Prebiotics

Prebiotic carbohydrates reach the colon intact, where bacteria with the right enzymes, chiefly Bifidobacterium and some Lactobacillus and Bacteroides species, ferment them into short-chain fatty acids (acetate, propionate, butyrate), lactate and gases. Butyrate is the main fuel for colon cells, strengthens tight junctions, reduces inflammation and may protect against colorectal cancer; propionate and acetate reach the liver and influence glucose and lipid metabolism and appetite hormones (GLP-1, PYY). The acidic environment created by fermentation inhibits pathogens, improves solubility and absorption of calcium and magnesium, and favors cross-feeding of bifidobacteria to butyrate producers such as Faecalibacterium. Prebiotics also increase mucus production, modulate immune responses toward tolerance via regulatory T cells, and in infants GOS and HMOs shape a bifidobacteria-dominant microbiome linked to lower allergy and infection risk.

Prebiotics have a large trial literature with consistent microbiome effects and moderate clinical effects. Dozens of randomized trials confirm that inulin, FOS and GOS at 5–20 g daily increase fecal bifidobacteria within 1–2 weeks. Meta-analyses show modest improvements in stool frequency and consistency in constipation, increased calcium absorption in adolescents (with small bone density gains in a year-long trial), and small reductions in fasting glucose, insulin, LDL cholesterol and body weight in overweight adults, with effects strongest for inulin-type fructans. GOS and HMO-supplemented infant formulas bring the infant microbiome closer to that of breastfed babies and reduce some infections and possibly eczema. In IBS, results are mixed: low doses (3.5 g) of GOS improved symptoms in one trial, while higher doses of inulin and FOS worsen bloating in many patients, which is why low-FODMAP diets restrict them. Trials in anxiety and cognition (GOS reduced cortisol and attentional bias to negative stimuli in one study) are small. Lactulose has strong evidence for hepatic encephalopathy. The main limitations are short trial durations, heterogeneous products, and individual microbiome variability in response.

Inulin and fructooligosaccharides (FOS), galactooligosaccharides (GOS), human milk oligosaccharides (2'-FL), resistant starch, partially hydrolyzed guar gum, beta-glucans, arabinoxylans, lactulose, polyphenols with prebiotic effects

By definition prebiotics are not absorbed; their 'bioavailability' is to colonic bacteria. Short-chain FOS ferments rapidly in the proximal colon, producing more gas quickly; long-chain inulin and resistant starch ferment more slowly and further along the colon, which is gentler and delivers butyrate to the distal bowel, where most colorectal disease occurs. Response depends on the individual microbiome: people whose gut lacks bifidobacteria respond less to GOS and may need a probiotic first. Fermentation products (SCFAs) are absorbed efficiently by colon cells and the portal circulation. Gradual dose escalation over 2–3 weeks allows the microbiome to adapt and reduces gas.

• Chicory root, Jerusalem artichoke, dandelion greens (inulin)
• Onion, garlic, leek, asparagus (FOS and inulin)
• Slightly green bananas and cooked-then-cooled potatoes, rice and pasta (resistant starch)
• Legumes (beans, lentils, chickpeas) — GOS and resistant starch
• Oats and barley (beta-glucan), whole wheat and rye (arabinoxylan)
• Human breast milk (HMOs) — for infants
• Cocoa, berries, pomegranate, green tea (polyphenols with prebiotic effects)