The gut microbiome: what is actually established

The gut microbiome is a real and active field, and the commercial claims have run far ahead of it. What is reasonably established:

Diet shapes the microbiome, and it does so within days. Fibre and fermented foods both change composition and function. In a controlled feeding trial, a diet high in fermented foods increased microbiome diversity and decreased markers of inflammation over ten weeks, while a high-fibre diet increased fibre-degrading capacity without the same diversity change. Two different mechanisms, and both are diet-level, not supplement-level.

Probiotics work in specific, narrow indications. The best evidence is for antibiotic-associated diarrhoea and for some infectious diarrhoea. For IBS the pooled effect is modest. For general "immune support" or "gut health" the evidence is weak and the products are unstandardised. Strain matters: a result for Saccharomyces boulardii does not transfer to "a probiotic".

Prebiotics are fermentable substrates, and they are also the FODMAPs. Inulin, oligofructose and galacto-oligosaccharides feed gut bacteria. In a person with IBS, the same molecules are the ones that cause bloating, which is why a prebiotic can improve one person's gut and ruin another's week. That is not a contradiction; it is the same pharmacology with two different hosts.

What is not established. Commercial microbiome testing is not validated for clinical decisions. "Leaky gut" is used to sell supplements; increased intestinal permeability is a measurable phenomenon in specific diseases, not a general diagnosis to treat with a powder. Specific bacterial "imbalances" being the cause of fatigue, weight gain or mood problems is a hypothesis being studied, not a finding to act on.

What a household does with this. Eat fermented foods you make yourself — they are cheap, they are diverse by nature because they contain live cultures and their metabolites, and the diversity may be the active part rather than any single strain. Eat plants, many kinds, with the fibre intact. Use a specific probiotic for a specific, evidenced problem. And treat the supplement aisle's claims about your microbiome the way you would treat a claim about your karma.