The six groups, and every food in each.

Fructans are chains of fructose that plants build to store energy: in wheat grains, in onion bulbs, in the root of the chicory plant. Human digestion has no enzyme that can cut those chains apart, so fructans travel through the small intestine intact and arrive in the large intestine whole, where the bacteria living there ferment them and produce gas. That is the entire mechanism, and because how much arrives depends on how much you ate, fructans sit on a scale rather than a yes-or-no list - how much you eat changes the picture as much as what you eat.

Lactose is the sugar in milk — cow, goat and sheep alike. To absorb it, your small intestine has to split it into two simpler sugars using an enzyme called lactase; when there isn't enough lactase for the amount that arrives, the leftover lactose carries on into the large intestine, pulling water in as it goes and feeding the bacteria there, which ferment it and produce gas. The amounts here range from a trace to the top of the scale, taking in the obvious milk and ice cream as well as things that don't taste of dairy at all.

Excess fructose means exactly what it says: the food carries more fructose than glucose. Fructose crosses out of the small intestine by a slow route with limited capacity, and glucose alongside it opens the way for more of the fructose to cross — so when the two sugars are roughly matched, most of the fructose is absorbed, and when fructose runs ahead, the surplus draws water into the small intestine as it goes, then travels on to the large intestine, where gut bacteria ferment it.

GOS is short for galacto-oligosaccharides: small chains of sugar, built around galactose units, that plants pack into their seeds. Human digestion has no enzyme that can cut those chains apart, so GOS travels through the small intestine untouched and arrives in the large intestine, where gut bacteria ferment it and release gas. That happens in every body — with IBS, the same gas is more often felt as bloating or pain.

Sorbitol is a sugar alcohol, or polyol — the P in FODMAP. Mannitol is the other one you will see measured most often, and maltitol, xylitol and isomalt belong to the same family. The small intestine absorbs it slowly and only up to a point, so whatever is left over carries on down: it draws water into the bowel as it travels, then gets fermented by gut bacteria further along, which produces gas. It reaches you two ways — grown into certain fruit and vegetables, and added to manufactured products, where it sweetens and keeps things moist. That second route covers the sweet aisle and the medicine cabinet alike.

Mannitol is a sugar alcohol — one of the polyols, the P in FODMAP, and the one usually measured alongside sorbitol. It occurs naturally in mushrooms, celery, winter squash, sweet melon and seaweed, and it's added to sugar-free products as a sweetener and bulking agent, where the ingredients list calls it mannitol or E421. The small intestine takes it in slowly and only partly, so whatever isn't absorbed carries on into the large intestine, drawing water along with it and giving the bacteria there something to ferment.