Unless you're someone who brags about your "iron stomach," you probably only think of your GI system when you're assigning blame, not distributing praise. We tend to think about stomachaches, gas pain, and bloating, sure, but you owe your GI tract a pretty big debt of gratitude: 80 to 90 percent of your immune system is inside your intestines. Mistreating them impairs the ability of your immune system to protect you. "A diet high in fat and low in vegetables prevents the body's ability to help," cautions Bontempo.