Ahh! let’s talk about ought!

Since humans store fat, one could see mechanistically we are setup to run on the energy we store: fat

When humans go more then 4 hours without eating glucose (skipping a meal, keto, fasting, or sleeping) they are running on fat, including the brain. If you want to prevent your brain from using fat you need to drip feed glucose all day (which some people try really hard to do), but when you sleep some of that fat will finally get to be used by the brain. One could reasonably argue the default energy of the human body is fat, hence why it’s used during sleep.

https://www.frontiersin.org/journals/molecular-neuroscience/articles/10.3389/fnmol.2016.00053/full#h7

Both short-term PET and arterio-venous difference studies in humans show that brain glucose consumption decreases as ketone availability to the brain increases. These results suggest that ketones are actually the preferred energy substrate for the brain because they enter the brain in proportion to their plasma concentration irrespective of glucose availability; if the energy needs of the brain are being increasingly met by ketones, glucose uptake decreases accordingly. This decrease in brain glucose uptake when both ketones and glucose are available supports the notion that ketones are the brain’s preferred fuel.

The body will use glucose when available, because glucose is so damaging to cells - glycation happens rapidly. As soon as any glucose elevations are seen in the blood stream insulin is immediately released to push glucose into fat cells and get blood glucose levels back to the low normal.

However, I’m open to being wrong: Why ‘ought’ the brain use glucose instead of fat?