SugarGlucose-free diet definitely made a huge difference for my brain. I thought it was placebo that my attention span declined unprecedentedly for the entire 5 months i've been dieting - felt like i was just trying so hard to convince myself that my attention span is actually in decline - I never submit to that, but recently, out of sheer curiosity, I decided to check out real research on this to validate my subconscious assumptions.
Turns out, lack of sugar in your blood does hamper your ability to focus, reason, and remember. Brain as the paper says is fueled by sugar, and its paucity can significantly reduce intra-brain neural connectivity. However, excess sugar is also extremely negative, something afaik tightly linked to type 2 diabetes.
So, now, I am trying to get my sugar levels to their normal amount after a nearly half-a-year break.
You can read the Harvard paper
here.