Sorry to startle you, but there’s a massive number of bacteria (trillions, actually!) in your gut. It’s called the microbiome, and don’t worry, it’s there for a good reason.

Although bacteria often get a bad rap, they’re really the unsung heroes of our health. In fact, the human body has relied on bacteria to survive since the early days of homo sapiens, explains Frank Lipman, MD, functional medicine doctor and Chief Medical Officer of THE WELL.

The main role of the gut is to help properly digest food, as you may already know. But your gut plays an essential role in many other systems of your body. Poor gut health can lead to a lot of not-so-great side effects and impact your overall health.

Here are six of the surprising things that happen when the gut microbiome is out of balance:

1. Improper Nutrient Absorption

When “bad” bacteria within the microbiome overtake the “good” bacteria, our microbiome becomes unbalanced. This happens for a variety of reasons, including stressful lifestyles, lack of sleep, antibiotic use, too much sugar, processed foods or alcohol and more.

When things go awry, your digestive tract won't properly absorb nutrients from the food you eat, which can lead to serious health problems down the line, including food intolerances and allergies, brain health issues and more.

Looking for ways to improve your gut health? Read this.

2. Weakened Immune System

Another key job of our microbiome is to nourish the cells of the gut wall, Lipman explains. This wall is only one cell thick and most of your immune system is just on the other side. So when your microbiome is off kilter, your immune system struggles, and you’re more likely to suffer from frequent colds, allergies, joint pain, acne and potentially more serious disorders.

3. Poor Mental Health

“Our gut helps us process thought and emotion — so much so that it is often referred to as ‘the second brain,’” Lipman says. In fact, about 95 percent of serotonin (a feel-good chemical that promotes emotional well-being, self-confidence and good sleep) is made in the gut.

When your microbiome is in good shape, your serotonin and other neurochemical levels are more likely to be optimal. As a result, you feel calm, balanced, optimistic, and confident, and you are more likely to sleep well. But when your microbiome is out of whack, your body's production of serotonin and other neurotransmitters drops, leaving you more vulnerable to depression, anxiety, self-doubt and sleep problems.

“We think of these emotional health issues as ‘brain problems,’ but in fact, the biochemicals that govern them are more densely populated in the gut, so really, they could be thought of as ‘gut problems,’” Lipman notes.

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4. Increased Food Cravings

A 2016 study shows that the microbes within us can affect our food choices: By releasing signaling molecules into our gut, the bacteria in our bodies can actually engineer our cravings, Lipman notes. As the study explains, “Microbes have the capacity to manipulate behavior and mood through altering the neural signals in the vagus nerve, changing taste receptors, producing toxins to make us feel bad and releasing chemical rewards to make us feel good.” In other words, the bacteria within us can trick us into eating less-than-healthy food that ultimately can lead to inflammation and/or weight gain.

What’s more: Due to a link between the gut and pituitary gland (which controls your hunger signals), an unhealthy balance in your gut microbiome can also confuse your brain into not knowing whether you’re hungry or full.

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5. More Inflammation

When you overfeed your bad gut bacteria, it boosts gut permeability, Lipman says. “This means more bacterial bad guys are slipping through the cracks in the delicate gut wall, and getting into the bloodstream, where they’re triggering system-wide inflammation,” he explains. “This inflammation produces weight gain as well as many other symptoms, including the gas, bloating, reflux, acne, and hormonal issues that bring so many patients to my door.”

6. An Unhealthy Heart

“There is growing evidence indicating that a healthy gut also plays an important role in heart health, making optimal gut health a potential lifesaver,” Lipman notes. The reason: Gut permeability appears to contribute to heart disease by triggering inflammation, which makes the plaque that sticks to your artery walls less stable. And unstable plaque can break off into the bloodstream, triggering a heart attack. In sum: “The less inflamed the gut, the better your odds of living a long, healthy life.”