It is an individual thing and it can affect non-IBS individuals too. Some people can consume significant amounts of HFCS and sweetners with no ill effects while many others cannot tolerate any. In some studies 40-60% of the IBS patients showed a significant reduction in symptoms when high fructose foods (e.g. HFCS) and sweetners such as sorbitol were removed from the diet.
Reference
Goldstein, R., D. Braverman, and H. Stankiewicz, 2000: Carbohydrate Malabsorption and the Effect of Dietary Restriction on Symptoms of Irritable Bowel Syndrome and Functional Bowel Complaints. Israel Medical Association Journal, 2.
-------------------- STABLE: ♂, IBS-D 50+ years - Science of IBS