Prebiotics
Food components your gut microbes use, to your benefit.
A prebiotic is a substrate that gut microbes use in a way that benefits your health. In practice that means specific fibres and related compounds, notably inulin, fructo-oligosaccharides and galacto-oligosaccharides, which pass undigested to the colon and are fermented there.
The definition matters because the word gets applied loosely. The international scientific association that maintains it is deliberately strict: a prebiotic must be selectively used by host microorganisms and must confer a health benefit. Not all fibre qualifies, and not everything labelled prebiotic has been shown to do either thing.
The distinction from probiotics is simple enough to hold onto. Probiotics are live organisms you swallow; prebiotics are food for the organisms you already have. Products combining both are called synbiotics.
The mechanism is fermentation. Gut bacteria break these substrates down into short chain fatty acids, chiefly acetate, propionate and butyrate, which feed the cells lining the colon, influence appetite signalling and enter the circulation with effects still being mapped.
Food sources are unglamorous and cheap: onions, garlic, leeks, asparagus, slightly under-ripe bananas, oats, barley, apples and pulses. There is no strong reason to buy a supplement for something a varied plant-rich diet supplies, and the supplement versions are frequently the cause of the bloating people then attribute to a food intolerance.
That side effect is dose and speed dependent rather than a sign of anything wrong. Increasing fermentable fibre gradually gives the microbial community time to adjust, and people with IBS may find these specific fibres are among the FODMAPs that trigger symptoms, which is a genuine exception worth respecting.
In practice, eat a wide range of plants rather than buying a powder, increase slowly, and treat total fibre intake as the target that matters most.