Vitamin C: Side Effects, Contraindications

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

in the case of normal dietary intake, vitamin C is absorbed in percentages varying between 70 and 95% of the dose taken. It is important to note that the capacity for intestinal absorption decreases with increasing dose; for example, if 1.25 grams of vitamin C are administered, the absorbed fraction can drop to 33%, while in the case of a lower intake (200 mg) almost all the ingested vitamin C (up to 98%) is absorbed. It follows that:
- if administered dose = 200 mg → absorbed dose = about 190 mg
- if administered dose = 1250 mg → absorbed dose = approximately 440 mg
The kidney contributes to the homeostasis of vitamin C by modulating excretion and resorption according to 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 plasma-filtered vitamin C; consequently, concentrations of ascorbic acid in the urine begin to increase.
According to 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 devoid of side effects and contraindications . Not surprisingly, the food & nutrition board sets the maximum level of vitamin C presumably safe in a healthy 25-year-old at 2 grams per day.

Contraindications

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 intake levels of 1-2 grams per day). This occurs both in healthy subjects and in subjects predisposed to the formation of renal calcium oxalate stones. Therefore, individuals subjected to oxalate kidney stones should avoid chronic megadoses of vitamin C, limiting any long-term integration 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 food iron, keeping it in the reduced state Fe2 + and preventing its oxidation in the Fe3 + form, less bioavailable. 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 urine analysis : as we have seen, when vitamin C is taken at high doses it is lost in important concentrations with urine. All this could distort the results of urinary analyzes that use reactive papers (dipsticks) to estimate the urinary levels of some metabolites. In particular, it has been shown that high levels of vitamin C in 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 high-dose vitamin C may appear falsely healthy; this can cause diagnostic delays with progression of the underlying disease. Although this aspect has been known for years and as such at least partially solved (sticks resistant to the influence of vitamin C, sticks that indicate the presence of vitamin C suggesting to repeat the test), it is important that:
- before the urine test, at least 10 hours should elapse after the last vitamin C intake (fruit, vegetables, medicines). For safety, vitamin C supplementation should be suspended the day before the urinary analysis.

Side effects

The side effects of vitamin C are mostly gastro-intestinal, and appear in case of high intake:
- The pure form, ascorbic acid, may be unpleasant to the palate due to the bitter taste. Moreover, due to its acidity, it can be badly tolerated at the gastric level, especially by subjects suffering from gastritis and stomach acid. The main advantage of pure ascorbic acid is that it is the most concentrated form, in which each gram of product contains a gram of vitamin C. However, especially when taken in particularly generous doses (> 2 grams / day) ascorbic acid pure can cause gastric 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 its salts (calcium ascorbate, sodium ascorbate, potassium ascorbate ...) . If you choose to take vitamin C in a salified form, you need to consider two aspects:
  1. how much elementary vitamin C is present in every gram of ascorbate:
  1. what effects does the mineral associated with vitamin C produce in the body