Onion

Onion is notable for FODMAPs, oligosaccharides, salicylate and fructose. Clear on histamine, tyramine, sucrose, lactose, sorbitol, nickel, oxalates and gluten.

Yes — onion is high in FODMAPs. Onion carries mostly GF2 - a short fructan chain of three units - concentrated in the fleshy layers rather than the papery ones. Short chains ferment fast, which is why onion acts quickly rather than hours later. Long browning does break some of it, since dry heat above about 135 degrees degrades fructan, but not reliably enough to make fried onion a safe form: what is left is concentrated by the water that boiled off. Two things were measured on the onion itself. Across Sturon, Hysam, Durco and Caribo, fructans are the main carbohydrate and the SHORTEST oligomer is the major one - which is what makes onion fructan so readily fermented. And STORAGE changes it: free fructose rises markedly over months in store, attributed to the fructans hydrolysing. The fifth variety, Grano de Oro, is simply different - low fructan and FOS, high reducing sugars - so a low-fructan onion is a real thing, just not one a label tells you about. One figure puts this in proportion, and it is about the diet rather than the plant: in a survey of more than 15,000 Americans, WHEAT SUPPLIED ABOUT 70 PER CENT of all the inulin and oligofructose eaten, and onions about 25 - two foods for roughly 95 per cent of the total, against a daily average of 2.6 g of inulin and 2.5 g of oligofructose. That share belongs to a particular cuisine, not to the crop: a diet built on chicory or artichoke would divide differently. A separate survey, measuring the three fructooligosaccharide oligomers by ion chromatography, places the onion second of the vegetables it investigated - behind the artichoke and ahead of everything else it measured.

Yes — onion is high in oligosaccharides. Onion carries mostly GF2 - three units, about as short as a chain gets while still counting. Short chains ferment quickly, which is why onion produces symptoms faster than the long-chain inulin in a fibre supplement does. Muir measured the fructans in 60 vegetables and put these five at the top — garlic, artichoke, shallot, leek bulb and onion — across a range of 1.2 to 17.4 g per 100 g fresh weight for the group. The paper does not rank them against one another. One figure puts this in proportion, and it is about the diet rather than the plant: in a survey of more than 15,000 Americans, WHEAT SUPPLIED ABOUT 70 PER CENT of all the inulin and oligofructose eaten, and onions about 25 - two foods for roughly 95 per cent of the total, against a daily average of 2.6 g of inulin and 2.5 g of oligofructose. That share belongs to a particular cuisine, not to the crop: a diet built on chicory or artichoke would divide differently.

Onion contains only low levels of salicylate. Onion measures 5.0 mg of salicylate per kilogram for the white variety and 12.9 for the spanish - the variety matters more here than on most foods, and the survey measured both. A 36 g serve of white onion carries 0.18 mg.

Onion contains only low levels of fructose. Onion carries free fructose, and how much depends on how long it has been stored: a study of five varieties found that storage raises free fructose markedly, as the onion's own fructans break down into it. An onion out of long storage is therefore not the same food on this axis as a freshly lifted one. Four of the five had fructans as their main carbohydrate, and Grano de Oro was the exception — low in fructans and high in reducing sugars from the start — so the variety matters as much as the age. Amounts vary by brand and recipe, so always check the label of the exact product.