Smelly gas is not the FODMAPs
Why FODMAP gas does not smell, where the odour comes from and what to log.
What smells is not what ferments. There is no odour table per food because nobody has measured one; what can be measured is yours, and that is what the diary is for.
What smells. In the study that measured the gas of sixteen people and smelled it, what correlated with bad odour was hydrogen sulphide, with other sulphur gases; hydrogen, CO₂ and methane, which are the volume, do not smell (Suarez, 1998). Volume and odour are two different things, and they do not always go together.
Where the sulphur comes from. Three places. From protein that reaches the colon undigested: in five men in a metabolic ward, faecal sulphide rose step by step from 0 to 600 g of meat a day (Magee, 2000); what was measured is the daily amount, not how it is spread, and nobody has measured single meals. From dietary sulphate and sulphites, part of which reaches the colon (Florin, 1991) and which are found mostly in breads, dried fruit, wines, beers and ciders, partly from how they are preserved (Florin, 1993). And from sulphur compounds of brassicas and of the garlic and onion family (Kalantar-Zadeh, 2019).
What lowers sulphide. In the laboratory, with human faeces, fermentable fibre and resistant starch greatly reduce the sulphide bacteria make (Yao, 2018); in people it has not been measured. A rise in stools suggests a rise in gas, but that has not been measured either.
What we do not know: whether the diet changes the odour. Nobody has measured it. We know separate things that pull in opposite directions. With fewer FODMAPs there is less fibre to ferment, and a low-FODMAP diet lowers bifidobacteria and raises a sulphide-producing bacterium (Huamán, 2018). But there is also less hydrogen, which is what those bacteria use to make it (Christl, 1992), and garlic and onion, which are sulphurous, are gone. Which one wins in each person is unknown. If you notice more odour when starting, it is not harm nor a reason to stop the diet: log it.
What to do with this. Protein is not cut: what was measured is the total daily amount, and cutting it has other risks. Keep the fibre you can: oats, kiwi, quinoa, carrot. Check labels for sulphites. And log “smelly gas” in the diary: it is compared with what you ate in the previous 24 hours, apart from FODMAP symptoms, and in a few weeks the app tells you after what you had it more often than on days without it.
When it is not about the diet. If the change in odour comes with greasy or floating stools, diarrhoea that does not stop, blood or weight loss, those are the warning signs from the start of the app and they are checked with a doctor before the diet.