Manganese

BACKGROUND

Manganese is an essential element involved in formation of bone. It is also involved in the metabolism of carbohydrate, cholesterol and amino acids. Manganese metalloenzymes include manganese superoxide dismutase, arginase, phosphoenolpyruvate decarboxylase and glutamine synthetase. Cereal products provide about one-third of the intake of manganese and beverages (tea) and vegetables are the other major contributors. Less than 5% of dietary manganese is absorbed (Davidsson et al 1988, Finley et al 1994). In excess, it can interfere with iron absorption (Finley 1999, Rossander-Hulten et al 1991).

Manganese is taken up from blood by the liver and transported by transferrin and possibly alpha 2 - macroglobulin or albumin to other tissues (Davidsson et al 1989, Davis et al 1992, Rabin et al 1993). Retention can be affected by immediately prior intakes of manganese, calcium, iron and phosphorus (Freeland-Graves & Lin 1991, Greger 1998, Lutz et al 1993). Low ferritin levels are associated with increased manganese absorption, thus exerting a gender effect on manganese bioavailability (Finley 1999). Manganese is excreted rapidly into the gut through bile and lost primarily in faeces. Low bile excretion can therefore increase the potential for manganese toxicity. Urinary excretion is low and not related to diet (Davis & Greger 1992).

Manganese deficiency in animals is associated with impaired growth, reproductive function and glucose tolerance as well as changes in carbohydrate and lipid metabolism. It also interferes with skeletal development. Clinical deficiency in humans has not been associated with poor dietary intake in otherwise healthy individuals. Skin symptoms and lowering of cholesterol were also seen in one experimental depletion study in young men (Krishna et al 1966). Accidental overdose has been shown to result in symptoms such as scaly dermatitis, hypocholesterolaemia, hair depigmentation and reduced vitamin K-dependent clotting factors (Doisy 1973).

The indicators for estimating the requirement of manganese include balance and depletion studies, serum, plasma, blood or urinary manganese concentration, arginase activity and manganese superoxide dismutase activity. However, none of these is reliable or sensitive enough to be used for setting recommended intakes. Balance studies are problematic because of the rapid excretion of manganese into bile and because balance studies over short to moderate periods do not appear to give results proportional to manganese intakes (Greger 1998, 1999).

Serum, plasma, blood and urinary manganese measures seem highly variable over the normal range of consumption and largely insensitive to moderate dietary change (Davis & Greger 1992, Friedman et al 1987, Greger et al 1990). Arginase activity is affected by a number of factors, including high protein diet and liver disease. Ethanol and dietary polyunsaturated fats can affect manganese superoxide dismutase (Davis et al 1990, Dreosti et al 1982).