What a randomised trial found when it fed people fermented food for ten weeks, why the fibre in your food beats the bacteria in a capsule, and what to do if your gut is genuinely unhappy.
Kratos Natural · Domain: Gut
1. What is in your gut?
Something like 40 trillion bacteria, mostly in the colon, doing chemistry you cannot do: fermenting the fibre your enzymes cannot break, producing short-chain fatty acids, and interacting with the cells of your immune system across a mucus layer.
A stool sequencing test tells you which species were present in one sample on one day. There is no established "good" microbiome to aim at and no validated home test that says what to do about yours, so a report with a matching supplement is ahead of the science.
2. What does the research say?
Stanford randomised healthy adults to ten weeks of either a high-fibre diet or a high-fermented-food diet, and measured immune status in detail.
The fermented-food arm increased microbiota diversity and decreased markers of inflammation: 19 inflammatory proteins fell, including interleukin-6[1].
The high-fibre arm did not increase diversity over that period. It changed which functions the microbes were performing, and the immune response depended on where somebody started[1].
So fermented food, meaning yoghurt, kefir, sauerkraut and kimchi, has a randomised trial with immune outcomes behind it, which is rare here. And ten weeks is too short for fibre to move a diversity score. Fibre works through what the bacteria do, not through how many kinds there are.
3. What do your gut bacteria eat?
The fibre case is made in full in the fibre guide. The part that belongs here is the mechanism.
Starve the bacteria of fibre and they eat the mucus. In mice on a fibre-free diet, the microbes that normally ferment fibre switched to grazing on the colon's mucus barrier, thinning it and increasing susceptibility to a pathogen[2].
Different fibre structures produce different end products. Which short-chain fatty acids come out depends on which fibres go in[3]. That is the argument for eating many plants rather than one supplement.
And variety tracks with diversity. In the American Gut Project, people eating more than 30 different plants a week had more diverse gut communities than people eating 10 or fewer[4]. Self-reported and cross-sectional, so a target rather than a threshold. It is also the reason the nutrition tracker counts different plants.
4. Do probiotics work?
For a healthy person with no complaints: no good reason to take one, and no trial showing it does anything you would notice.
For irritable bowel syndrome the picture is different but still awkward. A systematic review and meta-analysis in Gastroenterology found some combinations and some single strains appeared beneficial for global IBS symptoms, but the trials were heterogeneous, many were at high risk of bias, and no specific product could be recommended with confidence[5].
So something in this class may help somebody with IBS, and no trial says which product.
5. What do you do?
Four things, and none of them is a supplement.
- Eat something fermented every day. Yoghurt, kefir, sauerkraut, kimchi. This is the one with a randomised trial and immune outcomes behind it[1].
- Count plants, not grams. Thirty different ones across a week, which includes herbs, nuts, seeds and whole grains, not thirty portions of vegetables[4].
- Keep the fibre coming. A gut with nothing to ferment starts on the mucus layer[2]. The fibre guide has the grams per portion.
- Skip the probiotic unless you have diagnosed IBS and a doctor suggested one. For everybody else there is no trial showing you would notice.
6. What do you do if your gut is unhappy?
For diagnosed IBS the best-evidenced dietary approach is not a probiotic. A network meta-analysis in Gut found a low FODMAP diet ranked first for global symptoms among dietary interventions[6].
Two things belong with that finding. It is a diagnostic elimination followed by systematic reintroduction, not a way to eat permanently. The restriction phase lowers fibre and starves the same bacteria this guide is about. And it should be done with a dietitian, because the reintroduction is the part people skip and the part that matters.
7. What we honestly do not know
What a good microbiome looks like. There is no reference range, which is why the hub does not print one[4].
Whether raising diversity improves anything by itself. Diversity is a marker that travels with health; making it go up has not been shown to be a lever.
Which probiotic, for whom, at what dose[5].
Whether the fermented-food result holds beyond ten weeks and beyond 36 people. It is a good trial and a small one[1].
8. When should you see a doctor?
Straight to a doctor if:
- Blood in the stool, or black stools
- Unexplained weight loss
- A change in bowel habit lasting more than three weeks
- Pain that wakes you at night
- Anaemia with no obvious cause
- Difficulty swallowing, or vomiting that keeps happening
And before starting an elimination diet, get the diagnosis first. Cutting foods out changes what tests can find afterwards, and coeliac disease in particular cannot be diagnosed properly once you have stopped eating gluten.
9. Sources
- Wastyk HC, Fragiadakis GK, Perelman D, et al. Gut-microbiota-targeted diets modulate human immune status. Cell, 2021. PMID 34256014
- Desai MS, Seekatz AM, Koropatkin NM, et al. A dietary fiber-deprived gut microbiota degrades the colonic mucus barrier and enhances pathogen susceptibility. Cell, 2016. PMID 27863247
- Deehan EC, Yang C, Perez-Muñoz ME, et al. Precision microbiome modulation with discrete dietary fiber structures directs short-chain fatty acid production. Cell Host & Microbe, 2020. PMID 32004499
- McDonald D, Hyde E, Debelius JW, et al. American Gut: an open platform for citizen science microbiome research. mSystems, 2018. PMID 29795809
- Goodoory VC, Khasawneh M, Black CJ, et al. Efficacy of probiotics in irritable bowel syndrome: systematic review and meta-analysis. Gastroenterology, 2023. PMID 37541528
- Black CJ, Staudacher HM, Ford AC. Efficacy of a low FODMAP diet in irritable bowel syndrome: systematic review and network meta-analysis. Gut, 2022. PMID 34376515
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