Nutritional yeast

Nutritional yeast is not a fermented food, which is the thing worth correcting: the yeast is GROWN on a sugar substrate and then killed by heat and dried, so there is no fermentation period during which bacteria break protein down. It carries amines all the same, because yeast is protein-rich and the drying concentrates it - but the mechanism is not the one the category implies.

Nutritional yeast is inactivated whole yeast, not yeast EXTRACT - the distinction MAOI guidance draws explicitly. The extract, made by autolysing the cells, is the concentrated one; the flakes are the cells themselves, and baker's yeast in bread is not a problem at all. Three things sharing a word.

Nutritional yeast carries mannan from the yeast cell wall, which does ferment - so low rather than none. What keeps it low is the quantity: a stock cube or a tablespoon of flakes is a small dose next to a serving of onion, the portion the FODMAP thresholds are set against.

Nutritional yeast is grown on a sugar medium, and the medium is sometimes barley-derived. Brewer's yeast, the by-product of beer, is a different thing again and does carry barley - the two are often shelved together. What a label may promise is set by regulation and not by a recipe: 828/2014 allows "gluten-free" at or below 20 mg/kg, sets the conditions oats must meet, and admits wheat-derived ingredients in a gluten-free food provided the finished product stays under that ceiling. It measures nothing — it says what a food must meet to carry the claim, which is exactly what a food name cannot tell you.

Nutritional yeast takes up trace metals, nickel among them, from the medium it is grown on, the same way yeast extract does - but it is the dried whole cell rather than a concentrate, so it carries less, and it is eaten by the tablespoon.

Nutritional yeast is a dried fungus, and fungi are not free of oxalate: Savage measured cultivated mushrooms at 58.9 to 104.1 mg of total oxalate per 100 g of dry matter. It is eaten by the tablespoon, which is what keeps this low. No assay of yeast is published; what is known is the neighbouring fungi.