Vitamin C: Side Effects, Contraindications

L-ascorbic acid, better known as vitamin C, belongs to the group of water-soluble vitamins. While this feature prevents the body from storing long-term reserves of vitamin C, on the other it limits the risks of overdose and the side effects of a specific supplement. Some data supports these claims:

  • in case of normal dietary intake, vitamin C is absorbed in variable percentages between 70 and 95% of the dose taken. It is important to note that the intestinal absorption capacity decreases as the dose taken increases; for example, if 1,25 grams of vitamin C are administered, the absorbed fraction can drop up to 33%, while in the case of a more contained intake (200 mg) almost all the ingested vitamin C (up to 98%) is absorbed. It follows that:
    • if administered dose = 200 mg → absorbed dose = approximately 190 mg
    • if administered dose = 1250 mg → absorbed dose = approximately 440 mg
  • The kidney contributes to the homeostasis of vitamin C by modulating its excretion and reabsorption based on the body's reserves. There is a maximum threshold of vitamin C in the blood beyond which the kidney is no longer able to reabsorb all the vitamin C filtered from the plasma; as a result, the concentrations of ascorbic acid in the urine begin to increase.
  • For what has been said, it does not make much sense to resort to megadoses of vitamin C (> 2 grams per day), both because the body is unable to retain ascorbic acid, and because such a practice is not entirely free from side effects and contraindications. . Not surprisingly, the food & nutrition board sets the presumably safe maximum level of vitamin C in a healthy 2-year-old individual at 25 grams per day.


  • Vitamin C and kidney stones: Vitamin C does not produce significant changes in urinary pH, but significantly increases the urinary excretion of oxalates (approximately 20-60% at levels of intake of 1-2 grams per day). This occurs both in healthy subjects and in subjects predisposed to the formation of calcium oxalate kidney stones. Therefore, individuals subject to oxalate kidney stones should avoid chronic megadoses of vitamin C, limiting any long-term supplementation to no more than 300 mg per day, which is already more than sufficient.
  • Vitamin C and hemochromatosis: Vitamin C increases the intestinal absorption of dietary iron, keeping it in the reduced Fe2 + state and preventing its oxidation in the less bioavailable form Fe3 +. Although this aspect is particularly advantageous for the general population (given the widespread diffusion of iron deficiency anemia even in subclinical form), it can cause damage to subjects suffering from hemochromatosis or other iron overload disorders.
  • Vitamin C and urinalysis: as we have seen, when vitamin C is taken in high doses it is lost in important concentrations in the urine. All this could falsify the results of urinary tests that use dipstick to estimate the urinary levels of some metabolites. In particular, it has been shown that high levels of vitamin C in the urine can produce false negative results for various analytes - in particular glucose, blood, bilirubin, nitrites, leukocytes and urobilinogen - especially when present in only slightly excessive concentrations. As a result, the person who takes vitamin C in high doses may appear falsely healthy; this can lead to diagnostic delays with progression of the underlying disease. Although this aspect has been known for years and as such at least partially resolved (sticks resistant to the influence of vitamin C, sticks that signal the presence of vitamin C suggesting to repeat the test), it is important that:
    • before urinalysis, at least 10 hours should elapse after the last intake of vitamin C (fruit, vegetables, medicines). For safety, vitamin C supplementation should be stopped the day before urinary analysis.

Effetti collaterali

  • The side effects of vitamin C are mostly gastrointestinal, and appear in case of high intake:
    • The pure form, ascorbic acid, may not be pleasant on the palate due to the bitter taste. Moreover, due to its acidity, it can be poorly tolerated in the stomach, especially by subjects suffering from gastritis and stomach acidity. The main advantage of pure ascorbic acid is that it is the most concentrated form, in which each gram of product contains one gram of vitamin C. However, especially when taken in particularly generous doses (> 2 grams / day), ascorbic acid pure it can cause stomach upset and diarrhea.
    • To improve the gastric tolerability of vitamin C, L-ascorbic acid is salified with minerals such as potassium, calcium and sodium, giving rise to the relative salts (calcium ascorbate, sodium ascorbate, potassium ascorbate ...) . If you choose to take vitamin C in salified form, you need to consider two aspects:
      1. how much elemental vitamin C is present in each gram of ascorbate:
        • sodium ascorbate: 889 mg of ascorbic acid and 111 mg of sodium
        • potassium ascorbate: 825 mg of ac. ascorbic acid and 175 mg of potassium
        • calcium ascorbate: 886 mg of ascorbic acid and 114 mg of calcium
      1. what effects does the mineral associated with vitamin C produce in the body
        • sodium, although essential, is generally consumed in excess; those who take megadoses of vitamin C (> 2 g / day) should avoid sodium ascorbate in case of hypertension and a low-sodium diet
        • potassium does not cause problems for healthy people, but it can be particularly dangerous for individuals with kidney disease or on potassium-sparing diuretics.
        • calcium can help in the prevention of osteoporosis and at similar dosages it has no particular side effects and contraindications, not even for people suffering from kidney stones. In sensitive individuals, the most important side effect of calcium supplements is the onset of constipation.
          In general, the overall dietary calcium (diet + supplements) should not exceed 2000-2500 mg per day.
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