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
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.