We've all heard the phrase "trust your gut" -- and for good reason. The gut and more importantly, the healthy bacteria living inside it, are key players in what makes our bodies function at their best.

"There's a ton of research going on in this area right now -- smaller studies keep adding to the path and pattern that we're seeing -- supporting how important this gut microbiota is to our overall health," Liz Weinandy, lead dietitian at the Ohio State University Wexner Medical Center, said. "It really does have an impact on things from allergies to autoimmune diseases to even sleep disorders and metabolic disorders, all these different conditions that are becoming -- unfortunately -- more and more common in the U.S. it's definitely worth thinking about how a person's microbiome functions and what they can do to improve it."

What is gut health and the microbiome?

PHOTO: Abdominal organs are pictured in a stock illustration.
Sebastian Kaulitzki/Science Photo Library/Getty Images
Abdominal organs are pictured in a stock illustration.

Weinandy explained that most of the conversation around gut health is referring to bacteria.

"What we're really talking about is all these microorganisms -- generally we refer more to the bacteria in the gut, but there's lots of different microorganisms -- there's a whole lot of these little cells down in our GI tract (gastrointestinal tract) that really play a huge role in our health," she said.

"We have 10 times more cells in the gut than we do any other area of our body," she said, and cited numerous studies and scientific findings including the National Institutes of Health Human Microbiome Project. "We really want to try to think about this as a foundation for good overall health."

Dr. Will Bulsiewicz, MD MSCI, gastroenterologist, researcher and author who specializes in gut health, said: "There is an entire ecosystem living inside of us. There are 38 trillion microbes living inside our intestines," predominantly in the colon within the large intestine. "This is what makes up what we describe as our gut microbiome -- we have discovered that these living creatures are actually completely integrated with human health, human physiology so they play an absolutely critical role in our digestion."

"Seventy percent of our immune system is located in the gut," Bulsiewicz said. Microbiota, he said, are connected to other key aspects of our health. "They're connected to our metabolism, to our hormones -- our mood, our brain health, and even to the expression of our genetic code."

How to measure gut health

"One of the principal rules, if not the main rule of gut health is that these gut bugs thrive on fiber. This is their preferred food -- because fiber is not digested by human enzymes," Bulsiewicz said. Fiber reaches the colon fully intact, the microbes feed on it and digest it. "They will release something called short-chain fatty acids," which he said "are the most healing, most anti-inflammatory thing that I've ever come across."

A recent study from the Laboratory on Thymus Research published in Frontiers in Endocrinology detailed the relationship between SCFAs and the Central Nervous System. Though it's a brand-new area of research, scientists are learning that there may be a link between gut bacteria and the brain - and even mood disorders like depression and anxiety.

PHOTO: Natural foods can contribute to a healthy bowel.
Piotr Malczyk/iStockphoto via Getty Images
Natural foods can contribute to a healthy bowel.

"It starts in the gut. Fiber meets microbes, short-chain fatty acids come from this intersection. And then the short-chain fatty acids get into the bloodstream and they spread throughout and they provide healing benefits everywhere that they go," he continued.

The American Gut Project at UC San Diego School of Medicine, one of the largest studies on how diet and lifestyle affect the health of the microbiome, "found that the single most powerful predictor of a healthy gut was the diversity and variety of plants in your diet," Bulsiewicz said.

Fueling the microbiome with the right food

Dietitians and gastroenterologists agree that a range of plant diversity is the best way to keep the bacteria inside a healthy gut, happy and functioning.

"Minimally or unprocessed and plant-based foods" in a wide variety, with "fiber from many different plant sources are really what will help to feed and really grow and help the microbiome flourish," Weinandy said. "The main thing for people to think about is to add more plants to their diet ... specifically, fruits, vegetables, lentils, beans, nuts, legumes, whole grains, things that are going to be higher in fiber that help to feed those beneficial gut bacteria."

Underripe bananas, plantains and sweet potatoes are all examples of resistant starches, or fermentable fibers, which both Weinandy and Bulsiewicz said are very beneficial to gut bacteria. Whereas fermented foods like sauerkraut, kimchi, yogurt and kefir, have live and active cultures that act as gut-friendly fuel for microbiota.

"Artichokes have been shown time and time again to really help to promote the gut flora growth ... beans and asparagus, raspberries, apples," Weinandy suggested, adding that many other nutrient-rich foods like leafy greens, while maybe not the highest in fiber, offer other plant compounds like polyphenols, essential to a healthy microbiome.

Fiber, resistant starches and polyphenols are all found exclusively in plants and mushrooms, Bulsiewicz said, but "the fiber in kale is not the same as the fiber in a bean. Some microbes like kale, and some like beans – the way that we make sure that all our microbes get fed is by eating a wide variety of plants."

They both emphasized the importance of minimizing consumption of processed foods, especially inflammatory foods like added sugar, which can hide in an array of foods from processed bread to ketchup.

PHOTO: Gut health
Laurence Monneret/Getty Images
Gut health

"Probiotics are the actual beneficial microorganisms themselves that people can take, usually in supplement form," Weinandy said. But while they can be beneficial, "I tell people don't rely on these," since it depends on a person's lifestyle and overall health and "like a vitamin or mineral supplement, they're not the main way to get these microorganisms in your guts."

Experts say that for most people, the best way to feed your gut microbes is with real food - and not with store-bought probiotic pills.

"Prebiotics are actually food items -- fiber -- that really help to grow the beneficial gut bacteria," she said. "It's the food for the beneficial bacteria. If we don't get enough of these prebiotics in, then what we're doing is we're starving our gut bacteria."

"What those fermented foods have in them are post biotics," Weinandy continued. "The compounds that are made when that good gut bacteria actually digest the prebiotics. It's the compounds that come from that digestion and those are actually active compounds. It's really full circle."

Bulsiewicz added that "in order to get the postbiotics, you need both prebiotics and probiotics. They're already there, we just need to feed them."

How to determine if it's time for a gut check

You should consult your doctor or a trained and licensed medical professional before self-diagnosing or making any changes.

If there is reason to believe your gut health needs a little boost getting into high gear, consult your doctor or a trained and licensed medical professional, experts advise, but they say there are some things to do in the meantime.

"The first thing is really looking at your GI tract and look at your bowel movements. How is your GI tract functioning? Do you have a lot of bloating problems with gas? Kind of what we think of those typical irritable bowel syndrome symptoms -- diarrhea or constipation," Weinandy said.

Bulsiewicz added that "bio-individuality" makes us all unique, so while studies can show "what is sort of normal among a broad population of people," he explained that "within those studies, there's going to be some people that something works for and others that it doesn't."

There can be "a laundry list of conditions" associated with an unhealthy microbiome he said, but "first and foremost if you have damage to the gut it is almost definite that you have some digestive health issue."

Bulsiewicz said if he looks at a person's medical history, he can see "not only are there the digestive problems but there may be metabolic or immune, neurologic or hormonal issues."

"Seek credible sources of information," Bulsiewicz also said. "There's going to be hyperbolic claims that come with the territory and anything that gets hot. There's gonna be people trying to like use this to make stuff happen or sell stuff so you've got to be careful."

Avoid any material from companies or people who are trying to sell gut-healthy foods or probiotic pills and if there are questions, ask your doctor, he said.