Of all the changes I've made strength and health wise over the last year nothing either compares or would have been possible without a change in nutrition. The ability to know how and what food affects your being is invaluable and the first step to becoming and living healthy.
Until I discovered the Paleo way to fuel myself, I constantly battled blood glucose fluctuations. Effects ranged from decreased post-meal productivity to full on shut-down mode depending on the amount of type of carbs I was eating. Mood swings and slowed mental processing are also common. Speaking from first hand experience, this not only affects your personal and professional life but also dramatically increases your risk of CV diseases and long term health. Pick up a copy of The Paleo Diet (link above or any bookstore) by Dr. Loren Cordain. You'll never view food the same again.
Check out this short summary from Oregon State on the Glycemic Load and Index. Great introduction to how blood glucose levels are measured and what effect they have. Link courtesy of the Big Apple Box.