Unless you have a specific health reason that causes your doctor to tell you otherwise, you should be including both garlic and onion in your diet. One of the best reasons for this is to help you ward off cancer. It seems the compounds that give both of these their strong flavor help to stave off cancer by neutralizing carcinogenic substances and they also speed the repair of damaged DNA. Garlic seems to be especially effective against cancers of the digestive system, including the esophagus and colon. As a survivor of colon cancer, let me assure you that is something you don't want to deal with. So spice up your meals with more garlic and onion!