Plant foods in general are good for preventing cancer, most people do not eat enough whole plant foods. Whether onions in particular have any special properties is more controversial.
general though is that foods high in antioxidants can help prevent cell damage which is what cancer is. Don't know if onions are specifically high in antioxidants, this page suggests there may be some benefits,
Originally Posted by c4rv: “general though is that foods high in antioxidants can help prevent cell damage which is what cancer is. Don't know if onions are specifically high in antioxidants, this page suggests there may be some benefits,
In the 17th century Nicholas Culpeper noted that "onions and garlic burn the fat that groweth around the heart".... so they knew back then that they were good for you.