The literature I've read says it should only be done under the supervision of a doctor. It is intended to help people with intractable seizures. The concept is they have seizures because their brain can not properly process glucose so they need ketogensis so the brain has energy. For them, the benefit is worth the risk.

The literature I found specifically states that the neuroprotective value of fasting and ketogenesis is limited to the first 24 hours after the injury. There was no noticed neuroprotective value of fasting at 48 hours.

Regarding sugar, the sugar content of carrots and many fruits is equal to or greater than many sweetened foods. The key is moderation.