Probiotics—and the buzz about their benefits—are pretty much everywhere. By now, you’ve probably even picked up a supplement or tried out a few fermented foods (like yogurt, pickled vegetables, sauerkraut, and kombucha) to boost these healthy bacteria that live in your gut. After all, who doesn’t want a healthier digestive system, more regularity, and super-strong immunity?

It’s important to get your fill of probiotics to keep your gut functioning at its best, but in order for these powerful critters to work their magic, they need a little help. That’s where prebiotics come in.

Prebiotics are basically the food probiotics need in order to thrive in your gut, explains Jenny Dang, R.D. This food helps the healthy bacteria do their jobs, so you can reap their health benefits. Prebiotics are a type of carbohydrate that your body can’t break down, says Toni Fiori, R.D., who specializes in digestive health. Chances are you’ve heard of this type of carb before: ‘insoluble fiber.’

Insoluble fiber, which isn’t digestible and passes through the body pretty much intact, helps food move through your system smoothly and wards off constipation. Because it also helps maintain the good bacteria in your gut, insoluble fiber is hugely important for your digestive and immune health.

We still have much to learn about the billions of bacteria that live in our guts—but if you ate only processed foods that lack prebiotics, it’s very possible that your healthy bacteria would take a hit, even if you did take probiotics, says Fiori.

The good news is, getting your fill of prebiotics isn’t that difficult. You can find them in whole grains, fruits, and vegetables, with some of the best sources being whole wheat, bananas, garlic, onion, and asparagus, says Dang. You can also find insoluble fiber in supplements, which are often made from chicory, a relative of garlic and onion.

To serve up the prebiotics your probiotics need to live their best lives, focus on eating a well-balanced diet that includes lots of whole grains, fruits, and veggies, says Dang. (That means eating about two to three cups of vegetables, two cups of fruit, and about two ounces of whole grains each day, according to the USDA.)

Related: 5 Foods That Are Packed With Probiotics

While women need at least 25 grams of fiber a day and men need at least 38, the average American gets just about 15, so chances are you need to up your intake. Just don’t try to go from zero to 60 in one day. When you eat much more fiber than you’re used to, the bacteria in your gut produce gas, which can result in major discomfort, bloating, and of course, gas. Instead, slowly increase your fiber intake to avoid tummy troubles.

If you have a health condition like irritable bowel syndrome, for example, you may need to get your prebiotics from gentler sources (think bananas, oats, and honey) since garlic and onions may upset your stomach, says Fiori. A dietitian can help you sort out which prebiotic foods might settle best with you, while picky eaters who don’t get their fill of fruits, veggies, and whole grains may want to try adding an insoluble fiber supplement, like oat bran, to their routine.

Become a prebiotics vs. probiotics whiz with a little help from this infographic: