This is a study done on malnourished children which found that improving gut bacteria "reaches well beyond the wall of the gut to influence many systems," - This is one reason why feeding pellets is not good for parrots. Pellets, because they are so processed, do not have any of the phytonutrients needed to 'feed' their gut bacteria. Raw food, raw food and more raw food is what they need!
https://www.livescience.com/microbiota- ... 3I0VGGWWWI