Ketogenic diet (“keto” diet for short) is a catch-all term for any diet that pushes your body into the natural metabolic state of ketosis, which means burning fat for fuel instead of carbohydrates. Though there’s no set formula for keto, generally, the diet works by cutting back on carbohydrates, to about 20 g of net carbs to start, and replacing those with mostly fat and a moderate amount of protein, according to the popular website Keto Connect. Net carbs are the total number of carbs minus the fiber and sugar alcohols, according to the Atkins website. (More on this diet later.)
When carbohydrates are used by the body as an energy source, the blood sugar levels become unstable. As the energy source is not consistent it is difficult for your brain to stay focused for long periods of time. On the other hand, when you are in ketosis and the brain uses ketones as a fuel source, which has a consistent fuel source and you can focus for longer periods of time. Hence, you also tend to feel more active and alert.
Insulin is a hormone that lets your body use or store sugar as fuel. Ketogenic diets make you burn through this fuel quickly, so you don’t need to store it. This means your body needs -- and makes -- less insulin. Those lower levels may help protect you against some kinds of cancer or even slow the growth of cancer cells. More research is needed on this, though.