Why should we be wary of some additives in our food?

I am not a vegetarian tho considering to begin purchasing organic type foods. Why, you might ask? I have learned a couple years ago (it has taken me a while to sink into my head) that all of these foods with preservatives in them do exactly what they are supposed to do: preserve food. When an autopsy is done on a person, pockets of foods can be found in the intestines that are so well preserved ... and who knows how long they are there. They believe food can stay in little areas of the intestine for years and due to being stuck in a pocket area, natural digestive enzymes I guess, dont get at the food in the pocket.... and pockets keep filling up... while the food stays in a great preserved state. So, I figure that if the preservatives are keeping old food intact, they are also doing something unatural to our bodies. We have so many cancers and health problems such as ADH that can be contributed to unhealthy preservatives that food makers have placed within our bodies. This all just grosses me out. I am close to a healthy eating lots of fresh foods lifestyle, more and more each day. Ocean salmon vs farm fed, etc. I am kinda-sorta watching for the preservatives/dyes and have been gradually changing to better foods for my body. If you are young, read all of your answers here and start now. You will have a more healthy future.