In 2008 I started having big-time problems with diarrhea after eating most anything. Various lab tests found nothing that could be causing it, so my doctor ordered a colonoscopy. Oh, joy! I was in my mid to late 60s by then, so my doctor thought it was about time I had my first one anyway.
The gastroenterologist found nothing in the colonoscopy that would account for it, so diagnosed me with IBS. He suggested a change in my med for my lactose intolerance. I used the one he suggested for several years but have since changed back. Unlike some people with lactose intolerance, I can eat or drink all the dairy products I want as long as I take one of those generics for Lact-Aid with each serving.
The gastroenterologist also recommended that I take a specific probiotic that I could find in our local health food store - PB8. I did, but told the doctor at a follow-up about two weeks later that I hadn't seen that it made any difference in what I could eat. However, I kept on taking it. And much to my surprise, within six months I was back to eating the way I had been before all the problems arose. And I can still eat whole dinner-plate servings of salad, as well as foods like beans without any major problems. In my experience not many others in their mid-70s can eat all the foods I can without some digestive problems. I don't know if I even have IBS. Though I don't actually know that the pro-biotic is what made the difference, I still continue to take one pill a day most days.