There are a number of reasons to eat organic and local. Here's a
study that reports organic foods are, on average, more nutritious than conventional-chemically farmed foods. What's interesting about this study is that it's actually a study of other studies. The researchers screened reports from as far back as 1980, and selected 97 reports to base their research on.
As most people realize, and Michael Pollan
says best, nutrition science is changing rapidly. Various elements of foods, such as
polyphenols and
antioxidants, were unimportant to the public mind 10 or 20 years ago, and now are widely recognized as essential to good nutrition.
There are reasons to eat organic that are easy to understand: pesticides & chemical fertilizers leave residue that is harmful to the land, and potentially harmful to the people who have to work with them. It seems logical that those chemical residues shouldn't be ingested, and there is evidence that organic food is more nutritious.
Organic food, in the
US certification system, can't be irradiated, which potentially kills the healthful enzymes in the food that aid in digestion.
However, it is the system, the gestalt, the whole, of the food and the farmland that is still tricky to know and perhaps the best reason to choose organic: we understand food and nutrition less than we should to experiment with it on our own bodies. I know that if I eat organic foods, they are the closest to what my body has evolved to thrive on.