No, it grades high. Plain mayonnaise is fine on FODMAPs, but tartar sauce adds sweet pickle relish and finely chopped onion or shallot, which are concentrated fructan sources, plus the sugar in the relish. Because the pieces are small and evenly distributed, even a modest spoonful delivers the load. A plain lemon and mayonnaise dressing gives you a similar tang without it.

Not by nature, but it grades borderline because brands differ. Egg, oil, lemon and cucumber are all gluten free, yet some recipes use malt vinegar, which is brewed from barley, and some commercial jars use wheat-derived starch or thickener. Cross contamination in a fish and chip shop fryer setting is a separate risk. Treat the label, not the recipe, as the answer.

Because the fish is the clean part of the plate. Tartar sauce combines a high FODMAP load from relish and onion with a heavy refined-oil base, and fat slows gastric emptying while the fructans ferment further down. That combination of delayed emptying and gas production is exactly what produces bloating and cramping an hour or two after eating, long after you have forgotten about a condiment.

It grades moderate on sulfites, and they arrive with the pickled ingredients. Commercial relish, gherkins and capers are commonly preserved with sulfites, and some vinegars carry them too. It is not a high-sulfite food like dried fruit or wine, but if you are already limiting sulfites it is an easy one to overlook because you think of it as mayonnaise.