Could be the vitamins and minerals, could be the antioxidants, could be the fiber, or could be (and probably is) some synergistic effect of all three. Check out Michael Pollan's "In Defense of Food." The point he makes is this: every time we think we've "solved" human nutrition, we later found out there was something we were missing. First it was the discovery of macronutrients in the mid-1800's, and so conventional wisdom of the time said, "as long as you're getting enough protein, you'll be fine." But people weren't fine--scurvy is a good example. Then scientists discovered vitamins, and said, "okay, okay guys--this time we've got it." But it turns out that's not the whole answer either. The latest cure-alls are antioxidants, fiber, and omega-3's. All good things, admittedly, but how good are they individually and how much does the whole package matter? Until that answer materializes, I will keep eating a rainbow of fruits and vegetables, because there's something going on there that's clearly working.