Oat

Oats are high in nickel, and it sits in the bran - which is the part oats are eaten for. There is no low-nickel way to eat a whole oat.

Oats carry fructans, which is easy to miss because the grain itself is so often treated as the safe one. Biesiekierski measured 0.11 g of fructan per portion as eaten — small, and not zero.

Oat fructan is the oligosaccharide this axis asks about, so its answer agrees with the FODMAP one: 0.11 g per portion as eaten, less than wheat and more than rice.

Oats are not free of oxalate: Israr measured the highest soluble oxalate of the study in oat bran, 79 mg/100 g. The bran is the fraction and not the whole grain, so a bowl of oats carries less than that figure — but not zero. The oat is also high in phytate, which binds calcium and so keeps more of the oxalate soluble than the total alone suggests.

Oats sit in two frames at once, and both are true. Biochemically they carry AVENIN, a prolamin related to but different from the gluten of wheat, and most people with coeliac disease tolerate it. Legally they are gluten: Annex II lists 'wheat, rye, barley, OATS, spelt, kamut' among the cereals containing gluten, and the gluten-free regulation defines gluten as a protein fraction from those same four grains. That is why an oat can only be called gluten-free when it has been specially produced and tested below 20 mg/kg. They are flagged because they are grown in rotation with wheat and milled on the same equipment, so ordinary oats routinely test above the threshold. Certified gluten-free oats are grown and milled apart and do not. Certification answers contamination and not everything else: both associations say uncontaminated oats suit MOST coeliacs rather than all, because a minority react to the avenin itself.