Speaking only for myself, I’m not really worried about diabetes/cancer/obesity when eating rice, but if I eat a large quantity of rice (e.g. the amount typically served to each person in a Chinese restaurant in the US) I invariably feel very tired about 40–80 minutes later, and it throws off my mood and my ability to focus for a few hours. The same thing happens for a plate of spaghetti, a large bowl of oatmeal, or too much bread or cake.

I haven’t found brown rice to be much better though.

If I stick to 1/3 or so of that portion, there’s not much noticeable effect. (e.g. eating a sandwich or hamburger made with 2 pieces of bread, or putting curry on a smallish amount of rice, or eating some dumplings doesn’t seem to be a problem).