Probiotics.

Inulin passes undigested to the colon where bifidobacteria and lactobacilli ferment it, so it feeds the organisms the capsule delivers. Supplying strain and substrate in one dose is the defining synbiotic combination.

PHGG is a low-viscosity fibre fermented gradually along the colon into short-chain fatty acids, which lowers luminal pH in a range that favours lactobacilli. It is chosen over inulin when gas production needs to be gentler.

Glutamine is a preferred fuel of the cells lining the small intestine and supports the tight-junction proteins that hold that lining together. It acts on the host side of the barrier while the strains act on the luminal side.

Colonic bacteria ferment beta glucan into short-chain fatty acids, so it acts as a carbohydrate source for the strains supplied alongside it. The pairing follows the same substrate plus organism logic as other synbiotics.

Pectin is fermented by colonic bacteria into short chain fatty acids, giving supplemented organisms a substrate to grow on. Feeding the organism is the classic synbiotic pairing.

Psyllium is only partly fermented, so it both feeds some fermentation and holds water to slow transit, giving organisms longer contact time.

Butyrate is the short chain fatty acid colonic bacteria produce from fibre and the main fuel for colonocytes. Supplying it directly delivers the end point that fermentation normally provides.

Lactic acid bacteria carry their own beta-galactosidase, which is the same activity supplemental lactase provides. Both act on lactose in the small intestine.

Carvacrol and thymol disrupt bacterial membranes without distinguishing supplemented organisms from resident ones. Taken at the same time they reduce how many viable organisms arrive, so dosing is separated.

Charcoal adsorbs a wide range of organic material in the gut lumen and can bind organisms and their substrate. Taking it with a probiotic reduces what is delivered.

Clay binds material in the gut lumen by surface charge, including bacterial cells. Separating doses is standard practice.

Berberine has broad antibacterial activity in the gut and shifts microbial composition on its own. Given together with a probiotic it can reduce the viable count delivered.

Iron that is not absorbed reaches the colon, where it favours iron-scavenging enterobacteria over lactobacilli and bifidobacteria. Higher unabsorbed iron shifts the population away from the supplemented organisms.

Several bifidobacteria and lactobacilli synthesise folate in the colon, adding to dietary intake. The organism and the vitamin sit on the same supply route.

FOS passes the small intestine undigested and is fermented by lactobacilli and bifidobacteria to short-chain fatty acids. It is the substrate most often paired with live strains in synbiotic formulas. Larger amounts increase gas and bloating in people who are sensitive to fermentable carbohydrate.

Resistant starch escapes amylase digestion and is fermented in the distal colon, where butyrate-producing species use it. That is further down the gut than most oligosaccharides reach. The pairing shifts where fermentation happens, not just how much.

Partially hydrolysed guar gum is a slowly fermented soluble fibre that gives a gradual fermentation profile rather than a rapid one. Fast-fermenting oligosaccharides are more often reported to produce gas at the same gram dose, though tolerance varies between people. It gives resident and supplemented species a substrate along the length of the colon.

Konjac glucomannan is a viscous soluble fibre fermented by colonic bacteria to short-chain fatty acids. It is used mainly for viscosity and satiety, with fermentation as a secondary property. It needs to be taken with adequate water because of how much it swells.

Oat beta-glucan is both viscous in the small intestine and fermentable in the colon, so it feeds resident and supplemented bacteria. The fermentation end products include propionate and butyrate. This is a substrate relationship described in fermentation studies rather than a clinical outcome measured with a live strain.

S. boulardii is a yeast, not a bacterium, so it is unaffected by antibacterial agents that suppress bacterial strains. Formulas often pair it with bacterial strains for that reason. The two act through different surfaces of the same gut environment.

Lactoferrin sequesters free iron, which limits the growth of iron-scavenging organisms while bifidobacteria, which have low iron requirements, are less constrained. It is a component of human milk that sits alongside the oligosaccharides shaping the infant microbiota. The pairing is mechanistic and has not been resolved into a specific clinical effect here.

Bovine colostrum carries immunoglobulins, lactoferrin and oligosaccharides, several of which are fermentable or bind bacterial surfaces. It is used alongside live strains in gut-focused formulas. The evidence is mechanistic and composition-based rather than a trial of the combination.

Zinc carnosine adheres to the mucosal surface and has been studied for its effect on epithelial integrity markers. Live strains act at the same interface through mucin stimulation and tight-junction proteins. The two are formulated together on mechanistic grounds, with no combination trial cited here.

Menaquinones are made by bacteria, not by plants, and gut bacteria including Bacillus and some lactic acid species produce various menaquinone forms. That is why MK-7 in supplements is produced by fermentation. Colonic menaquinone production is real but its contribution to human vitamin K status is difficult to quantify.

Cobalamin is synthesised only by bacteria and archaea, and certain lactic acid bacteria and propionibacteria produce it during fermentation. Colonic synthesis sits below the ileal absorption site, so it does not reliably reach the bloodstream. The relationship explains where dietary B12 comes from, not that a probiotic supplies it.

Urolithin A is not present in food. It is produced when gut bacteria metabolise ellagitannins from pomegranate and certain nuts. Only some people carry the microbial capacity to make it, which is why the metabolite is sold directly. This is one of the clearest examples of the microbiota determining what a person actually absorbs.

Curcumin is poorly absorbed, and much of an oral dose reaches the colon where bacteria convert it to metabolites including tetrahydrocurcumin. The bacterial population present therefore shapes which metabolites appear. The direction and size of that effect in people has not been settled.

Catechins that escape absorption are ring-fissioned by colonic bacteria into smaller phenolic acids, and catechins in turn have antibacterial activity against some species. The relationship runs both ways. It is characterised in fermentation work rather than by a clinical combination study.

Supplemental proteases and carbohydrases break substrate down in the small intestine, which reduces what reaches the colon for fermentation. In a shared capsule, proteases can also act on the bacterial cells themselves during storage if moisture is present. Formulators generally separate the two for that reason.

Gastric acid is the main barrier live organisms have to survive, and betaine hydrochloride is taken specifically to lower stomach pH. Taking the two at the same moment works against the delivery of the strain. Separating them by an hour, or using an acid-resistant capsule, is the usual approach.

Allicin and related thiosulfinates have broad antibacterial activity in laboratory systems, including against lactic acid bacteria. Whether a culinary or supplemental dose meaningfully reduces a live strain reaching the colon has not been quantified. It is worth separating the doses when both are being taken deliberately.

Chicory root inulin is a fructan of longer chain length than FOS, so it ferments more slowly and further along the colon. It is the most widely used prebiotic in synbiotic products. Tolerance is dose-dependent and gas is the usual limiting factor.