Prebiotics are types of fiber that go undigested through the GI tract, are quickly fermented by the bacteria in our gut and stimulate their growth and activity (think of them as their favorite food). This is, in general, a good thing, as these friendly gut bacteria confer a number of health benefits. For example, they keep at bay the “bad” bacteria, and can aid in the absorption of minerals and benefit the immune system. The downside is that, as bacteria feast on this food, they also produce gas. In healthy people this is no more than a “nuisance” but in people with IBS, the bacteria produce excess gas or flatulence, which can be painful and life-disrupting.
The low FODMAP diet minimizes the intake of prebiotics to reduce IBS symptoms. As such, it reduces the amount of food available to the friendly bacteria in our gut, leaving them potentially "starving".
Studies that have looked at the impact of a low FODMAP diet on gut bacteria have shown that after only 3 or 4 weeks on the elimination phase, both the amount and variety of good bacteria are decreased. One study also showed that there was an increase in bad bacteria.
Should you be worried? Yes and no. For starter, no-one should be in the elimination phase for more than 4-6 weeks. While you are in that strict phase you are not supposed to completely eliminate all probiotics but to minimize them. You can eat small portions of prebiotic-rich foods and keep your gut bacteria from "starving" too much.
Here are a few examples of foods that confer some prebiotics but are still considered “low FODMAP” (at the “green” serving size, according to the Monash University Low FODMAP Diet App):
Vegetables: potatoes (cooked and cooled as in a potato salad), savoy cabbage, eggplant, canned baby corn
Fruit: green or firm bananas, kiwi, citrus fruits, and pomegranate seeds
Whole grains: raw rolled oats; buckwheat groats, brown rice, and quinoa, cooked and cooled, as in a grain salad.
Legumes: canned chickpeas and lentils (1/2 cup)
Nuts: almonds and hazelnuts; tigernuts - not really a nut but a tuber
Finally, once you start re-introducing higher FODMAP foods, you may find that you can re-introduce some of the prebiotic fibers (oligosaccharides). The goal is to find a good balance between keeping symptoms at bay and eating some prebiotics to achieve the right equilibrium of gut bacteria. The good news from research in this field is that, once people start eating more prebiotics, the gut population grows back strong!