Do Live Microbes Drive the Health Benefits of Fermented Foods? The Answer Might Surprise You

 Key Takeaways 

• Fresh and pasteurized sauerkraut showed similar health effects. 
• Live microbes may not be essential for the health benefits of fermented foods. 
• Sauerkraut consumption was associated with slight reduction in systolic blood pressure. 
• No significant improvement in gut barrier function, inflammation, or blood sugar control were seen in healthy adults. 
• More research is needed to understand which fermentation-derived compounds are responsible for these health effects.

Today if you look at any grocery aisle, you'll see kombucha, kefir, kimchi and sauerkraut being touted as products that promote gut health and a balanced gut microbiome and contain live cultures. The rationale behind this billion-dollar wave of fermented food craze is straightforward that live bacteria are the key players and more of them means better health. However, what if this assumption is partially correct?

Fermented foods like sauerkraut and kimchi are popular for their live cultures and gut health benefits (Beatrice B / Pexels)

1. Why Scientists Keep Studying Fermented Foods: Fermented Foods, Inflammation, and Gut Health

Low-grade, chronic inflammation is one of the hallmarks of today's diseases. It has been linked to obesity, type 2 diabetes, cardiovascular disease and many other non-communicable diseases that are on the rise around the world. Many of these have been associated with the "Western diet", which consists of a diet rich in saturated fat, salt and refined carbohydrates, low in fiber and alters the gut microbiota and compromises the intestinal barrier. If that barrier is compromised, sometimes a person is described as having “leaky gut”, where bacteria or other components of the intestines enter the bloodstream such as lipopolysaccharide (LPS). LPS is a strong stimulator of inflammatory signaling and increased levels of circulating inflammation has been linked to insulin resistance and poor glucose control. This is a theoretical mechanism: damage to the gut leads to inflammation which leads to metabolic dysfunction.

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 In the course of this conversation, fermented foods came into play as they contain live and inactivated microorganisms many of which are considered to be live microorganisms that when administered in adequate amounts confer a health benefit on the host (the definition of the term probiotic). Sauerkraut is a fermented cabbage that's long been known to provide anti-inflammatory and gut-supportive benefits. But credit is not the same as proof. Over the years, interest has been strong but scientific evidence is still lacking and contradictory.

2. The Journey to the Breakthrough: From Probiotics to Postbiotic Research

This trial didn't come out of nowhere but it's really the next step in a story that's been building for a while. It starts with the probiotic idea itself which took hold in the early 2000s that live bacteria settle into your gut (or at least pass through it) and that's where the benefits come from. Since then, that belief has influenced the design of these studies as well as the marketing of these products.

Then researchers started poking holes in that assumption. When the foods were fermented, but only heated to the point of killing the majority of the bacteria, the benefits began to appear as well. That's peculiar, when bacteria are supposed to do everything. It spurred the notion of postbiotics which are bits of the bacteria's cell walls and the metabolic products it produced that could still be working after the bacteria are dead. An early hint that live cultures didn't pan out was from an earlier pilot study of people with irritable bowel syndrome, who found that both fresh and pasteurized sauerkraut helped to improve symptoms.

 In parallel, the number of clinical trials yielding inconclusive results, especially in the realm of inflammation, was increasing. At the same time, there was increasing evidence from clinical trials that were giving conflicting results, particularly with regard to inflammation. In a meta-analysis of 26 randomized controlled trials, a correlation between fermented foods and reduced levels of one inflammatory marker TNF-α was observed, but not for two other markers, CRP and IL-6, which had varying effects by age, weight, and overall health status. In short fermented foods fight inflammation turned out to be more of an it depends than a rule.

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Take those threads and you have an obvious next question that has not been properly tested that If you remove the live bacteria but keep everything else, do the benefits go away? This is what this sauerkraut trial was looking to discover.

3. The New Discovery: Fresh vs. Pasteurized Sauerkraut: Live Bacteria vs. Postbiotic Compounds

Rather than the typical fermented food versus some unfermented control, the researchers tested fresh sauerkraut containing live Lacticaseibacillus paracasei, versus pasteurized sauerkraut, heat-treated to ensure bacterial counts were undetectable.

Eighty-seven healthy adults ate 100 grams of one type daily for four weeks, took a four-week break from all fermented foods, then switched to the other type for another four weeks. Everyone had experienced both of these so each person served as their own control, reducing the variability of people just tasting different food items.

The study compared fresh sauerkraut, which retains live bacteria, to pasteurized sauerkraut, which does not.

 Along the way, researchers kept an eye on four things: 

• Leaky gut markers (zonulin, FABP-2, LBP): How intact the gut lining stayed 
• Inflammatory markers (IL-6, hsCRP, TNFR-2, sRAGE): How much systemic inflammation was present 
• Metabolic markers (glucose, insulin, fructosamine): How well blood sugar was being managed 
• Blood pressure (systolic and diastolic)

4. What They Found: Fermented Sauerkraut and Blood Pressure Results

Both forms of sauerkraut lowered systolic blood pressure by a slight but statistically significant decrease of 1.5 to 2.5 mmHg. What was remarkable was how the two effects were alike, despite the fact that one sauerkraut was teeming with the bacteria and the other was almost devoid of them. Beyond blood pressure, the results were underwhelming across the board:

Marker Category	Fresh Sauerkraut	Pasteurized Sauerkraut
Systolic blood pressure	Significant decrease	Significant decrease
Gut barrier integrity	No significant change	No significant change
Inflammatory markers (overall)	Trending lower (hsCRP, TNFR-2), not significant	Not significant
Glycemic markers (overall)	Modest, not significant	Slightly bigger drop, still not significant

It was found that age and fiber intake were more significant than was anticipated. The adults who were less than 50 years old had more positive results with blood pressure and blood sugar levels after consuming pasteurized sauerkraut and those who already consumed a high amount of fiber had better results with hsCRP. The influence of sex and BMI was smaller. But no matter how the data was broken down, gut barrier markers remained unchanged. 

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The healing gut lining effect which is commonly thought to be responsible for fermented food health benefits were not observed here.

Both fresh and pasteurized sauerkraut were linked to a modest reduction in systolic blood pressure.

The number of bacteria in the fresh sauerkraut used in this study was between 100,000 and 300,000 bacteria per ml which is considerably below 1 billion bacteria per serving, the level that has been cobbled together as the threshold for a probiotic effect. That gap alone might explain why the live version didn't outperform the dead one.

5. What This Means Across Fields: Implications for Nutrition, Food Industry, and Public Health

For clinical nutrition and dietetics: This is good news for anyone who can't safely eat unpasteurized foods and people who are immunocompromised, pregnant or in a hospital setting. Pasteurized sauerkraut doesn't have to be a substandard choice in terms of nutrition as it may retain the same blood pressure advantage as fresh sauerkraut. 

For the food industry: Live and active cultures have been the selling point for years. This research suggests a second, lesser-studied frontier the postbiotic compounds such as organic acids and peptides that may warrant the marketing of pasteurized, longer shelf-life fermented products as true functional food as well as a compromise. 

For public health and dietary advice: There's some benefit in a modest real blood pressure reduction, even if it's not spectacular. The fermented foods such as sauerkraut might be considered a low-risk food item, instead of a therapy in its own right, in conjunction with diet plans such as the DASH Plan. 

For biotechnology and future product design: If a certain peptide or organic acid is determined to be the actual active ingredient, the possibility of concentrating and isolating it arises which would result in a postbiotic supplement containing much more consistent dosage of the active ingredients than say "eat 100 grams of sauerkraut a day" would. 

In the field of agriculture and food science: The phenolic and bioactive peptide level in fermented cabbages changes according to the type and storage method of the cabbage. That's a real chance to breed or process cabbage just to increase the levels of these, rather than to improve its taste or shelf life.

6. What Does This Actually Mean?: Postbiotics and the Fermentation By-products Theory

Since fresh and pasteurized sauerkraut lowered blood pressure by roughly the same amount, something other than live bacteria has to be doing at least part of the work. Fermentation is responsible for the production of more than microbes (such as organic acids, antioxidants, and peptides), some of which are similar to the ACE-inhibitory compounds that are known to lower blood pressure in other fermented foods, such as lactose-fermented soybean.

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However, it is important to remember the scope of this. It's real and a 1.5-2.5 mmHg drop at the population level could have a meaningful impact on cardiovascular risk but this will not be the same for each and every individual and it will not replace medication to control blood pressure. The lack of movement of the gut barrier markers is a big blow to the buzz surrounding the leaky gut concept in popular food marketing. At least over the course of 4-weeks, if sauerkraut doesn't visibly help to strengthen the gut lining here, then those who claim to heal with diet alone should be treated with skepticism at least in healthy individuals.

This study should not be taken as proof of anything else. No one among the trial participants was obese, diabetic, or h ad inflammatory bowel disease, all of which have elevated levels of baseline inflammation and gut permeability, and where a nutrition regimen may have a meaningful effect. There was only one food tested during a relatively short period, so it is not safe to assume that this will apply to yogurt, kimchi, kefir, or any other food that is fermented.

7. Future Research: Open Questions on Fermented Foods and the Microbiome

There are some questions left unanswered: 

• Which compounds are actually responsible? It would be good to test individual organic acids, peptides or phenolics in sauerkraut individually to see own would help confirm what's really driving the blood pressure effect. 
• Would sicker populations respond more? However, there is less room for improvement among healthy participants. The same trial run in people with hypertension, obesity, or IBD might show a much bigger effect. Is more time used to make a difference? Four weeks is a short time for microbiome research more may be needed to see if there was any change in the integrity of the gut or inflammation. 
• Does anything else ferment the same way? The microbial and chemical composition of kimchi, yogurt and kefir differs, and it is not known if pasteurization is the cause of the loss of benefits in all three cases.

9. Conclusion: Rethinking the Live Cultures Claim

This trial complicates the usual fermented food story in a genuinely useful way. Fresh sauerkraut and pasteurized sauerkraut lowered blood pressure by about the same amount. That's not to say probiotics are not important, but it does mean that the fermentation process itself and whatever it produces is just as worthy of being taken seriously as the microbes it is producing. Clearly, fermented foods aren't a magic panacea and in healthy people as their positive effects seem small and targeted. However, the live cultures tag is likely overdue for an update as the actual benefits of these foods in controlled trials are not necessarily the same as the benefits implied by a label, a distinction worth knowing before choosing fermented foods for gut health.

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10. Limitations: Study Scope and Caveats

• The results are valid for healthy adults and may not be the same for individuals with metabolic, gut or inflammatory disorders. 
• Four weeks is a short amount of time, particularly for changes in gut barrier function or microbiota. 
• The results only apply to sauerkraut, and should not be taken as being representative of other fermented foods. 
• A few markers showed non-significant trends interesting to watch, but not confirmed effects. 
• Blinding wasn't possible, since fresh and pasteurized sauerkraut simply look and feel different.

Frequently Asked Questions

Do fermented foods need live bacteria to be healthy?

Not necessarily. In a randomized trial, fresh sauerkraut (with live bacteria) was shown to reduce blood pressure by the same amount as pasteurized sauerkraut (with no live bacteria), indicating that it is likely that compounds that remain after fermentation that are not bacteria help some of the benefits.

Does pasteurization destroy the health benefits of fermented foods?

Not entirely. Pasteurization eliminates live bacteria but in this study, there was no difference in the reduction of SBP between fresh and pasteurized sauerkraut, suggesting that some beneficial components might persist after pasteurization.

Can pasteurized sauerkraut still lower blood pressure?

Yes, according to this trial. There was a slight, but statistically significant reduction in systolic blood pressure with both fresh and pasteurized sauerkraut following four weeks, with no differences seen between the two.

What are postbiotics?

Even after the live microbes are removed or inactivated, the resulting post-biotics are bioactive compounds like organic acids, peptides and antioxidants. Others are like some of the drugs used to treat high blood pressure that are known as ACE inhibitors.

Is sauerkraut good for gut health?

The gut barrier function, inflammation and blood sugar level were not significantly improved in healthy adults after four weeks of sauerkraut supplementation in this trial, but earlier studies have associated this with anti-inflammatory and gut health benefits.

Are probiotics necessary for fermented foods to work?

This study indicates that probiotics are not all they're cracked up to be. The fermentation process does not seem to create beneficial compounds in the absence of live bacteria, but further research is required to determine specific compounds that are important.