Prebiotics that build up gently

Healthy state

Prebiotics Prebiotics Food components, usually certain fibres, that specifically feed beneficial gut bacteria, such as inulin or GOS. are fibres that specifically feed beneficial gut bacteria. Well-known examples are inulin and fructo-oligosaccharides (FOS), for example from chicory, onions or Jerusalem artichoke. Humans have no digestive enzymes of their own for these sugar chains: neither saliva, nor stomach acid, nor the enzymes of the pancreas break them down. They travel unchanged through the stomach and small intestine and reach the large intestine. Only there do they meet bacteria that carry the right enzymes.

Fibres differ in how quickly they are fermented. Short chains such as FOS are used up within a few hours in the first part of the large intestine. Long-chain inulin, partially hydrolysed guar gum (PHGG, guar gum cut into shorter pieces), resistant starch or certain arabinoxylans from cereals are fermented more slowly and further along. In the lab, short fructans were fermented mainly in the first few hours, long inulin chains more steadily over a longer period. Besides chain length, branching, solubility and packaging matter: in whole foods, fibres sit inside cell walls that bacteria first have to open up. A pure powder is attackable at once. That is why the same amount of fibre often acts more strongly as a powder than in wholegrain bread or pulses.

The place of fermentation matters. In the first part of the colon plenty of sugar is available, the pH is slightly acidic at about 5.5 to 6, and the bacteria work fast. Further along, food runs short, the pH is closer to neutral, and the bacteria switch to protein. This produces substances such as ammonia, phenols and branched fatty acids, which in larger amounts tend to burden the lining. Slowly fermentable fibres carry carbohydrate into this far section and keep sugar fermentation going there. The result is short-chain fatty acids Short-chain fatty acids Acids such as acetate, propionate and butyrate that gut bacteria make from fibre. They nourish the gut wall and act throughout the body. More in the chapter “Butyrate and other short-chain fatty acids” such as butyrate Butyrate A short-chain fatty acid and the main energy source for the cells of the colon wall. It strengthens the barrier and dampens inflammation. More in the chapter “Butyrate and other short-chain fatty acids”, the main fuel of colon cells, which also lower the pH and support the uptake of water and salts.

Every fermentation produces gas: hydrogen and carbon dioxide, in some people also methane. Part of it is used up straight away by other microbes in the gut, for example archaea (methane-producing single-celled organisms), and part enters the blood and is exhaled. What is left leaves as wind: ten to twenty times a day is normal. Whether this gas bothers you therefore depends not only on the amount but also on where it is made and how fast it moves on.

Disrupted state: causes and effects

People with a sensitive gut often tolerate fast prebiotics poorly. In lab experiments with stool samples, fructans such as FOS and inulin produced a lot of gas in the first few hours, PHGG only moderate amounts. This explains why prebiotic powders often make bloating worse at first in IBS Irritable bowel syndrome Recurring abdominal pain linked to bowel movements, without tests finding another cause. Today it is understood as a disorder of gut–brain interaction. More in the chapter “The gut-brain axis” or small intestinal bacterial overgrowth Small intestinal bacterial overgrowth (SIBO) Too many bacteria in the small intestine, where normally few live. Bloating and diarrhoea are typical; it is usually diagnosed with a breath test. More in the chapter “When the small intestine is too densely colonised”. Short chains also draw water into the gut, because as small particles they act osmotically. This can soften the stool.

Four mechanisms lie behind a bloated belly, and from the outside they feel the same. First, simply a lot of gas, because a great deal of fast food arrives at once. Second, gas in the wrong place: if many bacteria already sit in the small intestine, they ferment the prebiotic there, before the ileocaecal valve (the closing muscle between small and large intestine). The thin gut is stretched, and that hurts more. More on this in the chapter on small intestinal overgrowth. Third, a sensitive gut wall: in IBS the nerves react with pain to normal amounts of gas, and in some people the diaphragm tightens instead of relaxing while the abdominal wall bulges forward. The belly visibly grows although there is no more gas. Fourth, slow transit: in constipation the food stays longer, the bacteria have more time, and the gas moves on sluggishly. In the small intestine the cleaning wave between meals (MMC) moves leftovers and bacteria onward. Frequent snacking interrupts it.

Rapidly fermentable fructans are also FODMAPs FODMAP Short for fermentable sugars and sugar alcohols that are poorly absorbed in the small intestine, such as fructans, lactose, fructose and sorbitol. More in the chapter “Doing low-FODMAP right”. If you avoid them because of symptoms, you feed your gut microbiota Microbiome, gut microbiota The community of all microbes in the gut, mainly bacteria, plus viruses, fungi and archaea. Most live in the colon. More in the chapter “Building microbial diversity” less in the long run. In studies of the low-FODMAP diet, the number of bifidobacteria often fell. That is why this diet is meant as a phase, not a permanent state, and slowly fermentable fibres can close the gap.

The effect depends on your own gut microbiota. The same fibre clearly raises butyrate in some people and hardly at all in others. Transit time counts too: on average it is longer in women than in men, often also in older age, and many women notice more bloating before their period.

Three misunderstandings: “If it makes me bloated, it is working.” Not true, because the benefit lies in the fatty acids; gas is a by-product. “Slowly fermented means trouble-free.” That is plausible, but barely compared directly in humans. “More is better.” Also wrong: many studies show the benefit at moderate amounts, and too much at once overwhelms the gas disposal. A diary helps with observation: if the belly swells two to three hours after eating, that points more to the upper digestive tract or small intestine, many hours later more to the colon. This is only a rule of thumb, not a test.