Fermented foods are a hot food trend right now. But it has been an overnight success thousands of years in the making. In this first part of an ongoing series theme on all things gut health, gut bacteria and the gut microbiome, I’ll explore the world of fermented foods, describe the popular ones and most importantly: look at what science has to say about their health benefits.
Fermented foods hold a firm place in cuisine from almost every culture in the world. Beer and wine are classic examples of fermented foods where yeast convert sugars to alcohol. Other types of fermented foods use bacteria, such as Lactobacillus, to make foods like yoghurt, sauerkraut, kimchi and many others. In the West, there has been a surge in popularity of fermented foods in recent years. The major reasons for this exploding interest are the potential health benefits of eating fermented foods.
Any mention of fermented foods quickly brings up the topic of gut health and with that gut bacteria. You may also hear words such as gut microbiota and gut microbiome thrown in the mix too. So just to define a few things to ease any confusion. Your gut microbiota is a complex system made up of your resident bacteria, viruses and other microbes that have colonised your gastrointestinal tract. You also may sometimes hear the term gut microbiome used. The microbiome is all of the genes inside the microbial cells which create the microbiota. The two terms or microbiota and microbiome are often used to mean the same thing, so I’ll stick with the term microbiota from now on.
The microbiota creates its own mini ecosystem in the same way that plants, animals and insects live together in their own delicate ecosystem in a rainforest. We have a skin microbiota, mouth microbiota and in females, a vaginal microbiota. Then there is the one that receives the most attention: the gut microbiota. The entire microbiota in our gastrointestinal tract weighs between 0.5 to 2 kilograms. That’s a lot of bacteria.
One-third of our gut microbiota is common between most people, while two-thirds are specific to just you. This makes our gut microbiota as unique as our fingerprints.
The gut microbiota is not a static thing. It changes throughout life after first colonising the gut shortly after birth and continuing to gather new members from the environment throughout life. Variation is highest during childhood, and it gradually decreases with age. Illness, antibiotic use, fever, stress, injury and dietary changes all affect the blend of microbes that make up the microbiome.
Probiotics and health
With that out of the way, let’s turn to the topic of the day and that’s fermented foods. And when we’re talking fermented foods and gut health, then it’s all about probiotics. A probiotic is any live micro-organism which when consumed in adequate amounts, offers some form of a health benefit. The clinical evidence for probiotics places treatment of diarrhoea (especially that caused by antibiotics) at the top of the list. A potential benefit in treating irritable bowel syndrome looks likely too. After this, inflammatory bowel disease, prevention of certain infant allergies such as atopic dermatitis, and an overall general protection against infection deserves considering, but the science is still evolving.
Any mention of fermented probiotic foods immediately brings something such as yoghurt to mind. But there are many other probiotic foods now being re-discovered.
Some types of yogurt contain live bacteria that were either a part of the starter culture or added after pasteurisation. Now, pasteurisation does kill the beneficial bacteria in yoghurt so that should mean the yoghurt is no longer considered a probiotic right? But just to show how complex the entire area of the gut microbiota is, there is now some emerging research to show that even pasteurised bacteria can have an effect in the gut. While counter-intuitive, ‘dead’ bacteria can have beneficial effects because they can act as a food source for fermentation by other bacteria so that makes them technically a prebiotic. Much more about prebiotics in an upcoming episode. And these ‘dead’ bacteria have also been shown to exert other physiologic effects that science is only now starting to understand. It’s crazy stuff, but….science.
Some types of probiotics found in yoghurt, such as species of Bifidobacteria and Lactobacillus, have been shown to help with IBS with a few studies pointing in a positive direction, but there are studies that don’t show a benefit either so the results are pretty mixed at this stage. IBS has many different causes so it is unlikely that a probiotic boost from yoghurt will help everyone.
Today’s podcast though is not just all about yoghurt. There are a whole host of traditional fermented foods from cultures all around the world. Two well-known fermented foods with strong cultural ties are the German staple of sauerkraut and the traditional Korean dish of kimchi. Both these cabbage-based dishes are made by lactic acid bacteria fermentation. The main differences between them is sauerkraut is cut much finer and has no other ingredients apart from brine, while kimchi is cut into larger pieces and served with a variety of condiments such as chilli, garlic, pepper and fish sauce.
So how healthy is sauerkraut and kimchi? Cabbage itself is naturally high in fibre and contains compounds called isothiocyanates, which have cancer-fighting properties. And as long as you choose unpasteurised sauerkraut, you will gain a potential probiotic benefit.
For sauerkraut, it is one of the few fermented foods for which there is a clinical trial in a common bowel disorder such as IBS. A randomised double-blind trial compared the effects of sauerkraut containing viable lactic acid bacteria on gastrointestinal symptoms in 58 people with IBS. Each person ate either 75 g/day of either unpasteurised or pasteurised sauerkraut which served as the control group. The people were not told what type of sauerkraut that had been given. Both groups of people saw a significant reduction in IBS symptoms and there was no difference between groups. So at least for IBS, it shows that the perceived health benefit of sauerkraut may not be from a probiotic effect. This seems to agree with the mixed results that I spoke about for probiotic yoghurt before that IBS is a difficult condition to try and help with just one dietary approach.
For kimchi, several studies have found that it may be a potent food in lowering cholesterol and controlling blood glucose. A 2013 study involving 100 volunteers who followed a low or high kimchi diet for 7 days in a controlled housing dormitory found effects of kimchi on fasting blood glucose, total cholesterol and LDL-cholesterol. While this was a one-off study, the results look interesting.
I’ll put details of both the sauerkraut and kimchi study in the show notes. https://dx.doi.org/10.1039/C8FO00968F and https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3598433/
Kefir
Kefir is another interesting probiotic food as on the surface appears similar to yoghurt. Kefir can be made from the milk of any ruminant animal, but is fermented with a different variety of bacteria and uses a starter culture that also contains yeast. Traditional kefir, which originates from the Caucasus Mountains, is a fermented milk drink with a creamy texture, sour taste and subtle effervescence. It is produced by adding a starter culture termed ‘kefir grains’ to milk. Kefir grains contain symbiotic lactose-fermenting yeasts and non-lactose fermenting yeasts.
A 2013 review on the health benefits of regular kefir consumption found good evidence for its antimicrobial activity, improved gut health, anti-carcinogenic activity, control of blood glucose and cholesterol, improved lactose digestion and a stronger immune system. Quite an impressive list indeed. Grab the study details in the show notes. https://www.tandfonline.com/doi/abs/10.1080/10408398.2010.540360
Natto
Another popular non-dairy probiotic food is the Japanese staple of natto, which forms the base of miso soup and many other foods. It is made by the fermentation of soybeans with the bacterium Bacillus subtillis. Natto offers some of the health benefits similar to soy foods, but with additional value coming from its probiotic properties.
To date, there is limited evidence from RCTs suggesting natto might positively influence bowel frequency in people with infrequent bowel motions while it may also influence the gut microbiota. One small study in 8 healthy volunteers consuming miso soup for 2 weeks saw a favourable change in some bacteria such as more Bifidobacterium and less of potentially more harmful Clostridium. However, these require confirmation in high quality trials
Sourdough
Then we have sourdough bread. The sourdough starter culture is produced through the fermentation of flour by lactic acid bacteria and yeasts, that originate from the flour and surrounding environment. Once the starter is ready, a small portion is added to the sourdough base ingredients to initiate the sourdough fermentation process
Several studies have looked at the impact of sourdough bread on gastrointestinal function and disorders. In one RCT, 17 healthy adults were randomised to consume a single meal of two sourdough croissants or two croissants made with brewer’s yeast. Abdominal discomfort, bloating and nausea were significantly milder in those people eating the sourdough croissants suggesting they are better tolerated than brewer’s yeast croissants. Details of the study in the show notes. https://jn.nutrition.org/article/S0022-3166(22)10981-8/fulltext
Kombucha
And finally, we have….Kombucha the emerging ‘star’ on the fermented food scene if you go by food trends. With origins in China, kombucha is made from a sweet tea base that has been fermented with a symbiotic colony of bacteria and yeast. Traditional kombucha is produced through aerobic fermentation of black tea (although green tea may also be used) plus sugar and a combination of bacteria and yeast, known as the symbiotic culture of bacteria and yeast or SCOBY for short. Popular in the health food, juice and detox scene, it is sometimes called ‘mushroom tea’ – getting its name from the brown slimy crust that forms on the surface of the beverage. Delicious I know.
Although kombucha is a rich source of acetic acid and lactic acid bacteria and yeasts, there are no published studies exploring the effect of kombucha on the gastrointestinal microbiota composition or function in humans. That’s right – not a single controlled trial in humans to support any of the health benefits claimed by promoters of kombucha. https://www.sciencedirect.com/science/article/pii/S1047279718307385
Summary
While the long list of health claims from eating fermented looks impressive, the evidence for some of them is certainly trailing. People have been eating fermented foods for thousands of years and they certainly have a role to play in any diet. Some of the benefits of fermented foods may just come from the healthy food it is made from, rather than any probiotic effect. So fermented foods are not a silver health bullet, but when added to a healthy diet, have the potential to make it even healthier.
Research Wrap Up
Doing a quick research wrap for a study that has grabbed my attention during the week, I’ll keep the theme of the gut microbiota going. For this, a new study out of Oxford University finds evidence of a link between a person’s personality traits and their gut microbiota composition.
We know that the gut microbiota has a measurable impact on the brain, influencing stress, anxiety, depressive symptoms and social behaviour. This all comes about from the two-way communication between the gut and the brain called the microbiome–gut–brain axis. Much of the work looking at the link between the gut and brain has used animal models or for humans at least, in people with psychiatric conditions. Very little is known how this complex system and variation in the composition and types of bacteria in the gut could be related to personality and behaviour in the general population.
In this new study, which you’ll find in the show notes, a group of over 600 otherwise healthy adults took part in a study looking at gut microbial diversity and personality traits. Faecal samples were provided by each volunteer to assess the microbiome composition. Each person also undertook a battery of online psychological questionnaires. Information on diet and lifestyle habits was also collected. https://www.sciencedirect.com/science/article/pii/S2452231719300181
After using statistical adjustment to allow for lifestyle differences between people, the abundances of specific bacterial type were found to significantly predict certain personality traits.
People with larger social networks tended to also have a more diverse bacterial make up, suggesting that social interactions may shape the microbial community of the human gut in some way. It could even argue for direct human-to-human transmission of bacteria. And interestingly, the levels of certain bacteria found to be higher in people with high levels of sociability were the same types of bacteria that are known to be quite low in people with autism.
In contrast, people who experienced a high level of anxiety and stress were more likely to have a reduced microbial diversity.
Greater microbial diversity was also linked with diets high in fermented and prebiotic food. People taking probiotics as supplements were surprisingly more likely to have a decreased microbiome diversity. This last finding could be from the observation that people with pre-existing gut health problems are more likely to take probiotic supplements.
The study is one of just associations at this stage, and the research field is a long way off concluding that gut bacteria can directly alter a person’s personality traits. But it is very conceivable that the relationship is two way with a person’s personality and how they live their life also altering the gut microbiome.
Even if the link is confirmed, it would never be as simple as a person being able to blame a specific personality trait such as introversion on the presence of a single bacterial species. And likewise, a drink of fermented kombucha would likely not turn an introvert into an extrovert.
Question
And finally, I’d like to finish off with a question that came through from one of my listeners - Elise. She said that she heard that dairy products cause calcium to be leached from bones and so it actually depletes calcium in the bones while countries with the highest rate of osteoporosis have the highest rates of milk consumption. Is it game over for dairy? Not at all.
Such comments if you come across them are very often used to justify a vegan diet. Firstly, if you want to go vegan, fantastic. It can be a very healthy way of eating. And if you’re doing it for ethical or other reasons, that’s completely your call - this is a nutrition science podcast, I’m not ever going to preach to anyone on what ethics they should follow when it comes to food choices.
But when it comes to calling on nutrition science research to support a diet philosophy, then you best bring your A game and give a fair portrayal of the research, not just cherry-pick research that supports your agenda. I see the pro-vegan agenda used as a platform to call out any research that is funded by the dairy industry, but fail to acknowledge their own biases in only providing a very selective view of the research.
So on to the full answer to Elise’s question. Firstly, dairy most definitely does NOT leech calcium from bones - the calcium from dairy is absorbed very well and bioavailable studies show this. Yes, at a country level there is some link between countries that have higher intakes of milk having higher rates of osteoporosis, but this is some of the lowest quality evidence to do research as it cannot make allowances for all the other factors that are important in bone health – exercise is a big one. Also, it could be that dairy is mostly drunk by people who have a low risk of osteoporosis and not much milk is consumed by people who have a high risk – these ecologic studies cannot tell you if that is the case.
When you look at higher levels of evidence at the level of what individuals are doing, dairy consumption is beneficial for bone health. Looking at the most recent systematic reviews and meta-analysis – these are the best types of studies - there is a clear favourable benefit of dairy consumption on bone health. A 2018 review published in Nutrition Research Reviews found an increase in bone mineral density and a reduction in fracture risk with increasing dairy consumption. An even more recent review and meta-analyses published in Advances in Nutrition in March 2019 found that a healthy diet that included milk and dairy was linked to less chance of having low bone mineral density or of having a fracture. As always, grab these studies in the show notes. https://www.cambridge.org/core/journals/nutrition-research-reviews/article/dairy-products-and-bone-health-how-strong-is-the-scientific-evidence/3387593447B20D7AA60E722574E18D66
https://www.ncbi.nlm.nih.gov/pubmed/30657847
So, some of those studies showing countries with high milk intake having higher rates of osteoporosis are more-than-likely just measuring Western affluence of which low physical activity goes along with that.
If dairy is not your thing, then there is no need to be concerned. Many people, for a variety of reasons, cannot or do not drink milk. Some cultures do not use milk in their cuisines; some vegetarians exclude milk as well as meat, and some people are allergic to milk protein or are lactose intolerant. Others simply do not like the taste of milk.
Some brands of tofu that have been set with calcium, nuts such as almonds and some seeds (such as sesame seeds and tahini) can supply calcium for the person who doesn’t consume dairy. Canned fish with bones can also be a great source of calcium.
Calcium-fortified soymilk is also an excellent substitute for cow’s milk in terms of providing an equivalent amount of calcium. Calcium can also be found in good amounts in plant-based foods such as bok choy, kale, parsley, broccoli and watercress. Then there are supplements that contain both calcium and vitamin D which is a better choice than taking calcium alone.
So in the end, you don't have to have dairy to be healthy yet you can say the same thing about any food - no one food is essential. If you choose to include dairy foods in your diet, then there is nothing to be concerned about that these foods will somehow be bad for your bones. Dairy foods can be a part of any healthy diet as they provide many valuable nutrients that keep bones in good health.
So that’s it for today’s show. You can find the show notes either in the app you’re listening to this podcast on if it supports it, or else head over to my webpage www.thinkingnutrition.com.au and click on the podcast section to find this episode to read the show notes.
If you find this podcast of value, then please consider sharing it with your friends and colleagues. Or maybe even leave a review. This all helps increase the ranking and reach of the podcast which means a big win for credible evidence-based nutrition messages while helping to dilute out the crazy and making the world a slightly less confusing place.
I’m Tim Crowe and you’ve been listening to Thinking Nutrition.