While ketchup and tomato paste share a similar base ingredient of tomato, they are not the same product and cannot be used interchangeably in recipes. Tomato paste is made from cooked, concentrated tomatoes that have been strained of seeds and skin, resulting in a thick paste with a deep tomato flavor. On the other hand, ketchup is a sweetened and seasoned condiment that contains not only tomato paste but also vinegar, sugar, and spices. So, while it is technically possible to use ketchup as a substitute for tomato paste in some recipes, it may alter the overall flavor profile and texture of the dish. If you are in a pinch and need to substitute tomato paste with ketchup, use a ratio of 1 tablespoon of ketchup for every 1 tablespoon of tomato paste and adjust other seasonings accordingly.