​Health matters related to molybdenum in drinking water

Molybdenum in our Diet

Molybdenum is an essential element for the maintenance of good human nutrition. Molybdenum is required for many different enzymes that are essential for all tissues and organs in the body. An example of the importance of molybdenum for good nutrition is the required supplementation of formulas for premature infants with molybdenum to ensure their optimal growth and development (Bougle et al., 1991; Casey and Hambidge, 1985).

Molybdenum is a normal mineral found in such foods as milk and milk products, dried legumes, organ meats (liver and kidney), cereal grains and baked goods. Most of our daily requirements for molybdenum are met by the consumption of cereal grains such as wheat, oats, barley and rye.

A number of regulatory agencies (e.g., Natural Resources Canada {NRC}, Health Canada) have estimated typical daily consumption rates and healthy or safe consumption rates for molybdenum. The key representative values are:

 

SubstanceHealth Canada (1996)NRC (1989) Essential Safe and Adequate Daily Intake (mg/day)
Molydbenum0.05 to 0.350.075 to 0.250

 

Studies of intake from normal foods have demonstrated that there are significant differences in daily molybdenum intake between sexes and among different age groups. When the intakes are compared on a body weight basis (per kilogram body mass), differences between sexes tend to disappear. Differences in molybdenum intake are also observed at different ages. Among young individuals, the consumption of molybdenum is greater than adults, even on a body weight basis.

However, while intakes of molybdenum do differ, there is no evidence of differences in sensitivity to molybdenum between sexes or age groups.

Molybdenum in the Human Body

Everyone has "homeostatic" control systems to maintain a constant balance of essential nutrients within the body. Thus, the body is able to ensure that the required amount of a given substance for health and maintenance of normal body functions is retained in the body, and excess amounts are excreted to avoid build-up in body concentrations that could lead to toxic effects. For molybdenum, this "homeostatic" balance is achieved by clearance of excess molybdenum through the kidney and excretion from the body in urine. As molybdenum intakes increase beyond those required for normal nutrition, the rate of excretion from the body in urine also increases. This homeostatic mechanism operates to maintain the concentrations of molybdenum at the required levels in the body. If the intake of molybdenum is extremely elevated compared to the bodies' requirements, the clearance of the excess by the kidneys is overwhelmed, and molybdenum concentrations in the body increase to the point where toxicity can develop.

Since the kidney performs an important function in maintaining molybdenum homeostasis, changes in kidney function are important in the assessment of molybdenum safety. Understanding how molybdenum is excreted by the kidney into urine is the key to understanding the importance of various aspects of kidney function relative to molybdenum safety. A number of investigations have shown that molybdenum is excreted by the kidney by a process known as passive diffusion or passive filtration. Passive filtration processes occur via simple diffusion, and do not require any energy (i.e., molybdenum is simply excreted, rather than being "pumped" out of the body).

Passive filtration systems, by which molybdenum is excreted through the kidney, are relatively robust (are not readily affected by diseases and other disturbances) compared to active transport systems. Also, the passive filtration capacity of the kidney does not change appreciably with age, especially when expressed in terms of unit body weight (Holiday et al., 1994; Heibron et al., 1991).

Since molybdenum is excreted through the kidney by passive filtration, no changes in kidney excretion rates of molybdenum, per unit of body weight, would occur with age. As such, molybdenum levels established as safe for adults would also be protective of infants and the elderly.

Molybdenum Permitted for Trepanier Creek

A number of researchers and regulatory agencies have established recommended drinking water quality objectives or limits for molybdenum. Key values are listed below, together with the long-term, average permitted molybdenum concentration in drinking water from Trepanier Creek, for comparative purposes.

SubstanceLong-Term Average Concentration in Trepanier Creek (mg/L)Drinking WaterDescription of ObjectiveReference

 

Molydbenum

 

Less than 0.015

0.25No adverse effects in humansBC Environment
0.07aNon adverse effects in humansWHO, 1993