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Your second brain: How your gut affects your mental health

Nov 6, 2019

Read time: 13 min

You know that “knot” in your stomach you get when you’re afraid? Or have you noticed that feeling sad or anxious can affect your appetite or the number of bathroom trips you need to make? ? It’s not in your head—new research confirms just how much our gut and brain interact, and the connection is stronger than we’d ever imagined.

“In moments of anxiety, I can feel it in my gut. I get queasy and feel like my insides are mush. For example, when I have to do a mock trial for school, it can get pretty tense knowing [I] have to perform memorized lines and responses, and I feel it in my stomach,” says Joe M., a fourth-year undergraduate at College of the Holy Cross in Massachusetts.

The gut and microbiome

Illustration of a happy guy

Research shows that our gut (aka digestive system) plays an essential role in all aspects of health—including brain health and mental health. This is because it digests and absorbs nutrients from our food and gets rid of waste. ? When our gut does its job absorbing what we need—and keeping out what we don’t—it helps nourish every single cell in our bodies.

Our gut also houses “friendly” microscopic organisms, aka microbes. This community of microbes—known as the microbiome—includes mostly bacteria, along with yeasts and viruses too—a whole lot of them. In fact, we now know that one person can have more than 1,000 trillion individual microbes in their gut! Holy microbes.

So how do these friendly microbes enhance our health? They:

Help break down certain nutrients we can’t use (e.g., fiber) and turn them into nutrients we can use (e.g., short-chain fatty acids, the main source of energy for the cells that line your colon)
Crowd out bad microbes we ingest that can cause disease, which reduces the risk of serious gut infections
Make certain essential vitamins, like Vitamins B12 and K
Have a profound effect on other parts of our bodies—such as our brain and mental health

“The gut microbiome appears to be a rich arena for medical progress,” says Dr. Davis Smith, a physician at the University of Connecticut. “We’re all colonized by millions of bacteria. The relative abundance of different bacteria types seems to have a significant impact on physical and mental health.”

How do these microbiota-gut-brain connections actually work?

The microbiome-gut-brain axis is a complex one. It involves connections between nerves, biochemicals, and the immune system itself. This is a hotbed of research right now, so we should have more details in the near future, but let’s look at what we know so far.

The enteric nervous system, aka our “second brain”

Your gut has somewhere between 200 and 600 million nerve cells that together form the enteric nervous system. This is often referred to as our “second brain.” These nerve cells control the intricate functions necessary for your gut to do its job. For example:

The release of enzymes to help you digest
The movement of food through your digestive system
The blood flow around the digestive system that picks up the absorbed nutrients

Basically, the gut uses its own brain to function optimally.

Our happiness neurotransmitter—serotonin—and the gut

One of the most famous mood-affecting neurotransmitters—serotonin—is made in the gut. Serotonin is sometimes called the “happy” neurotransmitter because it appears to contribute to our well-being and tends to be lower in people with depression. Research shows that 90 percent of serotonin is in the gut, (not in the brain) and it plays the essential role of promoting the movement of food through the gut.

Stress hormones and the gut

Another important biochemical connection between our microbiome, gut, and brain is through stress hormones. Our HPA-Axis (Hypothalamic-Pituitary-Adrenal Axis) starts in our brains and uses hormones like cortisol to affect other parts of the body, including the gut. A number of studies have shown that stress can alter the gut microbiome. It also appears that the microbiome plays a role in stress-related conditions such as anxiety and depression, though researchers are still working on understanding this connection.

The immune system and the gut

Our gut plays a major role in defending our body against disease, which makes sense since our mouths are portals to the outside world. In fact, over half of our immune cells that produce antibodies—aka the cells that find and attack invaders—are located in the gut.

How food affects your mood

Illustration of gut with food It turns out that it may be possible to affect our brain and mood with the foods we eat, according to developing research. A healthy diet is linked with a lower risk of mental health issues. The good news here is we control what we eat. In fact, what we eat is the main thing that influences our gut microbes.

Components of a healthy diet include:

Fruits and vegetables
Whole grains, nuts, seeds, and legumes (e.g., lentils, peanuts, beans)
Fish, poultry, lean red meat, eggs
Healthy fats such as olive oil and avocados

Not surprisingly, foods associated with poorer mental health include processed, sugary, salty, fried, and fast foods, as well as sugary drinks. A 2017 randomized clinical trial found that when people with depression who ate these poor-quality foods improved their diets over 12 weeks, their depression symptoms (such as feelings of sadness and tension) improved.

“I find that junk food always makes me happy when I’m tasting it, but almost immediately afterward, I feel worse, and then I get grumpy and ‘blah’ feeling. On the contrary, when I eat clean, healthy food, I always feel much happier and less like a roller coaster throughout the day,” says Hilary P., a fifth-year undergraduate at the University of New Brunswick in Canada.

Can probiotics help our mood?

Probiotics are health-promoting microbes that we can eat, drink, or supplement with. They’re found in fermented foods like yogurt, sauerkraut, kombucha, kefir, miso, and kimchi.

One review of 10 studies found that there may be some mood benefits from taking probiotics. Another review looked at seven studies that compared probiotic supplements to placebos in healthy volunteers. The researchers concluded that there was a significant improvement in psychological symptoms and perceived stress in people who took the probiotics. This research is promising but still preliminary.

Pro tip: Always check with your healthcare provider before taking any supplements, including probiotics. Also, always read labels before purchasing a supplement to ensure that none of the cautions or warnings apply to you, and to ensure you’re taking it as directed.

illustration of stressed gut

Reduce stress for your gut

So we know that gut issues can affect your stress level and moods, but guess what? It works the other way around too. If you have gut issues, then reducing your stress may help.

Stress influences a whole bunch of gastrointestinal functions, such as how well food moves through your gut (motility), the secretion of important biochemicals, and how tightly the gut cells adhere to each other (permeability).

Irritable bowel syndrome (IBS) is considered a “biopsychosocial” disease. This means that it’s not just physical—stress plays a key role in it. In fact, people with IBS tend to have higher-than-normal levels of stress, depression, and anxiety. And people who report high levels of stress can go on to develop gut issues.

In other words, reducing stress can be a big step toward improving a lot of gut symptoms.

Mind-body therapies to improve gut health

Some experts say that the most effective treatments for IBS are mind-body therapies, such as hypnotherapy, mindfulness, and cognitive behavioral therapy. A 2017 review of 12 studies found that mind-body approaches were effective in helping people in China with some of their IBS symptoms.

To improve your gut health, try this

Identify what could be contributing to the problem. “Sometimes, associated factors or symptoms can be clues,” says Dr. Smith. “For example, have you experienced a change in diet or meal patterns? Other illness symptoms? Any recent antibiotic use, or increased stress, anxiety, or depression?”
Eat more fiber-rich veggies and fruit, and eat fewer processed foods.
Try to reduce stress through things like mindfulness, social support, counseling, time in nature—whatever works for you.
Eat more fermented foods (e.g., yogurt, kimchi) or talk to your doctor about supplementing with probiotics (remember, this is still an area that needs more research).
Get adequate sleep and exercise.