Retrograde potatoes is a modernist cuisine method to make the best possible mashed potatoes. 1st you simmer the sliced potatoes in 70-71C water for 30mins, then drain, rinse to cool, and simmer them until done. After that mash into a ton of butter, 50% ratio :) This low heat pre simmering somehow locks the starch molecules so they dont turn into glue even when beaten heavily with a whisk.
I was googling around and noticed that this retrograde mashed potatoes is used by ones suffering from diabetes also, it seems that they spike insulin less than regular mash. I dont know the science behind this, or even if this is correct, i must check somehow. Better mash and lower insulin spike, thats a win-win almost ;)
I know that if you cook potatoes and then cool them, it forms resistant starch, which resists digestion in the small intestine and functions as a prebiotic (for better or worse) in lower digestive system. That's probably why they don't spike insulin as much. The fat will also affect insulin spikes too.
I'm not saying that retrograged starch can or can't affect blood glucose, however adding 50% butter surely would. Even I only add about 15-20% butter to my mashed potatoes, most people more like 2-5%. You've effectively taken away half the carbohydrates and replaced them with fat.
I learned something new today! I'd never heard of retrograde potatoes and I'm fascinated. The process seems to be fairly exact so the practicality of it goes out the window a bit for many but for those who have issues with starch the extra work may be worth it.