SIBO, or "small intestinal bacterial overgrowth", and it's symptoms are kind of weird when you think about it.
Those with SIBO often feel better when they remove or severely restrict high FODMAP foods like apple, onion, garlic, peaches, cruciferous veggies, avocado.... Hey, wait. You have to remove really healthy foods (fruits, veggies) from your diet in order to feel better. What gives? Why would healthy foods make you feel worse?
First, I'd like to put this one to rest- it's not that high FODMAP foods are unhealthy. Quite the contrary, actually. One of the main reasons these foods are healthy (their fiber) is the exact reason they make SIBOers miserable. I think it's most helpful to understand what's happening in someone with SIBO vs a "normal" person.
Normally you ingest high FODMAP/fermentable fiber foods and your cells are unable to digest that fiber. No big deal, because you've got zillions of bacteria in your colon that are waiting for the opportunity to help you with this! Those bacteria (mostly in the colon) gobble up that FODMAP fiber and make cool stuff- short chain fatty acids* and some gas. The gas isn't a big deal to the colon- it'll just accommodate that gas until it's able to pass it as a fart. The relationship between the colon microbiota and the host (human) is a symbiotic one- we feed them and give them a home, while they make cool, anti-inflammatory stuff for us and educate our immune system. Win, win!
Short chain fatty acids are amazing. Among the things they do, they help heal the gut lining (prevent or treat leaky gut) and send good, anti-inflammatory signals to our immune system.
If a person has SIBO, this goes quite a bit differently. When they eat the FODMAP fiber it doesn't take very long to reach a hungry crowd of bacteria. The bacteria happily munch on the fiber (same story, different location) and generate a lot of gas. The small intestine can't accommodating this gas quite like the colon, though. The gas has nowhere to go and causes a great deal of discomfort, pain, and distention (the SIBO pregnant belly effect!). Worse still, these gases can really mess with the our motility, which manifests as diarrhea, constipation, or both. Because we're talking about somewhere in the middle of a 30 foot tube (the digestive tract), the gas has nowhere to go- it sticks around until it's slowly absorbed into the blood stream, filtered by the lungs, and exhaled.
One of the first things I talk to my SIBO patients about is reintroducing high FODMAP foods- why it's important, how to do it, when to do it. These foods are super important and healthy for us to eat on a daily basis. The key is to treat the SIBO and the root causes first, then you will find that your tolerance for FODMAP foods will improve.