1. Potassium benzoate You might think diet soda is a better alternative than regular soda-but resist that urge! Sodium and potassium benzoate are added to some diet soft drinks and fruit drinks. They can form benzene, which is a carcinogen when combined with vitamin C, the ascorbic acid in juice or soda. In addition, research shows that there may be a link between diet cola consumption and weight gain, as well as cancer and diabetes, so if you're a diet soda junkie, try to cut back.