Some foods make wind and output smell stronger. It is worth saying up front that this is usually the smallest of the problems — you are already in the bathroom often, and that is where most of it stays. This page exists so you know which foods do it and why, not because it is something to worry about.
This is separate from the GASP score
The GASP score measures how hard a food is on your output — wind, urgency, looseness and residue. Smell is a different thing, so we keep it out of the maths. Eggs are the clearest example: boiled egg whites score about 0.2 out of 100, which is right, they really are gentle. They also smell strongly. Both things are true, and blending them into one number would have made it less useful, not more.
Almost all of it is sulphur. Gut bacteria turn sulphur-containing compounds into hydrogen sulphide — the rotten-egg gas. Three groups of food carry a lot of it: sulphur-rich protein, the cabbage family, and the onion family. The food list in most pouch leaflets is really just those three groups written out.
The number is a ranking aid, not a measurement — there is no clean per-gram evidence for smell the way there is for FODMAPs.
Sulphur-rich protein
Very high in sulphur-containing protein. Gut bacteria turn it into hydrogen sulphide, which is the classic rotten-egg smell.
Eggs are one of the gentlest foods here for output — this is only about smell. Many people keep eating them and just open a window.
Alliums
Organosulphur compounds — the same chemistry that makes garlic smell strong on the plate carries through.
Garlic-infused oil gives you the flavour with far less of both the fructans and the sulphur compounds.
Alliums
Sulphur compounds plus a heavy fructan load, so it tends to bring both wind and smell.
Onion-infused oil is the usual swap. Green tops of spring onion are milder than the white bulb.
Brassicas
Glucosinolates break down into sulphur compounds during digestion.
Boiling and draining softens both the residue and some of the sulphur. Smaller servings help more than you would expect.
Brassicas
Same glucosinolate chemistry as the rest of the cabbage family.
Well-cooked and mashed is easier all round than raw florets.
Brassicas
Glucosinolates, plus fermentable fibre that feeds the bacteria doing the work.
Heads are gentler than stalks for residue, though the smell is similar.
Sulphur-rich protein
Fermented and concentrated, so the sulphur compounds are already well developed.
A small amount goes a long way — the load scales with how much you use.
Brassicas
Glucosinolates again — cabbage is the family the others are named after.
Cooked cabbage is usually easier than raw slaw, for smell and for residue.
Sulphur-rich protein
Oily fish carries sulphur-containing protein, and smoking concentrates it.
Fresh fish tends to be milder than smoked or canned.
Sulphur-rich protein
Sulphur-containing amino acids in an oily fish.
Scores well for output — worth keeping in the rotation if the smell does not bother you.
Sulphur-rich protein
Shellfish protein is rich in the sulphur amino acids bacteria ferment.
Fresh and simply cooked is milder than heavily sauced.
Sulphur-rich protein
Red meat is high in cysteine and methionine, and more protein reaching the pouch means more sulphide.
Smaller portions of red meat, more often, tend to smell less than one big serving.
Sulphur-rich protein
Sulphur-rich protein, same mechanism as other red meats.
Same as beef — portion size matters more than the cut.
Sulphur-rich protein
Red meat protein plus, usually, allium seasoning and preservatives.
Often carries onion and garlic in the seasoning too, so it can stack up.
Brassicas
A gentler brassica, but the same family chemistry.
Milder than green cabbage, and much milder cooked than fermented.
Sulphur-rich protein
Aged and mould-ripened cheeses carry sulphur compounds from the ripening itself.
Hard cheeses are lower in lactose than soft, if that is also a factor for you.
Alliums
Allium sulphur compounds, milder than onion or garlic but the same family.
Green tops are gentler than the white part, and infused oil is gentler again.
Other
Isothiocyanates — the sulphur compounds that give mustard its heat.
Used as a condiment the amounts are small, so this rarely matters much.
Other
Fermentation by-products plus sulphur compounds from brewing.
Also one to watch for looseness the next day, which for most people matters more.
Other
Famous for smell, but that is asparagusic acid changing the smell of your **urine**, not your output. Included so the difference is clear.
Worth knowing so a strong smell after asparagus does not worry you — it is coming from somewhere else.
Groups at a glance: Alliums — Onion, garlic and their relatives — organosulphur compounds. Brassicas — The cabbage family — glucosinolates that break down into sulphur compounds. Other — Different chemistry, same result. Sulphur-rich protein — Protein high in cysteine and methionine, which gut bacteria turn into hydrogen sulphide.
Rankings are modelled estimates drawn from pouch guidance and the sulphur chemistry behind it — individual experience varies a lot. Not medical advice.