Dr. Jay Wortman has been thinking about whether our bodies prefer to run on carbohydrates (as a source of glucose) or, instead, on fats. The standard American diet provides derives about half of its energy from carbs, 35% from fats, and 15% from proteins. So you might guess our bodies prefer carbohydrates as a fuel source. Dr. Wortman writes:
Now, consider the possibility that we weren’t meant to burn glucose at all as a primary fuel. Consider the possibility that fat was meant to be our primary fuel. In my current state of dietary practice, I am burning fat as my main source of energy. My liver is converting some of it to ketones which are needed to fuel the majority of my brain cells. A small fraction of the brain cells, around 15%, need glucose along with a few other tissues like the renal cortex, the lens of the eye, red blood cells and sperm. Their needs are met by glucose that my liver produces from proteins. The rest of my energy needs are met with fatty acids and these come from the fats I eat.
Dr. Wortman, who has type 2 diabetes, in the same long post also writes about oolichan grease (from fish), an ancestral food of Canandian west coast First Nations people.
Drs. Jeff Volek and Stephen Finney have done research on athletes using a low-carb, high-fat diet.
My opinion is that just about everybody can eat a high-starch diet if they are strictly in calorie deficit. But you can’t be in calorie deficit forever. If you are not in calorie deficit and you try to eat more carbohydrate than your glycogen usage, you will start to move toward metabolic syndrome and diabetes. And unless you are a committed non-stop athlete, carbohydrate intake at calorie balance that is equal to glycogen usage is just enough to turn off ketosis and gluconeogenesis, but not enough to truly power the muscles and nervous system, so you will feel worse than if you get carbs down to 50 grams or so and let the liver and muscles run everything from fat. But, if you have excessive fat mass, a high-starch diet with a calorie deficit is sometimes easier (plain potatoes are filling!) than a low-carb diet.
Jim, our bodies do an amazing metabolic balancing act continuously and automatically, much as you describe.