It's all over the news for therapeutic or weight-loss reasons, the question of intestinal microbiota has now reached the ears of tea sellers. What does tea have to do with microbiota? How does it affect it? What are the real effects on people VS those observed in the laboratory?
The intestinal microbiota: between simplicity and complexity.
The intestinal microbiota refers to all the micro-organisms present in the colon and small intestine (i.e. bacteria, yeasts, fungi, etc.). Its composition is unique to each individual, and its diversity varies according to our genetics and lifestyle.
The microbiota is at the end of the digestion chain, fermenting non-digestible elements and helping to "cut" molecules such as complex sugars, fatty acids and fibers. These biochemical reactions form molecules called metabolites, which contribute to the functioning of our metabolism (simple sugars, contribution to the formation of vitamins).
The majority of the intestinal microbiota is made up of several large groups of bacteria which will produce the positive effects.The proportion of each group is very precise, with ratios between certain groups (e.g. Firmicutes/Bacteroidetes).In certain pathologies, such as obesity or intestinal cancer, an increase or decrease in ratios can be observed, as the groups are no longer in equilibrium.Similarly, pathogenic microorganisms can affect group ratios, particularly in cases of chronic local inflammation, suggesting that the gut microbiota directly influences local immunity.When the intestinal microbiota is out of balance, we speak of dysbiosis. Dysbiosis occurs when there are too many pathogenic micro-organisms, when there are fewer beneficial populations, or when the ratio between certain beneficial populations is no longer adequate.
Tea's potential place in the intestinal microbiota
The polyphenols contained in tea that reach the intestines are poorly absorbed (around 10%), but are transformed by the intestinal microbiota into metabolites (phenolic compounds, release of complex sugars, etc.) that the microbiota can use.Among the most widely studied polyphenols are the catechins in green tea, the tannins in black tea (thearubigin and theaflavin) and fermented/dark tea itself for its micro-organism composition.Rare studies have also found that L-theanine can be utilized by the intestinal microbiota, but L-theanine is produced in only a small number of teas.Among the most interesting metabolites produced by the microbiota's use of polyphenols are small-chain free fatty acids (SCFAs).These SCFAs possess anti-inflammatory properties and modify the functioning of microorganisms in favor of beneficial populations.
Studies are therefore focusing on tea as a prebiotic, i.e. a set of nutrients (often fibers) and/or metabolites produced by microorganisms that are administered to boost the "good" microbiota.
A diagram summarizing the effects of the conversion of polyphenols into metabolites by the intestinal microbiota https://www.mdpi.com/2076-3921/11/6/1212
The laboratory bench informs ...
Experimental trials aim to determine the main effects of different tea families on the growth of micro-organisms, their mortality and their ability to produce the right metabolites.The polyphenol concentrations used are highly variable, ranging from levels close to the infused leaf (i.e. several hundred µg/infusion) to levels 1000 times higher.
It has been shown that the main beneficial bacteria exposed to polyphenols (Akkermansia, bifidobacteria, Bacteroidetes, Christensenellaceae, Firmicutes or Lacto) produce phenolic compounds and SCFA (particularly in the case of Akkermansia). In the case of beneficial bacteria, their growth tends to be enhanced, unlike pathogenic bacteria (Chlostridium, Bacillus cereus, Helicobacter pilori, Legionella penumophila, Mycrobacterium spp, Escherichia Coli) whose growth tends to be slowed down.However, studies have shown the opposite effect, with a slowdown in Firmicutes and growth in certain Chlostridium pathogenic strains. The hypotheses put forward are that the slowdown in growth mainly concerns "Gram-positive" bacteria, and therefore depends on the composition of the bacterial membranes. The dose effect has also been put forward in the case of certain strains, notably Firmicutes, where increasing doses would reverse their growth. The main limitation of these studies is that it is not possible to cultivate all the micro-organisms in the intestinal microbiota (mainly bacteria and yeasts), and some groups of bacteria are not viable.
Laboratory studies on animals, most often mice (with genetics conducive to overweight, development of inflammatory bowel diseases, diabetes, obesity and colorectal cancer), aim to determine the beneficial effects of polyphenols on sick animals. The various authors quantify the evolution of symptoms (glycemia, weight loss, tumor growth, etc.), populations in the intestinal microbiota and blood levels of metabolites. In the majority of studies compiled, polyphenols are present in the form of extracts of green, dark/fermented or black teas, or as artificially synthesized molecules.
Overall, the effects detected were as follows :
- A tendency to lose weight in obese and predisposed mice
- lower blood sugar levels in diabetic mice (one study even found blood sugar and circulating fatty acid levels close to those obtained with an anti-diabetic agent)- Improved intestinal symptoms thanks to reduced inflammation in mice with severe inflammatory digestive disorders
- Little effect on tumor growth, but reduced local inflammation
- The presence of SCFA in diabetic, inflammatory and overweight mice.
In each of the pathologies, the studies revealed an increase in the diversity of positive bacterial populations, and an overall decrease in many pathogen groups (particularly Chlostridium and Escherichia Coli) in the case of pathogen-detecting pathologies. On the other hand, variations in bacterial ratios and the over-representation of certain bacterial populations differ greatly depending on the pathology. For example, obese and diabetic mice do not show the same variations in microbiota, even though there is a beneficial effect.
Other studies have also shown that the chemical reactions and molecular signaling in the gut bacteria and cells generated can differ according to pathology and degree of inflammation, so it's very difficult to standardize "a microbiota map" by pathology.
You can see why it's difficult to get a standard, comprehensive overview of variations in gut microbiota ... and that's just pathology here! https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8271705/
A special case in microbiota studies is fermented tea. While most studies are only interested in the total polyphenols in these teas, a few studies have looked specifically at the influence of micro-organisms such as the Eurotium Cristatum fungus (the yellow residue) that grows on fermented tea cakes. At present, there is no clinical follow-up on this subject, but the few in vitro studies and studies on obese mice indicate a reduction in several beneficial bacterial populations, which would allow the recovery of a healthy ratio after 12 weeks in the presence of the isolated fungus. This effect would result in improved lipid metabolism and blood glucose tolerance. The use of extracts of liu Bao, a type of fermented tea, would show similar results, as well as a local anti-inflammatory effect regulating the overactivation of the intestinal immune system.
... The clinic makes you crazy !
There are now several clinical studies on the impact of tea on gut microbiota composition, and on the improvement of certain symptoms.However, there are several limitations to existing clinical follow-ups:
- The number of people included is highly variable, ranging from barely a dozen to rarely a hundred or so.
- The state of health of the people involved is highly variable (age and BMI).
- Tea was delivered mainly in the form of tea extracts or artificial molecules, often in doses 2 to 5 times higher than the average dose for moderate tea consumption (2 to 3 cups a day).
- Follow-up times vary widely, from 24 hours to 1 year.
The most marked and significant effects were observed on consumption lasting at least a few weeks, above 400mg (equivalent to more than 2-3 cups of tea). The bacterial populations that tended to be most affected were Bifidobacterium (increase) and the Firmicutes/Bacteroidetes ratio (increase).
On the other hand, there is no clinical follow-up to date that correlates the effects of tea and impacts on the microbiota ... with an improvement in pathological symptoms, unlike animal studies!
Only one study has shown an increase in the level of IgA antibodies secreted in the mucous membranes of healthy adult tea drinkers with positive variations in intestinal microbiota. Although the authors speak of a potential better resistance to aerodigestive infections, this result remains speculative since they chose healthy patients showing no signs of infection or inflammation for this study.
Studies of tea as a prebiotic to improve the quality of the intestinal microbiota are fascinating and would seem to support a preibiotic effect of tea on the intestinal microbiota.
Experimental results on bacterial colonies and a growing number of studies in mice offer valuable insights into potential mechanisms of action. However, these studies suffer from severe limitations that are difficult to overcome. Although the prebiotic effects of tea polyphenols and even fermented tea fungi have been observed experimentally (balancing beneficial populations, blocking pathogen growth, multiple anti-inflammatory/glycemic/hypolipid and cholesterogenic effects), they are difficult to standardize. There are wide variations from one pathology to another, and beneficial effects are obtained for different bacterial ratios and proportions, with doses often exceeding moderate tea consumption.
At present, follow-up studies in humans are still very recent (mostly less than 5 years old). We know that tea can modulate the intestinal microbiota, but at doses 2 to 5 times higher than recommended tea consumption.The benefits would take at least several weeks to show up, although we don't know whether this reduces the symptoms of pathologies.
The effects reported in the media are therefore based solely on experimental laboratory data, and not on clinical trials! We'll need a lot more follow-up before we have the beginnings of an answer!
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