ARSENIC

Although most commonly known for its toxic properties, arsenic (As) has been shown to have beneficial actions when fed in very small amounts to laboratory animals. Numerous studies with rats, hamsters, minipigs, goats, and chicks have provided circumstantial evidence suggesting that arsenic is essential, but its physiological role has not been clearly defined. However, there is evidence that arsenic intake affects taurine and polyamine concentrations in plasma and tissues.

Deficiencies: The most impressive reported sign of arsenic deficiency is decreased growth of goats, impaired success of the first service and conception rates, greater absorption of fetuses during pregnancy, and higher mortality rate during the second lactation. There is often sudden death, and the mitochondria of the cardiac muscle showed ultrastructural changes in deficient goats. The most consistent signs of arsenic deprivation in rodents are decreased growth, higher death rate of young, rougher and yellowish hair coats (in white rats), elevated erythrocytes osmotic fragility, elevated spleen iron and splenomegaly. However, the severity and variation of these deficiency signs depend upon several dietary factors including the zinc, arginine, choline, methionine and guanidoacetic acid content. These substances are interrelated because they are effectors of methionine metabolism. Arsenic-deprived chicks drank and excreted more water, exhibited slower growth, usually had leg abnormalities, and arginine-supplemented deficient chicks had elevated hepatic zinc levels but depressed content of arsenic, iron and manganese in this tissue. Arsenic may be important under certain circumstances in humans. For example, arsenic, independent of omega-3 fatty acids, increases bleeding time. This implies that it may be a plausible candidate for the unknown factor in fish responsible for increased bleeding time. In addition, a recent human study suggested that arsenic homeostasis is altered by hemodialysis, and that low serum arsenic is correlated with central nervous system disorders, vascular disease, and "possibly" cancer.

Diet recommendations: It is inappropriate at present to give dietary recommendations for arsenic for humans because of questions of its essentiality. Based on animal studies, however, amounts of arsenic in the diet that lead to signs of arsenic deficiency can be extrapolated to humans. The suggested arsenic requirement for animals is between 25 and 50 ng As/g (based on diets containing 4000 kcal/kg). Extrapolated to the human population, this dietary intake is equal to 12.5 to 25 µg As/day. Human diets normally contain 12 to 50 µg As/day, and thus the postulated arsenic requirement for humans apparently can be met by food and water normally consumed. However, there may be dietary situations where the requirement for arsenic is not met (for example, low dietary arsenic coupled with an altered methionine metabolism or hemodialysis).

Food sources: Arsenic (inorganic and organic) in the diet is contributed by various foods including cereals and breads, 18.1%; starchy vegetables, 14.9%; and meats and fish, 32.1%. About 20% of the daily intake of arsenic is inorganic. Arsenic in water is almost all inorganic and most drinking water contains less than 10 µg As/L and, typically, 2 to 3 µg/L. Drinking water can contribute 20 µg inorganic As/day.

Toxicity: Toxicity of organic arsenicals, including compounds such as monomethylarsonic acid, dimethylarsenic acid, arsenobetaine, and arsenocholine, is low. Inorganic forms are more toxic than organic forms of arsenic; generally arsenite (+3) is more toxic than arsenate (+5). Signs of inorganic arsenic toxicity include dermatosis, hematopoietic depression, liver damage, sensory disturbances, peripheral neuritis, anorexia, and skin and internal cancers.