Fermented foods are often among the top nutritional suggestions for gut health, since they contain probiotic bacteria that are believed to have beneficial effects on the microbes in our guts. But what counts as a fermented food, and how do you know if you’re eating the right kind? Let’s dig in.
What are fermented foods?
Definitionally, “fermented foods” are those in which microorganisms, including bacteria and yeast, have been grown in some sort of food product. These microorganisms partially consume the food, and in return, their metabolic waste products change the food’s chemistry.
That chemistry change is why fermented foods were invented in the first place. If you allow milk to ferment, it grows acidic, which gives it a sour taste. More importantly, it makes the resulting milk—which you could call yogurt—too acidic to allow most harmful bacteria to grow in it. Fermenting a food is sort of like planting a lawn: just as grass can take over and make it harder for weeds to gain a foothold, fermenting food makes it harder for other microbes to grow in the food and spoil it.
Fermentation is a way of preserving foods, but it’s also used to change the taste and texture of foods, and to make them easier or more pleasant to eat. For example, if you’re lactose intolerant, you’ll have an easier time eating a block of cheese than chugging a glass of milk.
What foods count as fermented? If we define them as foods in which microbes have deliberately been allowed to grow, we get a huge list of stuff from all over the world. Dairy products like yogurt, cheese, and kefir all count, as do fermented vegetables like kimchi, sauerkraut, and anything pickled. Kombucha, miso, tempeh, and even sourdough bread make the list. Beer and wine? Also fermented.
But are all these fermented foods equal from a health standpoint? Not really. Some of these items, if you buy them in a grocery store, aren’t necessarily fermented in the old-fashioned way. Others use fermentation as a step in their production, but don’t contain any of those beneficial microbes by the time they get to you. And as you may have guessed from the variety in this list, these are all just completely different foods that likely have very different effects on our body.
Why fermented foods are (probably) good for you
I’ll be honest—scientists are still working on the question of why and even whether fermented foods are good for us. But the consensus so far is that they should probably be considered part of a healthy, diverse diet. Beyond that vague statement, things get complicated.
Many fermented foods contain lactic acid bacteria. These little guys make lactic acid—the name literally means “milk acid”—when they’re allowed to chow down on our food. They’re in yogurt, kimchi, and many traditionally made pickles. Lactic acid bacteria are considered to be probiotic: they seem to have beneficial effects on our health.
While probiotics are often characterized as “good bacteria” that take up residence in the gut, the truth is that probiotics don’t usually move in and set up shop. We already have bacteria in our gut, and they’re not going to give up their cushy jobs just because some new guy shows up. But whatever the reason, there are health benefits linked to eating probiotics, which have been associated with improvements in digestion, cardiovascular health, and immune health, to name a few.
There is also research suggesting mental health benefits from probiotics and “prebiotics.” Prebiotics are types of fiber that our human enzymes can’t easily digest, but that the bacteria in our gut can. Vegetables and grains are sources of prebiotics, so a food that contains lactic acid bacteria and prebiotic fiber—like, say, kimchi—provides both.
Why fermented foods might not always be beneficial
Despite all the hype, we don’t have a lot of solid evidence about the benefits of probiotics, or about fermented foods in general. This 2019 review even concludes, “there is very limited clinical evidence for the effectiveness of most fermented foods in gastrointestinal health and disease.”
The probiotics and prebiotics in fermented foods can also cause or exacerbate certain health issues. Bloating is one common side effect, just because there’s a lot going on in our guts when we eat them. Not everybody’s personal ecosystem is chill about accepting newcomers.
Fermented foods also contain a cocktail of different natural chemicals that can be good (or sometimes bad) for us. Histamine, for example, is a compound you may know from its role in allergies. But histamine can also occur in fermented foods, causing allergy-like symptoms in people who are particularly sensitive to histamines.
There are also food safety issues, especially with home-fermented products. The traditional means of fermenting foods usually works out pretty well, but it really helps to have experience with the particular fermented foods you’re trying to make so you can recognize when the wrong microbes are growing. If you’ve ever gotten so into kombucha brewing that you’ve pored over photos on blogs trying to diagnose whether your SCOBY is doing something normal-weird or bad-weird, you know what I mean.
Which grocery store foods are actually fermented?
So you’d like to add more fermented foods to your diet. Cool! But just knowing that certain foods can be fermented doesn’t mean that the ones you picked up from the store actually are.
Take pickles. The traditional way of making any pickled vegetable is to soak said vegetable in brine. The salt in the brine is hostile to many of the microbes that would otherwise want to grow on your veggies, while holding space for those lactic acid bacteria we know and love. When they arrive and begin to multiply, those little guys eat the carbohydrates in the veggies and produce acid. Leave your pickles to pickle long enough, and you’ll get that salty-sour fermented flavor we all know and possibly love.
But if you’ve ever made quick pickles, you didn’t do it that way. You got some veggies, and you poured vinegar over them. Boom, you have something that looks like a pickle and tastes like a pickle—it was just produced an entirely different way. (Vinegar is also traditionally a product of fermentation, in which one set of microbes turns sugar into alcohol, and a second set turns alcohol into acid.) Perhaps some fermentation has happened somewhere along the way, depending on what kind of vinegar was used, but you aren’t getting a mouthful of probiotics when you eat quick pickles.
If you want the old fashioned version, look for “lacto-fermented” pickles from a specialty supplier. (Most grocery store pickles are the vinegar kind.) Fermented vegetables with live cultures, including pickles and kimchi, will be stored under refrigeration to slow microbial growth. If you leave them at room temperature—or even in the fridge for an unusually long time—they can build up pressure and even, in extreme cases, explode.
If you’re buying yogurt, the FDA requires the yogurt maker to tell you if the yogurt does not contain live microbes. (The phrase “contains live and active cultures” is optional if they’re in there.)
Sourdough bread uses fermenting microbes in its starter, but by the time the bread has been baked, those microbes are all dead. You’ll still get some of the products of fermentation—hence the flavor—but if you’re looking for live microbes, they’re gone.
It’s fine to eat fermented foods that have had their microbes killed, or foods that were traditionally fermented but that are produced in a different way today. They’re not bad for you or anything. They often contain some of the same components, like the prebiotics and the bacterial metabolites, that make fermented foods healthy. But if you want to be sure you’re getting foods that are actually fermented, check labels, and do your research.
