The Tolerable Upper Intake Level (UL) is the highest level of daily nutrient intake that is likely to pose no risk of adverse health effects for almost all individuals. Although members of the general population should be advised not to routinely exceed the UL, intake above the UL may be appropriate for investigation within well-controlled clinical trials. Clinical trials of doses above the UL should not be discouraged, as long as subjects participating in these trials have signed informed consent documents regarding possible toxicity, and as long as these trials employ appropriate safety monitoring of trial subjects. In addition, the UL is not meant to apply to individuals who are receiving molybdenum under medical supervision.
Molybdenum compounds appear to have low toxicity in humans. More soluble forms of molybdenum have greater toxicity than insoluble or less soluble forms. The UL applies to all forms of molybdenum.
There are limited toxicity data for molybdenum in humans; most of the toxicity data are for animals, especially ruminants. Ruminants are more sensitive to molybdenum than monogastric animals, but the basis for the toxicity of molybdenum in ruminants is not relevant for humans. In monogastric laboratory animals, molybdenum has been associated with reduced growth or weight loss, renal failure, skeletal abnormalities, infertility, anemia, diarrhea, and thyroid injury (Vyskocil and Viau, 1999). Since none of these effects have been observed in humans, it is impossible to determine which ones might be considered most relevant to humans.
Molybdenum toxicity in animals varies according to age, species, sex, and duration of exposure (Vyskocil and Viau, 1999). In ruminants, the relative amounts of copper and sulfur in the diet are also important determinants of toxicity (Rajagopalan, 1988), but the effect of molybdenum on copper metabolism in humans is not significant (Turnlund and Keyes, 2000). The data on adverse effects of molybdenum intake are summarized below.
Renal Failure. Mild renal failure has been observed in rats after subchronic ingestion (by gastric intubation) at 80 mg/kg/day but not at 40 mg/kg/day of a molybdenum salt (Bompart et al., 1990).