OK, it's an over-simplification, but according to The Hidden Half of Nature, a new book by a couple of married scientists, it's not far off either.
An article in the Guardian gives the background for the book. The authors, Anne Biklé and David Montgomery, started out feeding their soil with organic matter, and learned that the benefit went to microbes in the soil, which in turn fed the plants.
Anne Biklé and David Montgomery
Analagously, feeding the 2kg or so of microbes in the gut has far-reaching benefits for our nutrition, and our bodies. See a review of Gut: The Inside Story of Our Body’s Most Under-rated Organ for details.
This is a new field in health that looks like it will have far-reaching consequences.