are evil and must be held in check at all times.
This Puritan way of thinking—mind good, body bad—is an old Christian heresy brought into nutritional thinking by way of the Seventh-Day Adventist Church and their advocacy of veganism. The reality, however, is that eating to satiety is the best possible way of preventing gluttony, because the body is always fed the right amount—not too little, not too much—and this renders bingeing and purging completely unnecessary punishments that we can stop inflicting on ourselves. Granted, people accustomed to over-riding their bodies’ signals may have to work to re-learn what satiety feels like, but it is a fairly strong signal, one hard to ignore once one knows what it feels like.
TL/DR
At any rate, we tell people not to gorge after a fast, because it is dangerous, whereas telling newbies to trust their bodies and eat to satiety brings benefits and no known risks.
