I'm currently on a diet to help with pre-diabetes. After going through nasty bouts of brain fog, insomnia and skin issues for a few years, I eventually quit trying to manage the symptoms and did some research to try and figure what my body was up to. It was something of a surprise for me to learn all the symptoms could be linked to my blood sugar levels. I had a blood test done a couple of years ago and nothing abnormal showed up. Either my body goes off at lower thresholds or medecine hasn't quite cracked it all out.
At any rate, the symptoms have resolved significantly since I switched to my new diet a month ago. I'm also taking probiotics to keep Candida at bay as it's othen associated with diabetes some other symptoms were a match.