Contraindications And Precautions

In patients who are sensitive to iron overload, vitamin C supplementation may exacerbate iron toxicity by mobilising iron reserves. As such, vitamin C supplementation should be used with caution by people with erythrocyte glucose-6-phosphate dehydrogenase deficiency, haemochromatosis, thalassaemia major or sideroblastic anaemia.

Supplemental vitamin C can affect the results of numerous laboratory tests and n~ „ ,

should be stopped prior to:

• carbamazepine

• lactate dehydrogenase

• serum bicarbonate

• serum cholesterol

• serum creatinine

• serum creatine kinase

• serum phosphate

• serum triglycerides

• serum urea nitrogen

• theophylline

• urine 17-hydroxy corticosteroids

• urine 17-ketosteroids

• urine amphetamine

• urine and serum bilirubin

• urine and serum glucose

• urine and serum uric acid.

• urine barbiturate

• urine beta-hydroxybutyrate

• urine oxalate

• urine paracetamol

• urine protein

Clinical note — Is the kidney stone risk overstated?

Although widely assumed to increase the risk of kidney stones, recent studies suggest high-dose vitamin C supplementation does not significantly increase this risk. In 1994, researchers discovered that vitamin C (in doses as high as 10 g/day) does not increase the amount of oxalate produced in the body (Wandzilak et al 1994). Instead, urine testing used to detect oxalate levels were found to actually be detecting oxalate formed by the conversion of ascorbate during the test procedure. As such, increased urine oxalate as tested by this method does not genuinely represent in vivo oxalate levels when ascorbate is involved. Two prospective studies of over 85,000 women and 45,000 men found that doses ranging from less than 250 mg/day to more than 1 500 mg/day taken over 6-14 years did not correlate with occurrence of kidney stones (Curhan et al 1996, 1999). These two large studies

further support the lack of association between vitamin C supplements and kidney stones.

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