Mr. Olympia competitor Ben Pakulski advises:
Pre-workout. No carbs. Why no carbs? Neurological focus. Neurotransmitter focus. I find that I get much better brain focus in my training and ends up giving me better overall outcome.
An article from Forbes also says low carbs increase vigilance at the expense of hurting other traits:
When consuming low-carb diets in the short term, this is certainly true. In a 2008 study, psychologists placed 19 women on either a calorie restricted low-carb diet or a calorie restricted high-carb diet for 28 days. Throughout the study, participants’ memory, reaction time, and vigilance were tested at regular intervals. While those on the low-carb diet enjoyed a slight boost in vigilance, they suffered impaired reaction time and reduced visuospatial memory.
Neither of these sources explained why low carbs increase focus/vigilance. What is the reason?