For most people, you fall into ketosis just by eating under 50 grams of carbs/sugar a day. In contrast, some low-carb diet can be as high as 250 grams of carbs. The main goal for both keto and LCHF is that you restrict carbs, this, in turn, deprives your body of glucose. Due to the lack of easily burned glucose fuel, your body has to start burning fat as fuel. This period is commonly referred to as the keto flu (or low-carb/Atkins flu).
Once you get past the flu phase, there is a second phase which is known as becoming fat adapted. If you leave carbs at or below 20 grams a day your body will start to produce ketone bodies which are made from fat (both dietary and stored). The act of producing and use ketones as fuel is known as becoming keto adapted. As a side note, depending on how insulin sensitive you are you might be below or above 20 grams, but it is a good starting point get you ketotic.
There’s a big difference between burning fat and producing ketones from fat. Ketones, unlike fat, can pass the blood-brain barrier in a similar way to glucose. When your brain runs off of ketones it actually runs better, this is why one of the side effects of ketosis is mental clarity. Your body does need some carbs/sugar, however, i does not need to be dietary. If it is needed, your body can go through a process called, gluconeogenesis. This is where your liver breaks down non-carbohydrate carbon substrates such as glycerol and fatty acids (from fat), and to a lesser extent, lactate (from muscles), and glucogenic amino acids (from protein) so your body does not enter a hypoglycemic (too little sugar) state. On a keto diet, since your brain is metabolizing the ketones you become less glucose dependant which spares muscle mass from gluconeogenesis.
Two quick notes: Breaking down muscles and protein is not a bad thing, it uses the old junky lactate and amino acids so they can be replaced with new ones. Second, ketosis is both protein and glucose sparing so the protein that is used will not be a huge amount.
In layman’s terms, when you are fat adapted your liver creates energy from your fat stores. If you continue to keep your carbs low and fat high, you produce ketones and your body will use ketones and stored fat for fuel. What glucose your body needs is being created from fat stores and a few dietary carbs you eat.
So there is the science, now I want to also talk a bit about the composition of your keto plate. Like with everything thing else the goal is real food. Things that can be found in nature, like meat, avocado, green veggies, dairy, and nuts. When picking your meals it can easily be summed up as this: eating as few carbohydrates as possible, an adequate amount of protein and finish off with eating fat to satiety.
If numbers are more your thing, I will give you my percentages. In a 1400-1500 calorie breakdown, you may eat 5% carbs, 20% protein and 75% fat. This keeps crabs under 20 grams, protein would be around 70-75 grams, and fat would be 117-125 grams.
When getting keto-adapted, most people feel balanced and in control of their hunger and moods. It may take some time to tweak it, but when it is right, it feels great. It did for me, and I never realized how much food changed my thoughts, feeling, and physique. I can become a totally different person if I change the way I eat. I have realized, I don’t like who I become when I eat a Standard American Diet because it makes me feel sad, depressed and out of control. This is why I tweak my macros until I am feeling optimal, but not everyone needs to manage their diet the same way I do. You just need to see what approach works for you and run with it. If it stops working, tweak it again.