Nickel

What is it?

Nickel allergy is the most common contact allergy — jewellery, belts, coins. Systemic nickel allergy syndrome arises from nickel in food rather than on the skin, and shows as eczema flare-ups, hives, headaches and digestive upset. The foods that carry most of it are chocolate and cocoa, nuts, oats, soy, legumes, whole grains, canned food and shellfish. Cooking acidic food in stainless steel can add more.

What a day's diet actually supplies

Flyvholm (1984) gives the figures this axis would otherwise lack: an average Danish diet supplies about 150 µg of nickel a day, rising past 900 where the high-nickel foods are eaten in quantity, and provocation studies report hand eczema flaring somewhere between 600 and 5,600 µg. That last range is nickel sulphate given as a single dose, which is not the same thing as nickel spread across a day's meals. Smart and Sherlock (1987) put a second national figure beside it and land in the same place: mean dietary nickel of 0.14 to 0.15 mg a day in the United Kingdom over 1981-84.

Which foods, in what order

Flyvholm names the very high ones — cocoa, soya, oatmeal, nuts and legumes — and publishes a content figure for none of them. Noël (2012) puts that list in order on a population's actual plate: the Second French Total Diet Study analysed 1,319 samples by ICP-MS and found nickel highest in tofu, then in sweeteners, honey and confectionery with dark chocolate above all, then in cereals and cereal products, then in ice cream. It too publishes the ranking and not the values. Both work by food group, so a level on a food page is a placing on such a list rather than a measurement, and neither can separate one brand from another.

Where the processing adds some

Smart and Sherlock found nickel fairly evenly spread across food groups, with the three highest being canned vegetables, sugars and preserves, and bread and cereals — which the authors read as a contribution from processing equipment and possibly from the cans themselves. That is their reading of where the nickel came from rather than a measurement of equipment, and it is a group-level finding: it places no single tin.

Prepared dishes, measured as sold

Cabrera-Vique (2011) analysed 170 samples of 43 convenience and fast foods eaten in Spain and found 18.5 to 95.0 ng per gram in the fresh edible portion — 0.019 to 0.095 mg/kg, an order of magnitude below the high-nickel foods above. The highest were egg- and pork-based dishes and sauces, and what raised them was an ingredient rather than a process: spices and herbs, whole cereals, dried fruit, cheese, mushrooms. The figure that changes how to read all of this is the dialysable fraction — only 4.5 to 7.75 per cent crossed the membrane, so most of what a prepared dish measures is not what reaches the blood. That is an in-vitro fraction and not absorption in a person, and the study measures dishes as sold rather than raw ingredients.

Fruit, which the lists rarely reach

Nie (2016) measured nickel in China's five main deciduous fruits — apple, pear, peach, grape and jujube — and found it highest in jujube and peach, lowest in grape, with 2.2 per cent of samples above the permissible limit and a daily intake from all five of no more than 1.1 per cent of the tolerable daily intake. Zeiner (2018) looked at lingonberries, blueberries and rose hips from an unpolluted part of Croatia, found nickel mainly up to 25 mg/kg and concluded that no health threat is to be expected from eating them. Neither publishes a figure per species, and between them they cover eight fruits grown in two places: no citrus, no tropical fruit, and nothing about what a fruit picks up on polluted ground.

Additives, where the reason is legal rather than chemical

Regulation (EU) No 231/2012 sets purity criteria additive by additive, and the heavy metals it routinely caps are arsenic, lead, mercury and cadmium — not nickel. It specifies nickel only where nickel is the process catalyst, as with the hydrogenated polyols, or an ore companion. For most additives there is therefore no legal nickel ceiling at all, so a level on such a page is an inference from how the additive is made rather than the reading of a limit. And where a criterion does exist it is a ceiling and never a content: it says what an additive may legally contain at most, and nothing about what a given batch holds.

Source: Flyvholm M.-A. et al., Z Lebensm Unters Forsch 179 (1984) 427-431, for the dietary intake and the foods that carry most of it — it names which are high and publishes a content for none; Smart G.A. & Sherlock J.C., Food Addit Contam 4 (1987) 61-71, for the UK intake and the three highest food groups; Noël L. et al., Food Chemistry 132 (2012) 1502-1513, Second French Total Diet Study, for the ranking of food groups; Cabrera-Vique C. et al., Sci Total Environ 409 (2011) 1584-1588, for the convenience-food range and the dialysable fraction; Nie J.-Y. et al. (2016), for China's five main deciduous fruits; Zeiner M. & Juranović Cindrić I. (2018), for lingonberry, blueberry and rose hip; Commission Regulation (EU) No 231/2012, for which heavy metals the purity criteria actually cap

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