I pulled the jar from the fridge, still cold. Standing at the kitchen counter between two training sessions, I ate 300 grams of raw minced meat and 300 grams of raw sauerkraut. It felt like I was doing something for my body that most people were too soft to do.
In my twenties, I wrestled competitively and weighed every meal. The sauerkraut had to be raw and unpasteurized. I just knew, without needing proof, that the live cultures mattered most. Heat destroyed them. The canned, shelf-stable kind seemed inferior. So most days, I ate it cold and sour, alongside the raw meat.
I never questioned any of it, not during my wrestling years, not in medical school, and not in the twenty years since. Fermented food was good for me. My doctor said so, my social feed said so, and my mother said so. It was one of the few health beliefs I never examined, because it seemed too old and obvious to be wrong.
Last month, I did what I usually do for everything else, but rarely for beliefs I enjoy: I looked for proof. I read every human trial I could find on the four fermented foods people call medicine: sauerkraut, kimchi, kefir, and kombucha.
What I found was not the reason I ate all that raw cabbage.
The gap between what we believe about these foods and what research has actually shown is wide, wider than I expected. Fermented food is fine to eat, but its reputation got ahead of the evidence, and no one updated the story.
The Yogurt That Won a Nobel Prize
This story begins with a Nobel laureate and a mistake.
In 1907, Élie Metchnikoff, one of the founders of immunology, published a book called The Prolongation of Life [1]. In it, he argued that Bulgarian peasants lived unusually long because they drank soured milk full of lactic acid bacteria, and that these bacteria fought the rot in the gut, which he believed caused aging. He won the Nobel Prize the next year (specifically for his discovery of phagocytosis, the process by which certain cells engulf and destroy foreign bodies and bacteria). The idea that fermented dairy means a long life is his.
There was one problem. It was wrong. By the early 1920s, work at Yale by the bacteriologist Leo Rettger had shown that the bacteria in Bulgarian yogurt do not survive the human gut, let alone settle there [2]. Metchnikoff’s specific claim was dead within about 15 years of the book. His reputation and the reputation of the foods were not. The belief outlived its own disproof by a century.
That pattern runs through this entire field. You do not have to take my word for how weak the evidence is. Listen to the people who most want it to be strong.
In 2021, the International Scientific Association for Probiotics and Prebiotics, the field’s own leading scientific association, published a consensus statement [3]. Inside the careful language is a sentence that should stop anyone selling you a bottle: “with the exception of yoghurt and other cultured dairy products, few well-designed, randomized controlled trials on the health benefits of the array of fermented foods have been published.” For foods like kombucha, they wrote, the evidence “is mostly limited to chemical analyses and animal and cell culture models.” Cells in a dish. Mice. Not people.
So how did such a weak belief become so widespread?
Part of the reason is that lab results are often mistaken for proof. The science is real and interesting: fermented foods have live bacteria, and those bacteria make compounds that calm inflammation in test tubes. But what works in a dish or in a mouse quietly turns into ‘good for you’ by the time it becomes a headline.
Then there are population studies, which cannot separate cause from coincidence. People who eat a lot of kimchi or sauerkraut also tend to eat more vegetables, avoid junk food, exercise more, and smoke less. Their better health is often credited to the fermented foods. Even the researchers behind these studies say clearly that this kind of evidence “does not establish causation” [4].
Money also plays a role. Many of the existing trials are funded by the companies that sell these foods. In one well-studied example, reviews of artificially sweetened drinks written by authors with industry ties were much more likely to be positive than those without [5]. There is a big business in promoting kombucha, and little incentive to prove that cabbage does nothing.
The Part That Holds Up
So keep eating fermented food. I do most weeks. Just judge it by the same standard as anything else you eat, and the claims become more modest and honest.
I still train hard for a man in his forties, and what I am actually trying to hold down is the low, constant background inflammation that rises with age. Researchers call it inflammaging, and it is one of the shared roots of heart disease, dementia, and metabolic decline [6]. It is not abstract. In adults over 80, high levels of two inflammatory markers, IL-6 and CRP, roughly doubled the risk of dying over the following years [7]. And your gut sits at the center of this, because it holds as much as 70% of your immune system [8]. Anything that genuinely lowers that inflammatory load has my attention.
There is one good study suggesting fermented food does exactly that. In 2021, a Stanford team put 36 healthy adults on one of two diets for ten weeks. One group worked up to six servings of fermented foods per day. The other added fiber. The fermented group’s gut diversity rose, and 19 inflammatory proteins in their blood fell. The fiber group did not move [9]. A genuinely striking result.
To be honest, because you deserve it, that study only included 36 people. When another team ran a larger trial in 2025 with 147 participants, the fermented-food group did not show the same broad drop in inflammatory proteins. In fact, some markers of immune-cell activation even increased [10]. When researchers combined 26 controlled trials involving 1,461 participants, they found a real drop in the marker TNF-alpha, but no significant change in CRP or IL-6 [11]. The effect is real, but it is small. I will not pretend it is more than that.
There is one finding I trust most, and it is not the loudest. A study of over 9,000 people found that a healthy gut becomes more unique as you age, and this pattern predicts who lives into their late eighties [12]. Diversity, the thing fermented foods seem to support, goes hand in hand with healthy aging. That is the real reason to eat these foods. The label still promises more than the science shows.
Best to Worst
So which of the four is worth including? I read the human trials on each, and they are not all equal.
Kefir (unsweetened) has the strongest case. I make a point of buying it when I see it in the supermarket or think of it. It is closest to yogurt, the only fermented food with solid evidence. Pooled trials show kefir lowers fasting insulin fairly consistently [13]. Blood sugar drops by about 10 mg/dL in one analysis, but only across six small trials with wide error margins [13]. Beyond that, the numbers are quiet: long-term blood sugar and blood pressure stay about the same [13][14]. There are only six to eight small trials, none large. Still, kefir is the lowest in salt of the four and easy to add to a normal day, so it is the one I choose on purpose.
Kimchi has one strong modern trial, but there is an important detail. In a 2024 randomized, double-blind study, 55 overweight adults took kimchi for twelve weeks. The kimchi group lost body fat, while the placebo group gained it [15]. That is a real result. However, the kimchi was freeze-dried and put into capsules, with one version made low in sodium, and the World Institute of Kimchi ran the study (I swear this is real). A capsule of dried kimchi is not the same as a bowl of the real thing. And real kimchi is very high in salt. The same Korean data that show benefits also link heavy kimchi intake to a higher risk of stomach cancer, likely due to the salt and N-nitroso compounds formed during fermentation [16]. Enjoy kimchi, but do not treat it as a supplement or eat it in huge amounts.
Sauerkraut, which I have eaten my whole life, has the weakest evidence. I found only two small controlled human studies that used sauerkraut alone, with about 120 people in total, and not one well-powered trial on a major outcome [17][18]. Its reputation is based on tradition and theory, not on strong human results. I still eat it, but I no longer call it medicine.
Kombucha has the weakest case and the most hype. I found about eight human trials, with mixed effects on blood sugar and one promising result in just twelve people [19][20]. A 16-ounce bottle has about 12 to 19 grams of carbohydrates, most of it leftover sugar [21]. I do not drink much kombucha, and reading the studies did not change my mind. For the most part, it is a fermented soft drink with a health image. If you like the taste, choose the low-sugar version and do not count it as a health habit.
The Jar in My Fridge
Right now, there is a jar of sauerkraut in my Berlin fridge that I paid too much for because the label promised raw, unpasteurized, live cultures.
The reason for that purchase makes sense. Most fermented foods in stores are pasteurized to last longer, but the heat kills the bacteria you want. A shelf-stable can is mostly dead food [22]. If you want live cultures, choose the refrigerated jar labeled raw or unpasteurized, and eat it cold.
Here is what the label does not say: when researchers compared raw, living sauerkraut to pasteurized, dead sauerkraut in people, the raw version did not come out ahead. In one trial, the pasteurized one did more [18]. The label promises more than the science supports. Still, buy the raw jar if you want more live bacteria and less sugar than the pasteurized kind. Just know the word on the front is not the main reason.
In my Upward ARC framework, fermented food belongs in Activate, the part about what you put into your body. Also, it touches on Recover, which is about calming your system back to normal, since lowering inflammation helps you reset. That is the right place for it: a cheap, low-risk, enjoyable food with potential benefits. It is not a miracle, but it is not worthless either.
Try This Today
Try making your own sauerkraut. Here is how we do it at home: use a scale and a big glass jar, and get one of the kids to help stomp the cabbage. It is almost impossible to mess up, and homemade sauerkraut is better than anything you can buy. Shred the cabbage, weigh it, add 2% of its weight in salt, and pack it into a jar until the brine covers it. Keep it under the liquid and away from air. Let it sit at room temperature, ideally around 20 degrees Celsius, for one to four weeks [23]. For safety, Fred Breidt, a USDA microbiologist, says there has never been a recorded case of food poisoning from properly fermented vegetables, because the acid produced by the bacteria lowers the pH below what pathogens can handle [24]. Botulism is a risk with sealed, low-acid canning, not with salted open ferments, where the acidity keeps the bacteria from growing. A fresh homemade jar can have hundreds of millions of live bacteria per gram, especially in the first three months [25]. All you need is cabbage, salt, a jar, and time.
Make kefir your daily choice. Of the four, it has the most evidence from human studies and the least salt. Buy it plain and unsweetened, or make it at home with grains, and use it as you would use yogurt.
Read labels carefully, like a clinician would. For any fermented food, look for raw, unpasteurized, refrigerated, live cultures, and skip shelf-stable versions if you want live bacteria. For kombucha, ignore the health claims on the front and check the carbohydrate content on the back. Twelve grams of sugar in a health drink is still twelve grams of sugar.
Know who needs to be careful. Fermented vegetables are high in salt and biogenic amines such as histamine and tyramine. If you have high blood pressure, histamine intolerance, or take an older antidepressant called an MAO inhibitor, which can make tyramine dangerous, these foods can be a real problem instead of a health food [26]. For everyone else, a serving or two a day is enough; you do not need a whole jar.
My Mother’s Kitchen
I still eat sauerkraut most weeks without thinking about it, just as I have since I was a boy. That has not changed, and it will not.
Every time I visit my parents, I ask for the same dish: my mother’s sauerkraut, cooked slowly from her mother’s recipe, from the Bavarian farm where the cabbage fermented in a stone crock in the cellar. She always expects it and starts making it before I even ask.
For decades, I ate these foods as medicine, convinced of a benefit I never actually checked. When I finally did, my certainty did not last, but the food did.
Eat these foods because they are real, you enjoy them, and they might offer a small benefit. That has always been reason enough.
The rest was just a story I told myself, even though I am supposed to be someone who tests his stories. I let this one go unchecked for decades, in the very field where checking is my job. We all have a few beliefs like that. The real work is figuring out which ones they are.
Stay healthy.
Andre
PS: If you know someone certain their kombucha is doing something, forward this to them. Be kind about it. That is how this newsletter grows, and it is the only way I want it to.
References
[1] Metchnikoff, É. (1908). The prolongation of life: Optimistic studies (P. C. Mitchell, Ed. & Trans.). G. P. Putnam’s Sons. (Original work published 1907)
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Great article!
I am a little sad that sauerkraut does not necessarily have all the supposed benefits we all thought, but like you I will still continue eating it as it's just so delicious!