This is NOT scientific information; it is extremely simplified. These are my words, the way I understand it. For a scholarly explanation, see these video links from Drs. Jason Fung and Eric Berg.
Many vehicles can use CNG or gasoline for fuel. I understand that you decide the fuel you wish to use by flipping a switch.
Brains are energy junkies--they require 20% of the body's energy even in a resting state. There are two different fuels the brain and body can use for energy: Carbohydrates or Ketones made from fats. However, I cannot simply flip a switch. I either burn carbohydrates or ketones.
In this country, most people eat SADly, Standard American Diet, which consists of at least 50% carbohydrates. When I eat that many carbohydrates (or more), fats simply float around in my body, adding to my stored fat content and/or creating other nasty internal problems. Persons who focus on eating 5% or less carbohydrates eventually reach ketosis; that is, their bodies have created ketones from fat and are burning those instead of carbohydrates. When eating meals, it may take several days or even weeks to attain ketosis. In ketosis, excess fat already on the body is used for fuel and depleted eventually. Fasting (abstaining from food) is ideal for me because ketosis is achieved in about 24 hours and my body burns excess fat as fuel.
Ketosis continues while eating less than about 60 grams protein and a high amount of quality fats. It usually only takes one day of 50+ grams of carbohydrates to break ketosis and have to start over. Example: one hot dog bun contains 21 grams of carbohydrates.
Sometimes, when something very special comes up, like my recent family reunion, I want to eat SADly because the people I love are doing so. For me, this shouldn't be every Easter, other national holiday, birthday, Sunday, or the whole month of December. This should be only very special times. (We hadn't had a family reunion since the 1980s). On those days, and other special days which I choose, I will eat MUCH less protein and fat to keep my system from flaking out. I will still be out of ketosis, but hopefully not re-storing fat and not adding to mortality from stroke and heart disease (just diabetes II). This must be planned; it cannot be a spontaneous thing. If I have eaten to maintain ketosis all day, I won't want to mess that up by eating a high carbohydrate food. Therefore, I will avoid those social functions where there are a lot of carbohydrates. If I have already eaten a high-carbohydrate food and kicked myself out of ketosis, it won't really matter to me if I have more because I am already going to have to start over.
My family was very poor and I always had the impression that I might starve to death any minute. We ate whenever and whatever food was available (especially free food) because of that fear. However, history shows that none of my family died from starvation. My immediate family members died from heart attack with diabetes II, stomach cancer (from a history of bulemia), pancreatic cancer (a sugar addict, like myself), and heart failure with morbid obesity--all before age 61. I believe these premature deaths were preventable or the quality of life could have been extended by using a carbohydrate-controlled diet.
At any rate, these eating decisions are mine to make. The physical pain, emotional self-loathing, and mental confusion are mine when eating SADly. And the physical strength, emotional euphoria, and mental acuity are mine when in ketosis. It gets real now: I have three months before October 20th; three months to work on a great A1C reading as a 60th birthday present to myself.