Sugar is half glucose and half fructose so only half of it raises blood glucose and the rest goes via the liver to be laid down as fat there is not used. So in fact sugar raises the blood glucose less than carbohydrates (whether low GI or not).
Despite the fact that the brain runs on 140g glucose daily, you do NOT need to eat that glucose. Your body makes it in the liver (gluconeogenesis) otherwise we would not make it through the night or a bout of sickness where no carbs were taken in? I mainly eat less than 30g a day and am still functioning very well. Lots of people do ketogenic diets where less than 30g carbs (converted to glucose) are eaten and do very well on them. There may be no need to go that far and I think Precision Nutrition is good at getting people to make better choices rather than going for an extreme change that they will not stick to.
Go to DietDoctor's site for lots of science or watch Steve Phinney/Tim Noakes on YouTube both of whom are well respected academics advocating low carb diets.