I dont usually do post any more that serve primarily to link up or quote another's work, but I'm making an exception.
I structure my nutritional philosophy around the notion of the Paleolithic Principle. The principle is that the human animal has been around and eating a relatively consistent diet for a couple of million years with Homo Sapiens being around for almost 100k years. It was only really with the introduction of the neolithic age that technology brought new foodstuffs to consume such as dairy, grains, and the other modern cultivars of plants that we eat. The paleolithic principle states that we have not fully adapted to these new types of foods and hence they may be harmful to our health, on a case by case basis.
These new sources of food were introduced roughly 5000 - 7000 years ago, which is perhaps some 300 generations worth of time. Humans, being long-lived, evolve quite slowly. Humans, being highly social sentient apex predators, also don't necessarily experience the same degree of natural selection as other species. The question is then, just how well adapted are we to these neolithic food groups?
That's physicist and sometimes commenter here, Robert McLeod, blogging at Entropy Productions. Go read the whole thing. I know from recent comments and emails that there are a large number of "newbies" here, and Robert's post is a very sound introduction to both theory (the Paleo Principle) and practice. So, you newbies print it out, read it, practice, and then read it once per week. I guarantee that your understanding and practice will converge.
You shall, in time, become expert yourself, which is my ultimate goal for everyone. Damn all those who would keep you ignorant and needy.