It's hardly news that the gastrointestinal tract is important to human health: It transports food from the mouth to the stomach, converts it into absorbable nutrients and stored energy, and shuttles waste back out of the body. If you don't properly nourish yourself, you don't live. It's that simple.But in recent years, scientists have discovered that the GI system has an even bigger, more complex job than previously appreciated. It's been linked to...