Rather than a complete fast, going on an anti-inflammatory diet would be worthwhile. The stress to the brain of a full, no calorie fast could be counter-productive.

If you are tempted to try a fast, I suggest you make it a water and coconut oil fast. The coconut oil will at least provide some ketones for brain energy. So, water plus a 1000 calories of coconut oil per day. 6 or 7 tablespoons of coconut oil would be about 1000 calories. Maybe a dash of salt with it or electrolyte water.

I've seen what a fast does to a PCS brain when I was involuntarily fasting due to surgery. I also experienced a 30 pounds in 6 weeks weight lost due to stomach issues and my brain was a mess. I don't recommend either.

Elimination fasts can be very good. You eliminate specific foods for a week or two. You should be able to find a lot of info online. Just remember these elimination fasts are not in a context of PCS. I did one under a doctor's care 30+ years ago.