Is the Glycemic Index Important

Is the whole GI, Brown vs White food "bro" science

First, the glycemic index is "real" science and does reflect the impact of foods on blood sugar/insulin levels. As you point out, this is important for diabetics to understand.

In practical terms, the glycemic load of an entire meal, not just the individual components, is what really counts, and that is much harder to calculate. Eating any kind of fat with a high GI food, for example, radically changes the speed of digestion and reduces impact on blood sugar/insulin.

So, from a non-diabetic bodybuilder's perspective, the GI is not something I would worry extensively about. There is research showing that an intake of high GI carbs, post workout, does improve replenishment of muscle glycogen better, but again, I wouldn't fret unless you're riding the Tour de France or involved in a similar activity.

The biggest practical/real difference between the white vs. brown, you cite, is dietary fiber. Most folks don't consume nearly enough fiber, so from that perspective, the "brown" foods do have a distinct advantage.