As a group, these vitamins tend to have fewer toxic effects compared with fat-soluble vitamins, mainly because water-soluble vitamins are not retained in the body to the same extent as fat-soluble vitamins. When these vitamins reach urinary threshold levels, the excess is eliminated rapidly in the urine. Thus, discernible harm to the individual can be avoided because of these elimination factors preventing accidental or deliberate ingestion of water-soluble vitamins. Nevertheless, adverse effects associated with excessive intakes of some water-soluble vitamins are being reported with more frequency, which increases the need for more judicious use of supplements and for taking precautions to avoid adverse effects.
Side effects are associated with large therapeutic doses of niacin. Nicotinic acid in large doses (100 to 300 mg oral or 20 mg intravenous) can result in vasodilative effects. Symptoms include flushing reaction, cramps, headache, and nausea. Therapeutic levels of niacin have been used successfully to reduce serum cholesterol, but with other reversible side effects such as pruritis, desquamation, and pigmented dermatosis. On the other hand, high doses of nicotinamide (used to therapeutically treat niacin deficiency) have no side effects. Nicotinamide does not lower serum cholesterol.
The major concern with the use of high levels of folic acid is the masking pernicious anemia resulting from vitamin B12 deficiency. Prolonged masking can delay recognition of the neurological aspects of pernicious anemia arising from vitamin B12 deficiency, which if unnoticed can lead to severe neurological damage. High doses (>15 mg) of folic acid are associated with GI disturbance, irritability, malaise, hyperexcitability, disturbed sleep, and vivid dreams.
Vitamin B6 (Pyridoxine)
Pyridoxal hydrochloride demonstrates oral toxicity at ca. 2 g/kg body weight, or 20 times the therapeutic recommended dose, for peripheral neuritis. Such doses result in convulsive disorder and inhibition of prolactin secretion.
The remaining water-soluble B vitamins, pantothenic acid and thiamin, show few adverse effects, certainly none from dietary sources.
As regards the toxic effects of minerals and trace elements, the absolute level of intake is not the only circumstance involved with either acute or chronic toxicity. Toxic intake levels can vary considerably with individual circumstances. An element that is easily stored may accumulate in tissues over time, and therefore ingestion of a lower concentration may produce a toxic effect that would not occur in an indi vidual without any prior exposure. Other circumstances that may influence the toxicity of a mineral or trace element include absorption and excretion factors, immobilization or storage of the toxic element (bone storage), and detoxification mechanisms.
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