A vitamin is a chemical compound that is needed for the human body to work correctly . They include Vitamin A , many B vitamins ( like B1 , B2 , B3 , B6 , and B12 ) , Vitamin C , Vitamin D , Vitamin E , and Vitamin K. For example , citrus fruits such as oranges and lemons contain vitamin C. For example , ascorbic acid ( vitamin C ) is a vitamin for humans , but not for most other animals , and biotin and vitamin D are required in the human diet only in certain circumstances .