I first understood this when I noticed that by avoiding white rice for lunch, I was more energetic in the afternoon. Unfortunately, in Indian culture, it's very common to eat rice at every meal and the corresponding sugar drop is a huge contributor to brain fog / lethargy, as well as the extremely high levels of insulin resistance (and pot bellies) commonly found amongst most Indians. I had to undo a lot of my childhood programming to get out of the constant consumption of rice. Rice is fine sparingly and in moderation, but not in the way it is commonly consumed across asian cultures.

Because sugar is half fructose which is processed in the liver when over eaten. Insulin doesn’t come into it for fructose

Yeah and a peeled potato has a dramatically higher glycemic index than straight fructose. Glycemic index is an interesting metric, but it's far from the end-all be-all, especially if you aren't diabetic.

Yeah, that kinda underlines my point, that without actual numbers, even “moderate rice and no sugar” isn’t particularly meaningful. (FWIW, I don’t eat white rice or other non-whole-grains.)