All of your food is either converted to a form usable by the body for energy, or excreted when you use the bathroom.

Your body is not capable of using any food in the form you eat it. It is first broken down into "manageable chunks" by various digestive processes, depending on the type of food. Then the body takes those "manageable chunks" and decides what to do with them.

Your body converts fats to ketones because that is the process for processing fats (and has nothing to do with "calorie deficit". This is the same reason it converts bread to glucose -- because that is the process.


If I eat 5x my body's caloric needs for a day, my body still needs to convert what I eat to a form usable by individual cells. So if I'm eating < 20g carbs and lots of fats, I'll still produce ketones. The fact that I ate a lot of food does not alter the fact that the individual cells in my body need energy to survive. So my liver will do its job and convert that food into a form usable by the cells.

If I ate all that in carbs, my body will quickly convert it to glucose, and try to produce enough insulin to force all of that glucose into the cells (as opposed to simply using what's actually needed) and convert the rest into stored fat (starting with storing it in my liver). Because glucose is the form where that type of food is usable by the individual cells.