Nickel is an ultra-trace element that is known to be essential in some animals, but its role in the health of humans is not clear. Early studies carried out in the 1970s suffered from some methodological problems that makes the conclusions they draw questionable. These studies reported the essentiality of nickel in chicks, rats and swine. Some of the symptoms of deficiency reported included abnormalities in the hepatocytes of chicks, liver and reproductive problems, rough coat texture in rat pups, and sparse rough coat and poor growth in swine1. Nickel is found in higher concentrations in plant foods compared to foods of animal origin, although the concentration of plants will vary geographically based on the metal composition of the soil. Nuts and legumes have the highest nickel contents (>200µg/100g) with grains and vegetables also containing reasonable concentrations (0.3 to 200µg/100g). Nickel deficiency is unlikely to occur in humans feeding on a high quality balanced diet.
Extrapolation of data from animal studies suggests a human requirement of 50 to 80 ng/g diet. Analysis of diets for nickel show that content of the metal range from 0.19 to 0.41µg/g with a mean of 0.27µg/g. This would equate to a mean intake of 165 µg/day or 75µg/1000kcal4. Nickel is present in human plasma in the range of 0.01 to 0.27µg/g of plasma and from 0.00 to 0.31 µg/g in red blood cells. The mean plasma levels have been reported as 0.063 and 0.046 µg/g and the mean red blood cell levels as 0.057 and 0.036 in men and women, respectively2. Nickel levels in 46 samples of milk collected from 13 women between delivery and 38 d postpartum showed that nickel levels did not change over time but remained at 1.3 ng/mL. At age 1 month, the mean intake of nickel in the infants was 1.5µg.
Researchers investigating nickel metabolism fed four healthy subjects 10 µg/kg bodyweight of the stable nickel isotope 62Ni in water. The researchers then analysed the plasma, urine and faeces for nickel, over the course of 5 days. Faecal excretion of the nickel tracer was 66.7% of the dose, with the absorbed dose therefore being 33.3%. Plasma levels peaked 1.5 to 2 hours after the ingestion of nickel, after which they fell over the course of 2 days. Urinary excretion ranged from 51% to 82% of the absorbed dose, but there was no evidence of excretion of nickel into the gastrointestinal tract via the bile. Between 18.0 and 49.6% of the ingested nickel was retained by the subjects at the end of the study. The mean daily excretion of nickel was calculated as 288µg/d which is consistent with an estimated intake of a typical diet containing 140 to 150µg of nickel in the diet and 100µg of nickel from cooking utensils.
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