Doing low-FODMAP right

Healthy state

FODMAP FODMAP Short for fermentable sugars and sugar alcohols that are poorly absorbed in the small intestine, such as fructans, lactose, fructose and sorbitol. stands for fermentable oligo-, di- and monosaccharides and polyols Sugar alcohols (polyols) Sweeteners such as sorbitol, xylitol or maltitol, which also occur in fruit. They are barely absorbed and can cause bloating and diarrhoea. More in the chapter “Lactose, fructose, gluten: making sense of intolerances”. It covers four groups of short carbohydrates that are poorly absorbed in the small intestine: fructans and galacto-oligosaccharides (in wheat, rye, onions, garlic, legumes), lactose (milk sugar), fructose in excess (honey, apples, mango) and polyols or sugar alcohols such as sorbitol and mannitol (stone fruit, mushrooms, cauliflower, sugar-free chewing gum). Humans have no digestive enzymes at all for fructans and galacto-oligosaccharides; for lactose many adults lack part of the lactase, and fructose uptake has a limited transport capacity that is used up when fructose is in excess of glucose.

Because these substances stay in the small intestine, two things happen. They act osmotically, meaning they draw water into the gut until concentrations balance. And they travel on into the colon, where bacteria ferment them: into gases (hydrogen, carbon dioxide, and in some people, with the help of archaea, methane) and 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”. That is normal and intended, because these very substances are food for useful bacteria such as bifidobacteria. A person without symptoms produces considerable amounts of gas every day and hardly notices, because it passes or is absorbed.

What matters is that effects add up. A small portion of onion rarely causes problems, but onion, apple, milk and a sugar-free chewing gum on the same day can together cross the threshold. How much someone tolerates depends on amount, sum, gut sensitivity, stress level and daily form. The same food can be fine one day and not the next.

Disrupted state: causes and effects

With a sensitive gut, the same amount of gas and water triggers symptoms. In irritable bowel syndrome 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” it is often not the amount of gas that is higher but the perception: the pain fibres of the gut wall (“visceral hypersensitivity”) react to a stretch others do not feel, and gut movement reacts faster. With bacterial overgrowth in the small intestine, fermentation also happens earlier, in a gut section poorly built for it, and the gas arrives there sooner and stronger. More on the causes is in the chapters on bloating and 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”.

What the studies show: in a crossover study of 30 people with irritable bowel syndrome, a low-FODMAP diet over 21 days clearly improved symptoms compared with usual food, above all bloating, pain and wind; healthy controls noticed little. In a later study of 104 people with a sham diet, symptom scores also fell, but the share with adequate relief did not differ clearly in the overall analysis (57 versus 38 percent). That a large share also improved on the sham diet shows how big the expectation effect is. A systematic review concludes that the diet helps in the short term, but the guideline rates the evidence as very low quality because diets can hardly be blinded. Roughly half to two thirds of people respond.

The downsides: in the same study the number of bifidobacteria fell on the diet, the very bacteria that live on FODMAPs. If you avoid strictly in the long run, you also forgo many fibres, calcium from milk and variety, risk constipation and often eat in a one-sided way. In people with eating disorders or fear of food, the diet can reinforce fear of foods (see the chapter on fear of eating and underweight). Children, pregnant women, older people and people who are underweight also need support. That is why the diet is meant as a test, not a way of life.

Typical misunderstandings: low-FODMAP is not gluten-free, even though wheat is affected; with wheat it is mostly the fructans, not the gluten, that cause symptoms. And eating gluten-free before coeliac disease Coeliac disease An autoimmune disease in which gluten damages the lining of the small intestine. It is diagnosed with a blood test and usually a biopsy while you are still eating gluten. More in the chapter “Lactose, fructose, gluten: making sense of intolerances” is ruled out distorts the test. Low-FODMAP is also not “sugar-free” or “low-carb” but a targeted selection. It does not replace a diagnosis: if you have warning signs such as blood in the stool, unintended weight loss, night-time diarrhoea or iron deficiency, you need work-up first. Intolerances such as lactose and fructose can partly be tested individually, see the chapter on intolerances.