In the diet you describe the body would preferentially reserve all glucose for the brain and run the rest on ketones. The body can manufacture glucose when needed from other energy sources but it’s an energy intensive biochemical process and not ideal for efficient use of available nutrients.
Always happy to find out new understandings of physiology and biochemistry so I dove back into some research this morning after seeing your comment.
The brain does use ketones in starvation sitautions, but it always needs to use some glucose, so over time the available glucose ends up redirected there.
From what I can find it appears that the heart does utilize ketone bodies for fuel, contrary to what you are asserting. There is correlation to heart failure, possibly from stress in utilizing an energy source with less efficient bioavailability.
