I know it may come as a surprise, but next to quitting smoking and drinking sugar, getting off of grains can have the single biggest positive impact on your health. Grains are actually reproductive systems, and are in a fight to survive! Evolution has equipped them with several tools that make them quite unappealing to our digestive systems, actually making them toxic to our bodies. Not toxic the way drinking anti-freeze is toxic, but toxic in their own way.
Grains contain 3 compounds that you definitely want to avoid as much as possible: Glutens, phytates, and lectins.
Lectins will mess with your gut lining and your body's use of insulin. Glutens can wreak havoc in your gut lining and make it very difficult for your body to absorb the nutrients you eat. Your body actually sees gluten as a threat and produces anti-bodies to get rid of it. And phytates bind to nutrients from other foods and make them unavailable to your body.
Grains have evolved to keep predators (us!) away, so stay away!