A prebiotic is a non-digestible food component, typically a carbohydrate such as inulin, fructo-oligosaccharides, or galacto-oligosaccharides, that selectively feeds beneficial bacteria in the large intestine. Unlike probiotics, which supply live microbes, prebiotics fertilise the microbes already resident, promoting bifidobacteria and lactobacilli whose fermentation produces short-chain fatty acids that nourish the gut lining and modulate immunity. Natural sources include onions, garlic, leeks, asparagus, bananas, chicory root, and whole grains; concentrated forms appear in supplements and functional foods. The concept, defined in the 1990s, has matured into recognition that prebiotic effects depend on the individual’s existing microbiota. Excessive doses cause the familiar side effects of fermentable fibre: gas, bloating, and discomfort, a reminder that feeding gut bacteria has dose limits.