Molybdenum is an essential element in humans that is required for the enzymes xanthine oxidase, xanthine dehydrogenase, sulfite oxidase and aldehyde oxidase. The xanthine enzymes are involved in the hydroxylation of heterocyclic nitrogen compounds such as purines, whereas sulfite oxidase is involved in the metabolism of sulphur amino acids. The function of aldehyde oxidase is not fully understood but may be involved in drug metabolism. As with all minerals, molybdenum concentrations in plants greatly depending on the growing conditions, geographical location and soil quality. Despite this, molybdenum deficiency is rare although cases have been reported in individuals with malabsorption syndromes such as Crohn’s disease. Deficiency of molybdenum occurs at intakes below 25µg per day and the average US intake has been estimated to be 120 to 240µg per day with a mean intake for men reported at 109µg per day.
The kinetics of molybdenum in humans were investigated by researchers1 in a study using 4 healthy men consumed a low molybdenum diet (22µg/d) for 102 days, followed by a high molybdenum diet (467µg/d) for 18 days. The subjects were also administer oral 100Mo and intravenous 97Mo in order to assess molybdenum excretion in the urine, plasma and faeces. The results showed that during the high intake period, urinary excretion of molybdenum increased when compared to the low intake period. The fractional molybdenum storage in tissues was also lower during the high intake period suggesting that metabolic adaptation occurs during low intakes of molybdenum in order to conserve body concentrations. The authors concluded that the subjects required a mean intake of 43µg/d in order to conserve plasma levels suggesting that the current minimum requirement of 25µg/d is too low.
Interestingly the authors found that food bound molybdenum was 16% less available than that of a purified supplemental molybdenum. Excretion of molybdenum was presented in a model, whereby the authors suggested that the data can be explained by losses of molybdenum by excretion in the urine as well as transfer from the plasma to the gastrointestinal tract via the bile. Some molybdenum would also pass straight to the faeces never having being absorbed. Absorption in humans is estimated to be between 85 to 93%, although this might be dependent on the plasma and tissue reserves of an individual. In this study, the absorption efficiency was reported at 94% during the high molybdenum intake and 91% during the low molybdenum intake suggesting a consistently high absorption at both low and high intakes, with excess molybdenum being excreted in the urine.
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