The basic notion is that, by eating in a way that works with the body rather than against it, we don’t have to pay so much attention to our caloric intake, because hormonal regulation will take care of that for us. Dr. Stephen Phinney talks in his lectures about how the metabolism responds to both the kinds of foods we eat and how much we give it. The problem with cutting calories is that it signals the body that there is a famine going on, which means it responds by lowering the metabolism, cutting back on non-essential processes (nail and hair growth, for example), and holding on to fat stores for as long as possible. Better a pattern of alternate fasting and feasting, since that appears to be how the race evolved over the past two million years.
Since the body responds in the opposite way to caloric abundance, by ramping up the metabolic rate (including fat metabolism), finding ways to use energy, and feeling free to shed excess fat stores (and even by allowing fat cells to burn more energy than they need for survival), it becomes less important to worry about caloric intake, as long as we are not promoting fat storage by eating too much carbohydrate (since carbohydrate = glucose, and excess glucose => excess insulin, which is the fat-storage hormone). The recommendations for a well-formulated ketogenic diet are (1) very limited carbohydrate, (2) reasonable protein, and (3) enough fat to satisfy hunger. It has been demonstrated that on such a diet, most people with excess stored fat will shed that excess. Some people will simultaneously add lean muscle.
While I don’t believe that the interplay of appetite and satiety hormones has been fully worked out yet, we do know some things. For example, excess insulin blocks the receptors in the brain that are supposed to receive the leptin secreted by the fat tissue to say that we have enough energy on board and can stop eating for a while. For most people this works as it should, once insulin drops, and appetite can be a reliable guide as to how much food to eat.
Dr. Phinney says that the subjects eating ad libitum in his research studies generally ate about 1000 calories less than expected on a ketogenic diet, the deficit being made up out of excess stored fat. By the time their excess fat has been metabolised, they are getting their full caloric needs from their food intake, which at that point has risen to the expected level. But the point is that during the period while the subjects had excess fat to shed, their caloric deficit was handled by their hormones; it was not intentional. It is intentional calorie restriction that risks putting the body into famine mode.
Unfortunately, there are people with defects in their leptin signaling, for whom appetite is not a reliable guide, but they are rare. Another possibility is that some people simply need more food than others, such as the guy in the low-carb study who ate 3000 calories a day and still lost fat at the same rate as the other participants. Others are like the DuPont executive, documented in a case study published in the 1960’s, who lost a goodly amount of fat on a low-carb diet, but who would gain weight from eating a single extra apple. Still other people need more protein and will be hungry till they get it. The study that was used to determine the 0.8 g/kg lean body mass/day recommendation for minimum protein intake showed people over a wide range of protein needs, many of them below the average of 0.6 g/kg, and many above (0.8 is the recommendation, because it was felt to provide some cushion).
So some of the people who feel that their appetite hormones are broken may simply be hungry because they are not getting enough protein. Everyone has to experiment a bit to figure out their individual needs. If professors Raubenheimer and Simpson are correct, all mammals have an instinct for getting enough protein. The problem with human beings is that most societies condition us not to listen to our bodies, but rather to try to out-think two million years of evolutionary history.