Who Should Not Take Vitamin C Supplements
Vitamin C is often treated as the safest supplement there is, and for most people modest amounts are indeed harmless. But high-dose products are not suitable for everyone, and for athletes there is an additional twist: large doses may interfere with training adaptations. Our editorial team outlines who should be cautious and why.
Why vitamin C is usually safe
Vitamin C (ascorbic acid) is a water-soluble vitamin needed for collagen formation, immune function, iron absorption from plant foods and antioxidant defence. The UK reference nutrient intake for adults is 40 mg per day, an amount easily achieved by eating fruit and vegetables.
When intake rises, absorption becomes less efficient and excess is excreted in urine. This self-limiting pattern explains why vitamin C has a reputation for safety.
The UK Expert Group on Vitamins and Minerals set a guidance level of 1,000 mg per day of supplemental vitamin C, above which gastrointestinal symptoms such as diarrhoea and bloating become more common. EFSA has not set a formal upper limit, citing insufficient data.
However, "safe for most" leaves room for groups in whom regular high doses can cause harm or interact with conditions and treatments.
People prone to kidney stones
Part of vitamin C is metabolised to oxalate, which is excreted by the kidneys. Higher oxalate in urine increases the risk of calcium oxalate stones, the most common type of kidney stone.
In a Swedish cohort study, Thomas and colleagues found that men taking vitamin C supplements had approximately double the risk of kidney stones compared with non-users. Ferraro and colleagues, analysing large US cohorts, reported that supplemental vitamin C was associated with a higher risk of stones in men, while dietary vitamin C from food was not.
People with a history of kidney stones, particularly calcium oxalate stones, are generally advised to avoid high-dose vitamin C supplements. Those with conditions that increase oxalate levels, such as some bowel diseases, should also be careful.
Athletes who sweat heavily and become dehydrated already face an increased stone risk, which makes good hydration particularly important if they choose to supplement.
Iron overload and G6PD deficiency
Vitamin C enhances absorption of non-haem iron. For most people this is useful, but for those with hereditary haemochromatosis, a condition causing iron to accumulate in organs, it may worsen overload. A review by Fleming and Ponka in the New England Journal of Medicine discusses the dangers of iron overload, and people with haemochromatosis are typically advised to avoid vitamin C supplements, especially with meals.
Haemochromatosis is relatively common in people of Northern European ancestry, including in the UK and Ireland, and many people do not know they have it. Unexplained fatigue, joint pain and raised ferritin are reasons to ask for testing.
Glucose-6-phosphate dehydrogenase (G6PD) deficiency is an inherited enzyme disorder that makes red blood cells vulnerable to oxidative stress. Very high doses of vitamin C, particularly intravenous, have been associated with haemolysis in people with this condition, as described in a review by Cappellini and Fiorelli.
People of African, Mediterranean, Middle Eastern and Asian heritage are more often affected by G6PD deficiency. Anyone with a known diagnosis should consult a doctor before taking high-dose supplements.
Kidney disease, medicines and other situations
People with chronic kidney disease cannot eliminate oxalate efficiently, raising the risk of oxalate deposits in the kidneys and other tissues. High-dose vitamin C supplements are usually not recommended for them unless advised by a renal specialist.
| Group | Main concern | General advice |
|---|---|---|
| History of kidney stones | Increased urinary oxalate | Avoid high-dose supplements |
| Chronic kidney disease | Oxalate accumulation | Only under specialist guidance |
| Haemochromatosis | Greater iron absorption | Avoid supplements, especially with meals |
| G6PD deficiency | Risk of haemolysis at very high doses | Medical advice before use |
| Cancer treatment | Possible interference with some therapies | Discuss with oncology team |
Vitamin C may interfere with some laboratory tests, including certain glucose measurements, especially at very high doses. People who monitor glucose should check with their healthcare team.
Anyone undergoing chemotherapy or radiotherapy should discuss antioxidant supplements with their oncology team, as the interaction with treatment is debated.
Pregnant women can generally meet needs from food or a standard pregnancy supplement and should not take high-dose products without advice.
Athletes and high-dose antioxidants
For healthy athletes, the main question is not toxicity but training adaptation. Exercise generates reactive oxygen species, which act as signals that stimulate adaptations such as new mitochondria.
Gomez-Cabrera and colleagues found that 1 g of vitamin C daily reduced training-induced mitochondrial biogenesis markers and hampered gains in endurance capacity. Paulsen and colleagues reported that vitamin C combined with vitamin E blunted some cellular adaptations to endurance training, although performance measures were not consistently affected.
- Use food first for vitamin C; fruit and vegetables also provide fibre and polyphenols.
- Reserve higher doses for specific short periods, such as heavy travel schedules, after discussion with a sports dietitian.
- Avoid habitual gram-level doses during key adaptation phases of training.
Some studies suggest short-term vitamin C may reduce the risk of colds in people under heavy physical stress, so the decision involves a trade-off rather than a simple rule.
The editorial team's conclusions
Vitamin C from food is safe for virtually everyone, but high-dose supplements are not suitable for people with kidney stones, kidney disease, haemochromatosis or G6PD deficiency.
Athletes should be aware that habitual high doses of antioxidants may blunt some training adaptations.
When in doubt, aim to meet needs through a varied diet and seek professional advice before taking supplements.
Related reading: how vitamin C affects iron absorption, antioxidant supplements and recovery, and hydration strategies for preventing kidney stones.
References
- Thomas LD, Elinder CG, Tiselius HG, Wolk A, Akesson A. Ascorbic acid supplements and kidney stone incidence among men: a prospective study. JAMA Intern Med. 2013;173(5):386–388.
- Ferraro PM, Curhan GC, Gambaro G, Taylor EN. Total, dietary, and supplemental vitamin C intake and risk of incident kidney stones. Am J Kidney Dis. 2016;67(3):400–407.
- Cappellini MD, Fiorelli G. Glucose-6-phosphate dehydrogenase deficiency. Lancet. 2008;371(9606):64–74.
- Fleming RE, Ponka P. Iron overload in human disease. N Engl J Med. 2012;366(4):348–359.
- Gomez-Cabrera MC, Domenech E, Romagnoli M, Arduini A, Borras C, Pallardo FV, et al. Oral administration of vitamin C decreases muscle mitochondrial biogenesis and hampers training-induced adaptations in endurance performance. Am J Clin Nutr. 2008;87(1):142–149.
- Paulsen G, Cumming KT, Holden G, Hallén J, Rønnestad BR, Sveen O, et al. Vitamin C and E supplementation hampers cellular adaptation to endurance training in humans: a double-blind, randomised, controlled trial. J Physiol. 2014;592(8):1887–1901.
- Expert Group on Vitamins and Minerals. Safe upper levels for vitamins and minerals. London: Food Standards Agency; 2003.
