The ketogenic diet requires restriction of dietary carbohydrate intake below a critical threshold, at which the body switches to using an alternative fuel source, ketones.
For reference, most Americans could consume 250g/day of carbohydrates without considering this to be an odd or unhealthy behavior, especially if they opted for low-fat, low-sodium choices. A given pasta meal or chocolate muffin is likely to be around 70g of carbohydrates. One can of Coke contains 40g of carbohydrates. The upper end of the more egregious flavored lattes borders around 80g of carbs, and a bowl of cheerios, assuming any quantity a real human being would pour themselves, something above 50g of carbs or so.
The presence of insulin prevents fat metabolism.There are complicated reasons for this both metabolically and evolutionarily, but the basic idea is that consumption of carbs raises insulin, and during that time shuts off fat utilization. Fasting altogether forces insulin to drop, and fat metabolism to start, and certain ways of eating can also instigate this metabolic context, to allow fat metabolism while eating plentifully.
Basically, everything you've heard about the dangers of sugar has been about carbohydrates the whole time. The distinction is not particularly or impressively important, as far as health is concerned.
The idea of the diet is to restrict carbohydrate intake (both frequency and quantity) enough that insulin levels drop to a level that allows the body to "switch gears" to an alternate metabolic focus. Instead of utilizing ingested glucose, it can enter the state of ketosis, where fatty acids, either ingested through fatty foods or pulled from fat stores, are processed into a number of highly energy-efficient and anti-inflammatory compounds called ketones, which get used to fuel most of the body and brain. The basic switch can occur within a few days. However, further downstream cellular and metabolic effects of a low carbohydrate diet occur in a process called keto-adaptation, which more recent research has shown continues to produce novel changes multiple months into the diet.
While on the diet, the lack of insulin usage allows your body to regain insulin sensitivity (probably towards a more natural evolutionary level). Insulin sensitivity is a good thing. It means your body needs to use less insulin to perform the same glucose-job. Constant spikes in glucose cause you to become more insulin insensitive. Insulin has a lot of roles within the body, apart from just handling glucose, so requiring larger and larger amounts of it to handle eating carbs has all sorts of incidental effects around the body, like increasing inflammation (think soreness, aches, and pains).
If an insulin insensitive person eats a cracker, their body secretes more insulin to handle this, than a more insulin sensitive person, just like a coffee-lover takes more caffeine to experience the same kick. Since insulin is essentially directly antagonistic to fat metabolism, more insulin around means less access to fat stores on the body. Paradoxically, without access to fat stores, you start to rely on food a lot more than your body ought to. Pretty soon this means a near complete inability to even use the fat on the body. For most Americans, it gets this bad by their twenties or thirties, when they likely haven't experienced serious ketosis since they were a young kid.
You're supposed to be metabolically flexible, meaning that you can eat carbohydrates, handle the insulin, and transition seamlessly back into fat metabolism, without the experience of any serious hangriness inbetween. Most modern people have diets so focused on carbohydrates, that their body has done something along the lines of completely forgetting how to process fats. Their cells have tooled themselves for carbs, and thrown out the adaptations that make for the "well-oiled" machine of fat metabolism. While being highly "keto-adapted" and being highly "insulin sensitive" are not literally the same thing, and have some different implications, in practice they tend to coincide.
You may hear the term keto flu, which is basically a made up catch-all term for the uncomfortable effects of the initial transition into the diet.You should not be worried about these symptoms for two reasons: