When you are in ketosis, your body begins to burn stored fats for energy, which causes a buildup of acids in your blood known as ketones. The keto diet seeks to burn undesirable fat by forcing the body to rely on fat for energy rather than carbs. When you consume fewer than 50 grams of carbohydrate per day, your body will switch over to using fat instead. Ketones are chemicals that occur when there is an excess of acetyl-CoA in the body. Acetyl-CoA can be made from either eating carbohydrates or from breaking down protein into its component amino acids and then combining those with carbon dioxide and oxygen to make them accessible to cells for use as fuel. As long as you have enough oxygen available and your body is not under extreme stress, it will always choose this route instead of burning pure glucose. This is why having low blood sugar is dangerous - it can lead to the body choosing to run out of fuel instead of switching to another source because it's easier than processing more complex molecules. When you start the ketogenic diet, you become dependent on ketones as a form of fuel rather than sugars. Once you reach this stage, you no longer have any choice about whether or not you burn fat - you have to in order to survive. This is why it's important to include some foods that will restore balance to the body if it gets too low on nutrients.