I think that this is an interesting discussion.

In adults glucose is the preferred source of energy for the brain. It can use what are called ketone bodies (derived from breakdown of fatty acids) as an energy source when the storage form of glucose (glycogen, found in muscle and liver tissue) is depleted or under more severe conditions like starvation.

From the link to the ketogenic diet above, much of it seems sensible to me; emphasising eating a wide range of minimally-processed foods.

However, its stricture that grain foods should not be eaten does not seem rational. Wholegrain foods like brown rice and wholegrain bread are rich sources of many nutrients as well as glucose. They are also a source of fibre, which is protective against bowel cancer.

The glucose in wholegrain foods is released slowly during digestion which means that there is not an insulin spike, The contrast is with processed grain foods like white rice and bread; glucose is released rapidly from them during digestion, with a corresponding insulin spike. This can lead to insulin resistance which is a risk factor for Type 2 diabetes.

In summary, I can see no rational grounds for avoiding wholegrain foods as part of a healthy diet.