Can Probiotics Help Your Depression? What We Know, What We Don’t

Bifidobacteria are found naturally in the intestines of humans and are also common in probiotic foods.

Bifidobacteria are found naturally in the intestines of humans and are also common in probiotic foods. (David Mills/UC Davis)

What if your psychiatrist prescribed yogurt and vegetables as an antidepressant?

It may sound like alternative medicine, but researchers at the intersection of psychiatry and biochemistry think that adding certain beneficial bacteria to a person’s intestines could be the future for treating anxiety and depression.

Diet and Depression

Studies have found that a diet high in vegetables and low in processed foods correlates with lower rates of depression. But showing that what you eat actually affects your mental health has been more complicated, because people who are depressed may be less likely to eat healthier, as opposed to the other way around.

But now, in a recent study out of Australia’s Deakin University, scientists say they have used food to effectively treat depression. 

“It’s the first controlled experiment, to our knowledge, to show that dietary intervention can curb mood disorders,” says Dr. Felice Jacka, a psychiatrist at Deakin and the study’s lead researcher.

Deakin and colleagues recruited 56 people, all of whom met two criteria: They were clinically diagnosed with moderate to severe depression, and they had consumed a lot of sweets and processed meats at the expense of healthier foods like fruit, vegetables and fish.

The participants were then randomly assigned to one of two treatments: diet counseling or “befriending.”

Over the course of the 12-week study, subjects in the diet intervention group regularly met with nutritionists who counseled them to increase their consumption of vegetables, whole grains and fish, and to decrease their intake of junk food.

The patients who were subject to befriending met with trained research assistants to discuss topics like hobbies or board games; they did not receive any psychological therapy. This group served as a control to ensure that any improvement in the diet intervention group would not be due to positive social interaction with the nutritionist.

At the end of the 12 weeks, all of the participants were re-evaluated, using the same depression measures as at the study’s start. The results? While both groups showed fewer symptoms of depression, those who had received the diet intervention were significantly less depressed than those in the control group.

Furthermore, the more healthy changes that the subjects made to their diet, the less depressed they were at the end of the study.

“It was pretty remarkable,” Jacka says. “Their level of improvement correlated closely with the level of improvement to their diet.”

How Can Food Affect Our Mood?

At the end of the study, the researchers found similar levels of biomarkers like glucose and cholesterol in the diet and control groups. The groups did not differ in the overall amount of exercise they had engaged in. 

So what happened to the group with the improved diet to make them less depressed?

While many people intuit that they are what they eat when it comes to mental health, Jacka and other researchers believe there is another factor at work: our intestines, and the  signals they send to our brains.

“We are still only starting to tease all of this out,” says Melanie Gareau, a physiologist at UC Davis who specializes in understanding interactions between our brain and our gut. Given all that we know about that link, the Australian study results make sense, she says.

“We’ve known for quite a while that over 95 percent of the serotonin in our bodies is produced in the intestines,” says Gareau. As serotonin is one of the primary neurotransmitters mediating depression, she thinks it’s no surprise that what goes into our intestines can affect our emotions.

But it’s not just about the food we are eating, she says. It’s how that food interacts with the trillions of bacterial cells that live in our guts, collectively called our microbiome.

Gareau points to a small study out of UCLA that shows the effect of probiotics — micro-organisms believed to be beneficial to humans — on brain activity.  

In the study, 12 women over the course of a month were given yogurt containing Bifidobacteria and Lactobacillus. Both of these have been associated with decreased depression in rodents, and there have been suggestive links between those types of bacteria and mood in human studies as well. Although it’s not clear whether taking probiotics with these particular bacteria changes the overall profile of our microbiome for any extended length of time, ingesting them does increase their levels for shorter periods.

In the UCLA study, after four weeks of consuming these probiotics, the women completed an emotional response task in which they viewed pictures of angry and fearful faces, while their brain activity was recorded through functional magnetic resonance imaging, or fMRI. The procedure, which measures changes in blood flow within the brain, showed which areas were activated while the subjects viewed the images.

“The faces [we used] can trigger threat responses in people,” explains Dr. Kirsten Tillisch, the study’s lead researcher and a gastroenterologist at UCLA. “And we know that people with anxiety show increased responses to them.”

As it turned out, the women who took the probiotics showed less brain activity when viewing the emotional images than women who took a placebo. Dr. Emeran Mayer, a co-researcher in the study and the author of “The Mind-Gut Connection,” explains that this kind of dampened response resembles the pattern you might expect to see in someone who isn’t hyper-reactive to the environment.

“The brain’s reactivity to threatening stimuli is reduced. So you could speculate that these people might be less prone to anxiety,” Mayer says.

Is it the Food or the Bacteria?

But if our microbiome affects our mood, how so? Researchers think the process might occur through metabolites, a byproduct released by bacteria that feeds on food our bodies cannot fully break down.

These metabolites can enter into the bloodstream or nervous system, travel up to our brain, and influence how neurons talk to one another. Metabolites may also serve as messengers, signaling cells in the intestines to increase or decrease compounds like serotonin.

Carlito Lebrilla, a professor of biochemistry and molecular medicine at UC Davis, says you have to look at both the bacteria and the food to understand what’s happening.

Although there has been an increase in the marketing of probiotic supplements in recent years, especially for improving physical health, “probiotics are not doing all of the work here,” Lebrilla explains. Ingesting probiotics, whether through supplements or a food like yogurt, lays down some of that “good” intestinal bacteria, so that they are poised and ready to give off the right kind of metabolites. However, whether or not your gut bacteria produce those metabolites depends on the food you eat afterward.

So you can eat a probiotic food like yogurt all day and still not experience the potentially positive effects, Lebrilla says. That’s because we still don’t know which metabolites make our brains feel better, which bacteria give off those metabolites, and which kinds of foods feed those bacteria.

“That’s what we are trying to do right now,” Lebrilla says. He says that while scientists have identified a few types of bacteria that are likely to give off good metabolites, there are hundreds and possibly thousands of bacterial strains in our intestines. If we could map out the specific bacteria-metabolite combinations that reduce anxiety and depression, we would be a step closer to creating customized diets for our brains. It’s something that could take a couple of decades to accomplish, Lebrilla says, “but it’s not that far-fetched.”

In the meantime, both Jacka and Mayer point out that over tens of thousands of years, our bodies have evolved in concert with the microbiota in our intestines to function optimally with the foods we have been eatingFor millennia we fed off of a mostly plant-based and lean-meat diet. But in recent years there have been “profound changes to the kinds of foods we eat,” Jacka says, particularly in the reduced amount of vegetables and increased amount of sugar.

“It’s wildly different from what we were eating even a generation ago.” 

Taking that into consideration, what the findings from her study might really show is not a new diet to curb mood disorders, but rather how we might look back to the foods our ancestors ate in order to restore balance to our bodies and brains.

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