I’m in a crowded commercial kitchen, and everywhere I look I see bottles of colorful drinks and jars holding faded vegetables suspended in brine. The smell of fermented cabbage permeates the room. I open a mason jar, which lets out a loud hiss. I’d spent months researching the gut-brain axis during my PhD, hoping to understand the role that fermented food may play in our mental health. So I enrolled in a class on how to make fermented foods.
The teacher is praising these ancient foods as a magical cure for every ailment you can imagine. I’m uncomfortable – not because of the smell, but because I’ve never found a scientific article that definitively supported this idea. I’m subconsciously applying a fact filter and wondering what the other unsuspecting students must think. I let this slide, since I’m here to learn the art of fermentation. I bravely take a spoonful of sauerkraut. The salty brine overwhelms my senses. Gulp!
If you’ve ever eaten sauerkraut, kimchi, tempeh, kombucha, or kefir, then you’ve had a fermented food (or drink). The first time I gave them a proper go (with a mind open to enjoying them), I noticed the sour, vinegar-like taste and the noticeable absence of sugar. It didn’t take me long to get used to the taste. After a while of drinking my bubbly kombucha, I noticed that my palate had adapted and sweet flavors felt overpowering.
Fermentation is a natural process of curdling or culturing that has been used for thousands of years to preserve foods. Fermented foods and drinks are made through “desired microbial growth and enzymatic conversions of food components” (as opposed to undesirable microbial growth, which happens when your food spoils). Fermented foods are made either by the bacteria and yeast already present in the environment/food material or by introducing bacteria or yeast to help start the fermentation process.
For example, when I made sauerkraut, I shredded the cabbage, added salt, then pummeled and squeezed the cabbage until it released its own juices, which also allowed the “probiotic” lactic acid bacteria in the cabbage to kickstart the fermentation process. Probiotic bacteria like Lactobacillus and Bifidobacterium are considered probiotic good bugs, and are also present in many yogurts and cheeses.
We can’t necessarily call our sauerkraut a “probiotic food” because we don’t know the exact probiotic strains that are in our sauerkraut and whether they are present in the correct “probiotic” dose. It’s also worth noting that foods and drinks that are produced by fermentation don’t necessarily need to have live bacteria in them when you eat them to still be considered a fermented food. For example, sourdough is born from a bubbly live starter culture that contains yeast and bacteria, but once cooked it might no longer have any live bacteria in it.
So, what about the health claims?
Microbial fermentation may interact with health through multiple different biological pathways. It can enhance the nutritional composition of the final food, create bioactive compounds, and change the composition of the gut microbiota (potentially outcompeting harmful pathogens). The lactic acid bacteria in fermented food might also help to influence your immune system and strengthen your intestinal barrier. Some fermented foods, like tempeh, also contain prebiotics; these are fibers that escape your digestion and are broken down by your gut bacteria, including your lactic acid bacteria, which feed off prebiotic fiber to help grow their colonies. In a recent diet experiment, a high-fiber diet was compared with a diet high in fermented foods (eg, yogurt, fermented vegetables, kefir, fermented cheese); those who ate higher fermented food had lower markers of inflammation and an increased diversity of gut microbiota (which is thought to be a good thing in adults). So, in theory, fermented foods sound good.
Still wanting to understand more, and dispel a few myths, a team of researchers and I investigated what’s known about the link between fermented foods and mental health. We looked at the pathways by which fermented foods might affect mental health, such as by reducing inflammation and strengthening the intestinal barrier. These pathways are relevant because they might reduce your brain’s exposure to certain inflammatory molecules that can impact brain function and mental health.
Fermented foods also contain neurotransmitters that are important to mental health. Research about fermented food and mental health is still in its early infancy. Animal studies provide experimental evidence that fermented foods can help with symptoms of depression and anxiety – but that’s in animals. The problem is in knowing how the animal findings relate to our human experience.
We found eight studies in humans that experimented with fermented foods (for example, fermented milk products) to measure their impact on depression, anxiety, and stress in adults, but the studies were all so different that we were unable to make firm conclusions. It is still difficult to know what the active ingredient in fermented foods is. Is it the microbes? Is it the byproducts? Is it the nutrition? And how much of each is needed, and what are safe levels of each? We really need more studies, with detailed descriptions of exactly what is in each food being tested. At this stage, there is not enough human evidence to make firm clinical recommendations for eating fermented food to improve mental health symptoms.
I’ve since moved on from sauerkraut to making sourdough bread as a COVID lockdown project (as this involves a fermented starter culture). When my delicious fresh bread comes out of the oven, my world is paused for a few minutes, and my family mill around to enjoy the warm, fresh bread. While it may be too soon to tell whether fermented foods help our mental health, my sourdough itself has sure helped us.
Dr. Dawson is a nutritionist and bioinformatician research fellow at the Food & Mood Centre at Deakin University, Geelong, Australia. She disclosed no relevant financial relationships.
A version of this article first appeared on Medscape.com.