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.