Coffee triggers movement in the colon, and it does it fast, often within a few minutes of the first cup, well before caffeine could be absorbed and act on its own. Studies comparing drinks find caffeinated coffee produces the strongest push, decaffeinated coffee a clearly weaker but real one, and plain hot water much less than either. That is the giveaway: something in the coffee beyond the caffeine is driving it, which is why cutting to decaf usually softens the effect without removing it.

It aggravates them through two mechanisms at once. Coffee stimulates gastric acid secretion, and it reduces pressure in the lower esophageal sphincter, the muscular valve that normally keeps stomach contents down. Both effects persist to some degree in decaf, so if your heartburn survives the switch you are looking at the coffee rather than the caffeine. Drinking it with food instead of on an empty stomach, choosing a darker roast or cold brew, and moving your last cup earlier in the day all help more than switching to decaf alone.

It depends which symptom. For jitters, a racing heart, anxiety and disturbed sleep, decaf resolves most of it, because those are caffeine effects and decaf carries only a trace. For gut urgency and for reflux, expect partial relief at best, since both mechanisms operate independently of caffeine. If decaf changes nothing at all, the useful next test is whether you react to coffee at all or to the milk and syrups going into it.

Black coffee is, and it grades clean for lactose, gluten and FODMAPs generally, so the drink itself fits a low-FODMAP plan. What breaks it is what goes in: regular milk adds lactose, oat milk is high FODMAP in normal serving sizes, and sugar-free syrups often carry polyols. Note also that caffeine can worsen IBS symptoms through gut stimulation even when the drink is technically low FODMAP, so a clean FODMAP score is not the same as a symptom-free cup.