This article was written by Sebastian Catana, nutritionist-dietitian with a passion for educating. Sebastian has a Bachelor’s degree in Nutrition and Dietetics from the University of Medicine and Pharmacy in Timisoara, Romania. Apart from his practice and individual coaching, Sebastian is employed by companies to hold nutrition, health and lifestyle-related seminars (sebastiancatana.ro). He is also a trainer in a certified course for a Nutrition Technician degree. But above all, his mission is to stop the wave of misinformation found in traditional and especially online media.
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I. What is evidence
First we need to clearly understand what evidence means. Here is a step by step explanation of what kind of science we need to look at to be able to draw a conclusion:
1. First let’s split all scientific studies into theoretical research and practical/empirical research.
Theoretical research is used mostly to formulate only for the purpose of developing and refining theories, metabolic pathways, biochemical processes, so basically theoretical and laboratory work. In vitro studies also belong to this group and are often used in the field of microbiology (including the study of fermented foods) when observing the activity of certain strins of bacteria and fungi that colonize the foods that are being fermented.
However, most often in vitro studies do not translate to real-life environments. So for example a study may discover that a 100gram gherkin contains 5 trillion live bacteria when fermented in the lab in an optimal controlled environment. But this result does not give us any information regarding the number of living bacteria reaching our intestines after having passed through a highly acidic stomach whose main purpose (apart from digesting protein) is to kill anything that moves.
This is why we need to assess the effects of any product/drug/food (in this case fermented vegetables) on live organisms. So we need practical research done with fermented foods and live organisms. Lastly, these organisms need to be human and not animal, as the distinction between the two in this case is quite large.
Why especially in this case? Because we are talking about the digestive tract which differs considerably between humans and mice/rats (in length, digestion speed, types and volumes of enzymes, acidity, and especially gut flora composition). So again, results from experimental studies done on animals would not be translatable to humans.
2. Secondly research split: observational and interventional/experimental.
These two main categories need to coexist. All types of studies within these two categories have a purpose. But in what reliability of results is concerned, the top 3 most relevant studies are (descending order):
- any experimental study (especially randomized control double-blind crossover trials – but these are very rare due to logistical and financial barriers);
- Cohort studies (very large and lengthy observational studies);
- Case-control studies (are prone to some confounding but if done right, they might actually be very relevant);
So we need to look out for those. But no matter what, observational studies cannot prove causation. Observational studies can only find correlations (which surely can also be very relevant in some cases). So if we are looking at the effect of fermented vegetables on colon cancer, an observational study can only say there is or isn’t a correlation between the two. A correlation is usually not worth much (often due to confounding factors, study design flaws, biases, reverse causality, and other issues), and can at best lead the way for experimental studies to prove a hypothesis.
Interventional studies are the only ones that can identify causation (Eg: Fermented vegetables have a positive effect on immune system activity). These are the ones where you give people medicine or food and watch them react to it. When looking for cholesterol changes or similar blood markers it may take a few days, but if you’re looking at the effects on cancer, you need years or even decades of follow-up.
In conclusion, the interventional study called a randomized controlled double blind crossover trial (RCDBCT) is the flagship experimental study when it comes to reliability of outcome. But in the field of nutrition it’s impossible to make double-blind studies (because the participants will know what they’re eating) so nothing short of RCTs should be enough to draw a conclusion for any kind of nutritional intervention.
That being said, there are very few interventional/experimental studies done on fermented vegetables (let alone RCTs). There are many done on fermented dairy though.
3. Meta analyses and reviews.
Meta analyses (MAs) are powerful tools – with meta analyses of randomized control trials being the most powerful and relevant tool. In the eyes of most top-ranked researchers, the only tool we should be using to make decisions and draw conclusions about health guidelines. MAs take all data from all studies done up to that point, and create a new conclusion. For example, let’s say there are 5 interventional studies with data showing a decreased LDL cholesterol after ingesting sauerkraut for a week, 5 other showing no effect on LDL, and 5 more showing an increase in LDL cholesterol. These studies will differ in the number of participants, the values obtained and other parameters. What meta-analyses do is they create a mathematically accurate summary generated from all results of those 15 studies compiled into a single result. Pretty neat and useful. Especially once studies start to clutter up after decades of research.
Unfortunately, as you would expect if you’ve read the entire article up to this point, there are very few meta analyses and reviews referring to fermented vegetables.
4. Further distinctions: fermented foods vs probiotic supplements vs fermented vegetables exclusively
Ok. So we’ve done away with lab studies, theoretical research and most observational studies. We are left with very few interventional studies and even fewer relevant observational studies.
Now what? Well, we need to filter out the studies done on probiotic supplements and focus on the ones that study fermented foods. This distinction is often overlooked. The two are not the same at all. In fact they differ in every respect: quantity of live bacteria, variability of bacterial strains, viability (are they alive or dead), colonization strength (can they really colonize the gut wall), actual strain itself (probiotics only use laboratory-made bacterial strains, not natural ones found in fermented foods), ingestion method (supplements are mostly taken without food whereas fermented foods are foods themselves). So all studies on probiotics should be excluded.
No, we’re not done yet. One more step to go: we need to seek out the studies that look at the effect of fermented vegetables, and not of fermented foods lumped together. Different fermented food groups will have different effects (cereal, fruits, vegetables, dairy, meat and fish).
Also we need to exclude studies done on supplements based on secondary constituents or metabolites of probiotics (eg. vitamin K and short chain fatty acids like butyrate). There is absolutely no reason to believe that the effect found from taking these supplements are transferable to the consumption of fermented vegetables.
II. The Media Mirage effect
There is a plethora of newspaper articles and other specialty website articles, videos and documentaries praising the benefits of fermented vegetables. Unfortunately, a lot of them use very weak data to make a point. If you need a reason it’s to sell a story basically and to keep up with the trends. If the New York Times wouldn’t have at least an article about probiotics and the ‘amazing world of bacteria living in your gut‘ they wouldn’t be considered up to date with the current trends.
Yes, microbiology has gotten a lot more interesting since the complexity of the microbiome got a better look. Yes, there are very many living bacteria in your gut. Yes, there are some benefits from eating fermented foods. What benefits? How big are these benefits? Are there also drawbacks? How much do I have to eat to get those benefits? Are the results replicable and reliable? From my perspective, these are questions we can’t answer yet.
The media have attractive headlines, beware. They throw them around like they just don’t care. A little rhyme for you there. But it’s true. It’s as if they were trying to sell a story. Oh wait, they are.
What journalists do is they often use weak studies in the absence of high quality studies like RCTs or meta analyses. They also lump all fermented foods in one category (dairy, vegetables, fruits, cereal, meat and even fish) and very often they include probiotic supplements in the mix as well. They’re basically doing everything we need to avoid when forming opinions and this is very frustrating for many scientists and researchers out there. Media is like the shiny pink hat and golden $ necklace an introverted scientist is forced to wear while being exposed to crowds. Not the best fit (with some exceptions).
So let’s look at some examples:
Example A.
Everything you always wanted to know about fermented foods by Sciencebasedmedicine.org
Skipping the intro and the part about dairy, we get to the paragraph entitled “Fermented vegetables”. Here they state that:
“Lactic fermentation has been shown to […] reduce phytate content, a well-known inhibitor of iron and zinc absorption.”
Main issue here: the vegetables that are usually fermented, and vegetables in general do not have phytates (with the exception of some legumes), only cereals and nuts/seed do. Which is why they use a study done on fermented oats (link) to justify the benefits of fermented vegetables.
Further on:
“It has also been shown that fermentation of maize enhances the bioavailability of iron”.
Cereals? Again? So no benefits for fermented vegetables?
A 2015 study published in the European Journal of Nutrition found that the reason for the increased bioavailability of iron in lactic-fermented vegetables compared to fresh vegetables is due to an increase in the concentration of hydrated ferric iron (Fe3+) which may be more favourable for iron absorption.
Aaah finally, right? But when reading the actual paper, we discover it studies “In vitro digested fermented vegetables” and the effect on iron and zinc bioavailability. So basically it is the type of study with the lowest degree of reliability or even relevance for human health. By the way… in the conclusion “No effect on zinc bioavailability was observed“. Only on iron. And the effect isn’t new at all. Gastric acid or vitamin C will do the same thing (convert Fe3+ to Fe2+ which is the only absorbable form of iron).
Example B.
Try Fermented Vegetables To Boost Beneficial Gut Bacteria by The Huffington Post (Contributor Platform)
They report:
Studies demonstrate that eating foods that contain probiotics have benefits for both our bodies and our brains. These industrious bacteria can treat diarrhea and ulcerative colitis, boost the immune system and may even help prevent breast cancer. Probiotics are also effective in treating mental illness. A multitude of studies prove the effectiveness in probiotics in treating anxiety and managing depression.
Let’s look closer at the studies they cite as sources:
treating diarrhea – Based on probiotics, not fermented vegetables, and the study is weak and the conclusion is “effect unclear – more studies needed”
ulcerative colitis – No mention at all of fermented vegetables or even probiotics
immune system – Citation from the study abstract: This review is presented to provide detailed information about how probiotics stimulate our immune system. Not foods, just probiotic supplements. Conclusion: Probiotics that can be delivered via fermented milk or yogurt could improve the gut mucosal immune system (so not fermented vegetables)
breast cancer – citation from study: “There are not enough human trials where the application of probiotics as biotherapeutics against breast cancer was tested. Assays in humans are very important before the medical community can accept the addition of probiotic or fermented milks containing lactic acid bacteria as supplements for cancer patients.”
Furthermore, in the study probiotics are far from being in the spotlight. It’s mostly about flavonoids, essential fatty acids, micronutrients and other nutritional interventions. There are dozens of studies listed in the reference section, NONE of which are about fermented vegetables, and only 3 mention probiotics or yogurt as dietary intervention.
anxiety – this is actually an article from the Medical News website that talks about a study on mice being fed a specific lab-made strain of bacteria.
III. CONCLUSION
The main conclusion of my review (so far) is that there is far from sufficient high quality scientific evidence for most health claims related to fermented vegetable consumption. But it does look promising if we look at the amount of lower quality studies – in vitro experiments, animal models, (weak) observational studies.
There is a lot of bias, a lot of media hype and numerous study-design and interpretation flaws in this field. Also, there is much more scientific work done on fermented dairy products than on vegetables. Why is that?
Science needs money. Even your basic and small RCT can cost a few million dollars. So you need some incentive to invest in that direction. Now let’s think about the food industry for a second. Try to name 3 yogurt brands off the top of your head. Got them? Got more? Good. Now think of one kimchi or sauerkraut or fermented gherkins brand. Not that easy, right? So you see how easy it is to spot where the money is?
Why has yogurt spread far better than any other fermented food around the globe? Well, we can only assume… Could be the fact that more people find fermented dairy taste better. Could be the fact that large companies dealing in milk and beef could very simply integrate yogurt in their portfolio. Perhaps the fact that it’s easy to sell to kids (especially if you add some sugar and taste enhancers in there). Or it could just be a matter of food culture. Who knows?
One thing is certain though. There should be more research done on fermented vegetables. The potential benefits from the very large quantities of live bacteria in properly fermented vegetables like sauerkraut (theoretically much larger per volume of food than in dairy) might bring considerable advantages to human health, especially since we now know that we are walking bacterial houses.