For some people, grain products cause bloating.
Wheat contains a lot of FODMAP carbohydrates, which can cause stomach problems, especially for those with irritable bowel syndrome. Barley and rye, on the other hand, are very high in fiber, which can cause bloating in people whose intestines aren't used to a high-fiber diet.
Oats are the most stomach-friendly of the grains. A few days old bread might be easier on the stomach than fresh bread. This may be related to changes in the starch content of the bread as it dries.
Bloating caused by grains can also be due to celiac disease, where the gluten found in certain grain products causes inflammation in the small intestine lining and damages the intestinal villi.
