Fresh tomatoes contain fructose in very small amounts. They are FODMAPS-friendly. However, when tomatoes are processed into tomato paste, the fructose is concentrated a great deal. According to one of my favorite references, Cook's Thesaurus, just 1tablespoon of tomato paste is equivalent to 1 medium tomato! So tomato paste is NOT allowed during the elimination phase of a FODMAPS-elimination diet. Depending where you are in the world, tomato paste may be called tomato puree or tomato concentrate, so check food labels for all these terms.
Check the nutrition facts on the label to see how the product compares to fresh tomatoes (2 grams sugar per 1/2 cup) versus tomato paste (16 grams sugar per 1/2 cup). This tip assumes the product has no sugar added as an ingredient, and that the sugar listed on the nutrition facts panel is all coming from the natural sugars in the tomato.
After reading the label, if you're still not sure whether the tomato product you have is OK to eat during the elimination phase, you may have to open the can and look inside. Whole canned tomatoes or diced canned tomatoes with watery-looking juice in the can are OK to eat during the elimination phase of the diet. If the juice in the can is thicker than water, transfer the contents to a a freezer-safe container, stash in the freezer, and wait until the fructose challenge phase to eat this tomato product.