Ascorbic acid

What it is

Ascorbic acid is water-soluble vitamin C, which humans cannot synthesise and must take in from the diet. It is a required cofactor for collagen formation and acts as an antioxidant and a reducing agent in many enzyme reactions, including several in immune cells. It is used to correct or prevent deficiency and is taken for immune support.

A usual adult range, not a national label — the dose and the timing differ by country and by product.

Used for

• Deficiency
• Immune support

Approved uses differ by country — some of these may be off-label or unapproved where you live.

What to know while taking it

It is taken with food, which softens the stomach upset that the acid can cause. Ascorbic acid keeps dietary iron in its reduced, better-absorbed form, so it usefully increases iron absorption from a meal or from an iron supplement, an effect worth knowing about in either direction. Excess is cleared by the kidneys, partly as oxalate, and a sustained high intake therefore raises the risk of kidney stones, which is the main reason to keep intake moderate rather than open-ended.

When to see a doctor

Flank or back pain, painful urination or blood in the urine should be assessed by a doctor. Anyone with a history of kidney stones or kidney disease, or who is taking iron, should discuss vitamin C intake with a doctor first.

Worth knowing

• Raises kidney stone risk
• Improves iron absorption