“Caffeine can increase bowel regularity, including increasing the chances of diarrhea, a major symptom of irritable bowel syndrome (or IBS),” says Angel Planells, MS, RDN, Seattle-based registered dietitian nutritionist, and former president of the Washington State Academy of Nutrition and Dietetics and Greater Seattle Dietetic Association. “So if you have IBS, it is encouraged to limit/avoid caffeinated beverages.”