Corn CAN be a source of FODMAP carbohydrates (fructose, sorbitol and/or oligosaccharides), but not all corn products are significant sources. It helps to remember two things about corn: 1) the sweeter the corn product the more sorbitol it probably has and 2) corn products with no fiber have no oligosaccharides.
Corn oil--no carbs in corn oil, so no FODMAPs; suitable for the elimination phase
Corn starch--refined to eliminate all corn components, leaving only starch behind. Starch is not a FODMAP, so this is suitable for the elimination phase of the IBSFree diet
Sweet corn--not suitable for the IBSFree FODMAP elimination diet, in which the portion size for cooked vegetables is 1/2 cup, though other tools may use smaller portion sizes to help a food like sweet corn make it "under the wire." Sweet corn has been analyzed and reported out by the Monash FODMAP lab in two studies (2007 and 2009). It contains sorbitol. There is a little more of a question about oligosaccharide content. Sweet corn could be included in a polyol challenge.
Corn meal--has not been measure in the lab as far as I know. My research shows that corn meant for corn meal is bred for higher starch content, rather than sweetness, so until proven otherwise, corn meal is permitted on the IBSFree diet.
Corn flakes--Data published in 2011 show cornflakes contain a moderate amount of fructans, are suitable for the IBSFree elimination phase in the 1/2 cup portion shown in my book, IBS--Free at Last! Larger portions could be included in a fructan challenge.
Corn bread--unless gluten free or homemade from a special recipe, corn bread almost always has wheat flour in it so is not suitable for the elimination phase of the diet.
Corn syrup--excess fructose is the potential FODMAP problem in this product, but the USDA nutrient database shows it is mostly glucose, so until proven otherwise is suitable for the IBSFree FODMAP elimination diet. American brands of "corn syrup" such as Karo brand have not yet been analyzed by the Monash Uni FODMAP lab,
High fructose corn syrup (HFCS)--excess fructose is the potential problem here. HFCS is not a uniform product; some HFCS has excess fructose and some does not. The percentage of fructose actually varies considerably from one product to the next, but you can't tell from reading a food label which one was used. In sum, avoid it all on the elimination phase of the diet. Include it in your fructose challenge if desired.
Thanks for hanging in there with me for this rather involved discussion! If you are longing for a simpler explanation of FODMAPs, click here. If you just want to skip all the talk and go right to "What can I eat on a FODMAP elimination diet" check out Chapter 1 of my book, IBS--Free at Last!