IBS and GERD are probably the two illnesses that have most impacted my life. At their worst, I was missing a significant amount of school and not really able to participate in much, particularly since I was responding poorly to typical treatment.
These days I can honestly say that as long as I stick to what I consider a "reasonably" good diet, I don't have any issue with them. People often ask me where they should start in terms of tackling their own issues, so I'm creating this page, which is a work in progress.
I honestly don't believe IBS even exists. It's a catch all for stomach problems of unknown etiology. Because of that, there is no one way of dealing with it. The first step I would take would be to ask your doctor to screen you for celiac disease. This is done commonly in many countries, but not yet in the US. Celiac disease is increasing in prevalance and untreated it can be very dangerous.
Once you've gotten that out of the way, I think a lot of other cases are either small intestine bacterial overgrowth (SIBO) or carbohydrate malabsorption. A doctor can test for SIBO and sometimes antibiotics work, but antibiotics can cause their own problems, so if it's not serious I think diets like the specific carbohydrate diet or the GAPS diet can work pretty well though there are no current scientific trials on this method.
Carbohydrate malabsorption (CM) is very common, but also underdiagnosed. The main type people know about is lactose intolerance, but it is hypothetically possible with any carbohydrate. Fructose intolerance is probably the second most common type. With CM, certain carbohydrates are not properly absorbed by the upper digestive system and end up in large amounts in the colon, where they are consumed by bacteria that produce methane and other noxious by-products, producing bloating, diarrhea, and other issues. CM has even been tied to mental health issues. Some types can be tested for, but others might require a more rigorous elimination diet. I think the best approach for this is probably the FODMAPs approach ( fermentable – oligo- di- and monosaccharides and polyols). It is unlikely that you are intolerant to all of them, but it's wise to eliminate them all for a period and then slowly reintroduce certain groups of them. Using such an elimination diet I concluded that my IBS is not IBS, but fructan and galactan malabsorbtion.
Hypothetically, that's not necessarily a bad thing in all people. No one really digests fructans for example, their gut bacteria does. And this can actually be beneficially in some people, producing important short-chain fatty acids in the colon. But for me I guess I have a gut bacterial population that is a jerk about fructans and makes excess methane and hydrogen from the fructans. But luckily, it doesn't do that with all SCFA-producing carbohydrates and fibers, which is why I think an elimination diet is important, so you can figure out what fibers/carbohydrates don't bother you.
Intolerances and malabsorptions are also highly dose-dependent and can also vary based on what you consume the carbohydrates with or the health of your gut.