Prebiotic foods like asparagus help feed the good bacteria in the gut. (For Corewell Health Beat)

You have heard of probiotics, but what about prebiotics?

Probiotics may grab the spotlight these days—and space on store shelves.

But if we want them to do their best work and keep us healthy, we need to pay attention to what we feed them.

That is where prebiotics come in.

“They are so important,” said Krista Gast, a registered dietitian and health and wellness coach with Corewell Health’s Lifestyle Medicine program.

The two elements combine to create a healthy gut microbiome, which in turn supports our digestion, immune system, physical and mental health.

Gast explained how the role of the gut microbiome affects our health and suggested foods that can help keep it in top shape.

Support your immune system

“Prebiotics is an umbrella term that encompasses several things,” she said.

Mostly, it refers to fiber-containing plant foods, but it also includes resistant starch or fermented foods, as well as healthy fats.

“There is an ecosystem living inside our gut. There’s easily 90 trillion microbes,” Gast said. “It’s pretty amazing.”

Those microbes—the beneficial bacteria called probiotics—digest and ferment prebiotics.

They produce short-chain fatty acids, also called postbiotics.

“The postbiotics are very anti-inflammatory,” Gast said. “Seventy percent of our immune system lives right on the other side of our gut wall.”

The gut plays plays a crucial role in regulating the immune system, helping it to be active enough to fight infection but not over-active, which can lead to autoimmune disease.

The anti-inflammatory postbiotics also suppress the pathogenic bacteria—so-called bad bacteria—in the gut.

A diet high in processed food and low in fiber, on the other hand, allow pathogenic bacteria to thrive.

Brain boost

Between the brain and the gut runs the vagus nerve, like a highway that enables constant communication back and forth.

The gut microbiome plays a big role in the production of hormones, including serotonin, known as the feel-good hormone.

“Up to 90% of serotonin is produced with the help of the gut microbiome,” she said. “When we don’t have our gut working properly, we can see a shift in our ability to react to stress. It can also impact depression and anxiety.”

The short-chain fatty acids created in the gut can cross the blood-brain barrier and support brain function.

Researchers are studying the impact of the gut microbiome on cognition and even the development of chronic diseases such as Alzheimer’s.

“It can be another thing in our toolbox to prevent chronic disease,” Gast said.

Fueling with fiber

Some fiber is insoluble—it adds bulk to stools and helps keep the digestive system moving.

For prebiotics, look for foods with soluble fiber. Good sources include:

  • Onions
  • Garlic
  • Asparagus
  • Beans and legumes
  • Cruciferous vegetables such as broccoli and cauliflower
  • Leafy greens
  • Whole grains
  • Fruits, including apples and berries

Jerusalem artichokes, also called sunchokes, are another rich source of prebiotic fiber. They look a bit like ginger root and can be roasted like potatoes.

But if you try Jerusalem artichokes, Gast advises taking it slowly. “If you are not used to them, they can cause digestive distress,” she said.

In general, Gast suggests going “low and slow” in increasing fiber content in your diet. This holds especially true for those with constipation or inflammatory bowel disease.

More gut-friendly foods

Also helpful for the gut microbiome are fermented foods, including:

  • Sauerkraut
  • Kimchi
  • Kombucha
  • Yogurt and kefir with live cultures

Foods containing omega-3 fats also feed the probiotics. Good sources include:

  • Chia seeds
  • Flax seeds
  • Walnuts

Resistant starches also feed the good bacteria in the gut. They are carbohydrates that resist digestion in the small intestine and are fermented in the large intestine, acting as prebiotics.

Examples include:

  • Green bananas
  • Potatoes that have been cooked, cooled and reheated
  • Cooked and cooled rice

The more the merrier

Gast avoids labeling any one plant a “super food.”

Instead, she suggests including a wide variety of fiber-containing plants.

“One of the greatest indicators of a healthy and optimal gut is the diversity of plants coming into our gut microbiome,” Gast said.

Consider tracking the plant foods you consume for a week, and aim for a rich variety.

“Herbs and spices count, too,” she said. “They have really powerful antioxidants that microbes like.”

Researchers have found people who consume 30 unique plant foods in a week have much more diverse gut microbes than those who eat only 10.