We think our stomach hurts because we're depressed. Sometimes, it's actually the other way around, found Stanford researchers: Digestive problems can cause mood disorders
—at least for those who've suffered from frequent bellyaches since childhood. The gut communicates upward through the vagus nerve—and even if pain is blocked (as it was in lab rats)—its distress signals can alter brain function, leading to depression. Future therapies using "friendly" bacteria—like those in yogurt—may soothe the "gut-brain axis."