Hunger and satiety are two important mechanisms involved in body weight regulation. Even though humans can regulate food intake by will, there are systems within the central nervous system (CNS) that regulate food intake and energy expenditure. This complex network, whose control center is spread over different brain areas, receives information from adipose tissue, the gastrointestinal tract (GIT), and from blood and peripheral sensory receptors. The actions of the brain's hunger/satiety centers are influenced by nutrients, hormones and other signaling molecules. Ketone bodies are the major source of energy in the periods of fasting and/or carbohydrate shortage and might play a role in food intake control.
The second type of cellular fuel comes from fat and fat metabolism products called ketone bodies. The average sized human body can store hundreds of thousands of calories in the form of fat, so we could say that this system of energy is almost unlimited, depending on how long one goes without food. Eventually, it would get used up, but people have been known to fast for months and live through it.
This is why epilepsy patients have to get prescribed diets from profession nutritionists. Without getting into true ketosis, dieters risk ingesting an enormous amount of fat—and potentially a lot of saturated fat, if you’re eating animal meat—without any of the fat-burning effects of ketosis. "The fat is the thing that's problematic for a lot of people on keto," Fung says. "They basically give a pass for any types of fat and a lot of the recipes encourage saturated fats like butter." Dieters who are careful to focus on healthy, unsaturated fats like those in avocados may not have issues, but again Fung notes that you end up with a fairly monotonous diet that way, and thus a lot of people end up eating more saturated fats. "To me as a nutritionist, that's pretty scary."
One downside to a ketogenic diet for weight loss is the difficulty maintaining it. “Studies show that weight loss results from being on a low-carb diet for more than 12 months tend to be the same as being on a normal, healthy diet,” says Mattinson. While you may be eating more satiating fats (like peanut butter, regular butter, or avocado), you’re also way more limited in what’s allowed on the diet, which can make everyday situations, like eating dinner with family or going out with friends, far more difficult. Because people often find it tough to sustain, it’s easy to rely on it as a short-term diet rather than a long-term lifestyle.