“Ketogenesis” means “generating ketones”, which your body does naturally to generate energy from fat when carbohydrate sources are sufficiently low (e.g. after intense exercise, long periods of fasting, etc.). “Ketogenic diets” are thus ways of hacking your body into this starvation-like state.

Specifically, on “normal” diets (eating more than 100g carbs a day), before sending excess energy to fat cells, your liver and muscles will store carbs in a relatively simple storage mechanism called glycogen (these branches below are composed of glucose molecules, which is what most of the carbs you eat quickly get reduced to):

As these branches get sufficiently used up, some interesting changes start happening. Your brain normally runs on ~100g of glucose per day, so when your liver isn’t able to pump out sufficient glucose for this requirement, it starts producing ketone bodies to get the job done (and — yes — your brain is totally fine with getting up to ~70% of its energy from these ketone bodies, a process which gradually ramps up from 0% to 70% and takes about 3 weeks to occur).

What’s interesting during these ~3 weeks of “induction” is that all the other tissues in the body use a decreasing amount of ketones for energy, eventually using mostly fat (specifically, free fatty acids released from fat cells). Thus, after induction, your body is producing a relatively small amount of ketones (less than ~280 calories worth) to keep your brain happy.

So how do we get energy from ketones vs. glucose?

Under “normal” conditions, insulin helps glucose from the blood stream get into cells, where glucose is converted into pyruvate and sent into the citric acid cycle to generate energy like so (I know this is overwhelming, but pay attention to the NADH production):