The gut microbiome is essential for human health and prebiotics are essential for the microbiome.

Prebiotics are short-chain carbohydrates that are an essential food source for the microbiome and positively alter the composition or metabolism of the gut. They are the non-digestible fibre components of foods like onions, garlic, Jerusalem artichokes, chicory roots and beans, that travel undigested through the small intestine and ferment when they reach the large intestine (the colon) by bacteria such as bifidobacteria. The compounds produced by this fermentation process —short-chain fatty acids such as acetate, butyrate and propionate — encourage the selective growth and proliferation of the beneficial gut bacteria essential for a healthy microbiome.

Fibre in our foods is generally classified as either soluble or insoluble, but from a microbiome perspective the fermentability of the fibre is the critical component for health. Fibre that resists digestion, called digestive-resistant starch, in the small intestine that slowly ferments in the large intestine is critical for health. Due to their slow fermentation, prebiotics do not cause wind or digestive upsets, but feed the healthy bacteria, adding significant bulk to the stools and thereby improving digestive and systemic health.

Prebiotics are also called non-digestible fermentable carbohydrates (NDFC) and modulate the composition, structural diversity and metabolic potential of the gut microbiome and its role in the maintenance of health (including mental health), as well as the prevention and/or treatment of disease.

We grow the bacteria — the microbiome — that we feed.

Differences between prebiotics and probiotics

Probiotics are the live beneficial bacteria that are also naturally created by the process of fermentation in foods like kefir, yoghurt, kimchi, miso and sauerkraut among others, and are reinoculated into the gut when we eat these foods.

[Prebiotics are found in] foods like onions, garlic, Jerusalem artichokes, chicory roots and beans.

Probiotics are delicate organisms and can be easily killed by acids, time and heat, which is why probiotic supplements are freeze-dried with a short use-by date, and most of them need to be kept in the fridge. The balance of probiotics can change with changing conditions; for example, different foods (and different antibiotics) will impact on different gut bacteria, so the correct ones need to be provided when needed.

Prebiotics on the other hand are not nearly so fragile; they are not affected by acid, time or heat or cold, and the fermentation process is similar in most humans, so these are easier to incorporate into the diet than probiotics.

Prebiotics provide a medium for growth of selected probiotic strains, particularly lactobacilli and bifidobacteria, and with adequate prebiotics in the diet, probiotic microorganisms are better survivors, acquiring a greater tolerance to environmental conditions in the intestines.

Food sources of prebiotics

Prebiotics contain compounds such as inulin, fructooligosacharides (FOS), lactulose and derivatives of galactose and β-glucans.

Chicory root has the highest levels of inulin — 65 per cent of the root by weight is fibre and is a very rich source of prebiotic fibre. Other foods containing high levels of prebiotics are: Jerusalem artichoke (2 grams of fibre per 100 grams, with 70 per cent prebiotics), boiled unripe bananas (about 80 per cent prebiotics), onions, garlic and leeks (2 grams, with 17 per cent prebiotic fibre), asparagus, dandelion and spinach greens and oats and barley. Apples eaten with the skin contain about 2 grams fibre, of which 50 per cent is the prebiotic pectin. Seaweeds are also high in prebiotics.

The suggested daily fibre amount to be eaten for good health is 25–38 grams, containing prebiotic fibre of 5–20 grams.

Prebiotics’ effects

Digestive system

Epidemiological research suggests a link between imbalances in the gut microbiome and diets low in prebiotics, with a whole raft of chronic illnesses common today. Digestive conditions in particular include GORD (gastro-oesophageal reflux disease), inflammatory bowel disease, IBS (irritable bowel syndrome), diverticular disease and appendicitis.

Increasing prebiotic fibre in the diet significantly improves the microbiome, which in turn improves immunity, bone density, insulin management, weight management and brain health.

Anti-inflammatory

Increasing the ingestion of fermentable prebiotic fibre reduces inflammatory cytokines in the colon. The by-products of the fermentable fibre helps repair “leaky gut syndrome”, supporting the gut immune system and reducing the risk of inflammatory disorders such as asthma and cardiovascular disease both locally and systemically.

Bone health

Prebiotic consumption improves the gut–bone signalling axis and is a cost-effective approach for improved skeletal health and preventing fractures. Research has shown that prebiotics improve calcium absorption and bone metabolism in all ages from adolescence to the elderly.

Insulin management

As these prebiotic carbohydrates are not digested, they improve insulin regulation and reduce insulin resistance as well as improving lipid profiles, having potential benefits in resolving conditions like diabetes and metabolic syndrome.

Systemic health

Diets low in prebiotics, with associated abnormal gut bacteria, also play a role in systemic conditions including asthma, atherosclerosis, autoimmune diseases, cholesterol abnormalities, colorectal cancer, diabetes, high blood pressure, gallstones, non-alcoholic fatty liver disease, obesity and cardiovascular disease.

Prebiotics can be used as an alternative to probiotics or as additional support, and have enormous potential for beneficially modifying the gut microbiome to support many facets of human health.