WOW!! That is a lot of questions. Some of them HDM is the best source for answers, but I can tackle a few.
Probiotics are the good micro-organisms in the gut and they are a huge part of the immune system, because they compete with bad micro-organisms and thereby keep their numbers small. They are naturally occurring in tripe.
Prebiotics are food for probiotics, they are fibers that the probiotics love, they aren’t all created equal though, some prebiotics feed the bad stuff just as well as the good.
Enzymes carry nutrients where they need to go in the body and aid in digestion. There are already enzymes in raw meat, but in processing for kibble they are destroyed. Dogs make enzymes, but some do not make enough, because they were made to get them out of their food. And the body adjusts enzyme production to match the diet, so if a dog was getting a high carb, low protein diet for a long time, it may take a while for its body to get used to the different need for enzymes when the diet is changed. Digestive enzymes are the ones that are specific to digestion.
Spirulina is a blue green algae that is chockful of micronutrients, which are all the little things that a body probably needs, but science hasn’t determined how much of what or what for, so dog food companies don’t have to add them back into the food.
Fish oil, fish body oil are sources of omega 3s and need to be fresh so putting it in a bag of kibble may not be a good thing. Krill are little shrimp, otherwise the same applies to fish oil. Krill oil is supposed to be better. It has some of those micronutritents in it and a higher level of quality omega 3s.
As for why some say use them, don’t use them etc. There are what I consider raw food purists, that don’t feed anything except whole carcass raw prey. Period. They believe that since that is how wolves evolved, that is what you should feed dogs. Then there are those that recognize that we don’t live in that world anymore. That the world we do live in has pollution and food animals that were not raised as prefectly as we could want and therefore are lacking some of those things that if we lived in a perfect world they would have. They feed antioxidant rich foods to help rid the body of toxins and other foods to help make up for the perceived lack in the prey animals. And then there is every shade in between.
My fingers hurt, someone else will have to provide more. Or I will later.
