Key points
Why the body needs vitamin C
Vitamin C (ascorbic acid) is needed to make collagen — the protein of skin, blood vessels and bones — and carnitine, which helps cells get energy from fats. It takes part in forming noradrenaline and serotonin, works as an antioxidant and protects vitamin E and folic acid from oxidation (Burgerstein, Handbuch Nährstoffe 2015, p. 84–86).
Vitamin C and the common cold
Vitamin C and iron
Iron from meat and fish (heme iron) is absorbed well, iron from plant foods (non-heme iron) less well. Here vitamin C is a strong enhancer, and it also weakens the inhibiting effect of tea and calcium (Lynch SR Ann N Y Acad Sci 1980. PMID 6940487[2]). The handbook says that vitamin C considerably increases iron absorption (Burgerstein, Handbuch Nährstoffe 2015, p. 85, 89, 114).
How much you need and where to get it
What the studies showed
| What was tested | Who took part | Result |
|---|---|---|
| Regularly, 200 mg a day or more, Cochrane 2013 | 10,708 people from the general population | Risk of getting sick barely lowered (0.97) |
| The same under heavy exertion | 598 marathon runners, skiers, soldiers | Risk of a cold roughly halved (0.48) |
| Duration of a cold with regular intake | 9745 cold episodes | Adults: −8%, children: −14% |
| Starting at the first symptoms | 3249 cold episodes | No consistent effect shown |
| Iron with vitamin C (200 mg every 8 hours) and without, 2020 | 440 adults with anemia, nearly all women | Rise in hemoglobin and ferritin comparable |
| Vitamin C and kidney stones, 2016 | 156,735 women and 40,536 men, about 11 years | Men: 500–999 mg — risk +29%, 1000 mg or more — +43% (association); in women no association found |
| 1000 mg twice a day for 6 days, 2005 | 48 people with and without past stones | Urinary oxalate and the calculated stone risk index rose in 40% |
Not everything is equally proven: taking it from the start of a cold has been studied little (Hemilä H Cochrane Database Syst Rev 2013. PMID 23440782[1]), and the data on stones come from an observational study — this is an association, not a proven cause (Ferraro PM Am J Kidney Dis 2016. PMID 26463139[6]).
How much is safe
The tolerable upper intake level in the USA is 2000 mg a day (Institute of Medicine 2000). The European agency EFSA has not set an upper level for vitamin C (EFSA, Tolerable Upper Intake Levels for Vitamins and Minerals 2006). Signs of excess, according to the handbook, are nausea, bloating and diarrhea (Burgerstein, Handbuch Nährstoffe 2015, p. 91).
Who should be more careful
- People who have already had kidney stones: at 1000 mg twice a day urinary oxalate rose in 40% of participants (Massey LK J Nutr 2005. PMID 15987848[10]); in men, high intake is associated with stones (Ferraro PM Am J Kidney Dis 2016. PMID 26463139[6]).
- People with iron overload, for example in haemochromatosis: vitamin C enhances iron absorption (Lynch SR Ann N Y Acad Sci 1980. PMID 6940487[2]). More in the article on haemochromatosis.
- People who take anticoagulants: according to the handbook, high doses of vitamin C may weaken their effect (Burgerstein, Handbuch Nährstoffe 2015, p. 91).
- People who let vitamin C dissolve in the mouth or take it as a powder: the acid erodes tooth enamel, and the handbook advises rinsing the mouth (Burgerstein, Handbuch Nährstoffe 2015, p. 88).
Bottom line
Vitamin C is needed for collagen and iron absorption, and fruit and vegetables usually provide enough of it. Regular intake makes a cold slightly shorter, and during short periods of heavy exertion it halves the risk of getting sick. In men, large doses are associated with kidney stones. You can choose a dose that takes iron, kidneys and medications into account at a consultation.
This material is for information only and does not replace a doctor’s consultation. The dose of vitamin C and the need for tests are determined by a doctor.
References
- Hemilä H Cochrane Database Syst Rev 2013. PMID 23440782
- Lynch SR Ann N Y Acad Sci 1980. PMID 6940487
- EFSA NDA Panel EFSA J 2014. PMID 42125126
- Carr AC J Nutr Sci 2012. PMID 25191543
- Levine M Proc Natl Acad Sci U S A 1996. PMID 8623000
- Ferraro PM Am J Kidney Dis 2016. PMID 26463139
- Cook JD Am J Clin Nutr 2001. PMID 11124756
- Li N JAMA Netw Open 2020. PMID 33136134
- Lykkesfeldt J Food Nutr Res 2023. PMID 38187788
- Massey LK J Nutr 2005. PMID 15987848
Key facts
- Regular vitamin C intake of 200 mg a day or more shortened the duration of the common cold by 8% in adults and by 14% in children (Hemilä H, Cochrane Database Syst Rev 2013).
- In marathon runners, skiers and soldiers on subarctic exercises, regular vitamin C lowered the risk of a cold roughly by half: relative risk 0.48 (Hemilä H, Cochrane Database Syst Rev 2013).
- Vitamin C started only at the first symptoms showed no consistent effect on the duration or severity of a cold in 7 comparisons (Hemilä H, Cochrane Database Syst Rev 2013).
- In 15 young men with low vitamin C levels, one gold kiwifruit a day was enough to raise the blood level above 50 µmol/L (0.88 mg/dL) — a healthy level (Carr AC, J Nutr Sci 2012).
- In men, a total vitamin C intake of 500–999 mg a day was associated with a 29% higher risk of kidney stones, and 1000 mg or more with a 43% higher risk, than an intake below 90 mg; in women no association was found (Ferraro PM, Am J Kidney Dis 2016).
