Vitamin C

• Vit. C is a water-soluble antioxidant vitamin that acts as an electron donor for numerous biochemical reactions in the body. Vitamin C plays important roles as a cofactor for enzymes involved in collagen hydroxylation, plus carnitine and catecholamine biosynthesis. Vit. C also aids in iron absorption. A deficiency of Vit. C is rare given relatively low recommended dietary intake (45 mg per day) and wide distribution in fresh fruit and vegetables.
  • As an antioxidant, Vit. C reacts with potentially damaging reactive oxygen species (ROS) and reactive nitrogen species (RNS) and has beenshown to protect plasma lipids against oxidative damage.¹ Vit. C also strengthens the cellular antioxidant network by helping to maintain Vit. E and glutathione levels.^2-4
  • Vit. C is very labile and thus content in food varies according to season, transport, shelf life and storage time, cooking practices and chlorination of water. Cutting, bruising, heating and exposure to copper, iron or mildly alkaline conditions can destroy ascorbate. It can also be leached into water during cooking.⁵
• Vit. C may enhance immune function through effects on epithelial barriers, white blood cells and inflammatory mediators.6 Through enhancement of immune cell function, Vit. C may be able to prevent and treat respiratory and systemic infections.⁶ Vit. C supplements are promoted to reduce the duration and severity of colds, although the evidence supporting this is mixed:
  • Vit. C supplementation (200-2000 mg per day) has been found to reduce the duration of the common cold by 8% and reduce the severity of cold symptoms.⁷ Neither acute nor chronic Vit. C supplementation influences the incidence of upper respiratory tract infections.
  • Some studies in military personnel and school boarders have found a reduction in incidence of pneumonia by 80-100% with Vit. C supplementation⁸, although findings of these studies cannot be extrapolated to other groups.
• Vit. C may have benefits for athletes undertaking intense exercise. During exercise, our muscles produce increased amounts of ROS and RNS. Excess ROS and RNS can promote damage to proteins, lipids, and DNA, and potentially impair physical performance, recovery, and immune function. Vit. C supplements may act to neutralize some of the damaging effects of exercise-induced ROS and RNS, although studies report mixed findings on exercise-related outcomes:
  • A systematic review found that regular Vit. C supplementation (250-1000 mg per day) reduced the risk of the common cold by over 50% in athletes such as marathon runners and skiers who are exposed to short periods of extreme physical stress.7
  • Some evidence from randomized controlled trials indicate Vit. C supplementation (500-2000 mg per day) can prevent exercise-induced bronchoconstriction.⁹
  • Studies of effects of Vit. C supplementation on muscle function following a bout of intense fatiguing exercise have yielded mixed findings^10-15, drawing into question its use for recovery of muscle function.
  • There is compelling evidence to suggest chronic ingestion of single high dose antioxidants such as Vit. C (1000 mg per day for 8 weeks) can impede training adaptations¹⁶, yet when the same amount of Vit C is ingested via while food sources, performance may actually improve.¹⁷ Collectively this supports a food first approach to achieving Vit. C and other antioxidant nutrient needs, except in unique circumstances

• Vit. C is found naturally in a wide range of fruits and vegetables, including citrus fruits, berries, kiwifruit, tomatoes, broccoli, potatoes, capsicum, and sprouts. Vit. C food sources are presented in Table¹.
• Vit. C (as ascorbic acid and/or sodium ascorbate) is also widely available in oral supplement forms including capsules, tablets, powders or drops. Vit. C may also be intravenously infused if medically indicated. Oral Vit. C powder (as ascorbic acid) is white to off-white or light yellow in colour.
• On average, Australian adults consume approximately 110 mg Vit. C per day5, ~40% of which comes from vegetables, 19% from fruit and 27% from fruit and vegetable juices. Oral Vit. C supplements typically contain Vit. C in the range of 200 – 2000 mg per capsule or tablet.

• The integration of unprocessed, Vit. C rich foods into the daily meal plan is key to achieving not only daily Vit. C needs, but other important nutrients like fibre and phytochemicals.
• Only when directed by a sports dietitians or sports doctor, should an athlete consider acute Vit. C supplementation.
• A daily total dose of between 500-1000 mg supplemental Vit. C may be safe to consume acutely (during illness) to support immune health for most athletes undertaking intense exercise.

Table 1: Dietary sources of vitamin C

Possible impairment of exercise training adaptations

Some of the biological adaptations to training are stimulated by exercise-induced production of ROS and RNS. Antioxidant supplements that act to reduce ROS and RNS may therefore blunt these signals and make the training process less effective. Current uncertainty of evidence warrants an athlete discussing their training and performance goals with their coach and sports dietitian to manage the potential trade-off between possible acute immune-related benefits of Vit. C supplementation and possible impairments in training-induced adaptations. Achieving daily Vit. C intake goals via whole food sources should be a priority.

Side effects at higher doses

Gastrointestinal effects such as bloating and osmotic diarrhoea are the most common adverse effects associated with high doses of Vit. C (i.e. 2-6 g per day) given over a short period of time.¹⁹ However, these effects are attenuated through reduction of intake and adaptation to increased doses. International bodies have imposed a prudent upper limit of intake of 1000 mg Vit. C per day based on these side effects.²⁰

Increased risk of kidney stones or worsening kidney function

There is a concern that high dose Vit. C supplements might promote kidney stones. However, studies in humans using doses between 30 mg and 10 g per day have provided conflicting results^21,22 and it is unclear if Vit. C plays a role in kidney stone formation. Nonetheless, it might be prudent to limit intake to <1000 mg Vit. C per day in individuals who are known kidney stone formers.²³ High dose Vit. C is probably contraindicated in patients with existing hyperoxaluria and end stage renal disease.²²

Excess iron absorption in genetically prone individuals

Concerns have been raised in relation to use of high dose Vit. C supplements by individuals with genetic iron-overload disorders (e.g. haemochromatosis). Since Vit. C is a known enhancer of dietary iron absorption, it has been suggested that excessively high iron levels could damage the liver and heart and promote diabetes. Moderation (or omission) of supplemental Vit. C intake to no more than 500 mg per day is prudent for those individuals with genetic iron overload disorders.²⁴ On the contrary, co-ingestion of 50-100 mg Vit. C with non -haem iron sources significantly increases iron absorption, and thus may be an important to include fresh fruit and vegetables at most meals od the day amongst those with a history of impaired iron status.

Supplement safety information www.sportintegrity.gov.au/what-we-do/anti-doping/supplements-sport, opens in a new tab

1. Frei, B. (1991). Ascorbic acid protects lipids in human plasma and low-density lipoprotein against oxidative damage. Am J Clin Nutr, 54(6 Suppl), 1113s-1118s.
2. Halpner, A. D., Handelman, G. J., Harris, J. M., Belmont, C. A., & Blumberg, J. B. (1998). Protection by vitamin C of loss of vitamin E in cultured rat hepatocytes. Arch Biochem Biophys, 359(2), 305-309.
3. Henning, S. M., Zhang, J. Z., McKee, R. W., Swendseid, M. E., & Jacob, R. A. (1991). Glutathione blood levels and other oxidant defense indices in men fed diets low in vitamin C. J Nutr, 121(12), 1969-1975.
4. Johnston, C. S., Meyer, C. G., & Srilakshmi, J. C. (1993). Vitamin C elevates red blood cell glutathione in healthy adults. Am J Clin Nutr, 58(1), 103-105.
5. Australian Bureau of Statistics (ABS). Australian Health Survey: Nutrition First Results - Foods and Nutrients, 2011-12. Canberra: ABS; 2014 May 9. Report No.: 4364.0.55.007. Available from: http://www.abs.gov.au/ausstats/abs@.nsf/mf/4364.0.55.007?OpenDocument, opens in a new tab
6. Carr, A. C., & Maggini, S. (2017). Vitamin C and Immune Function. Nutrients, 9(11), 1211.
7. Hemilä, H., & Chalker, E. (2013). Vitamin C for preventing and treating the common cold. Cochrane Database of Systematic Reviews, 1.
8. Hemilä, H. 2004. Vitamin C supplementation and respiratory infections: a systematic review. Mil Med, 169(11), 920-925.
9. Hemilä, H. 2014. The effect of vitamin C on bronchoconstriction and respiratory symptoms caused by exercise: a review and statistical analysis. Allergy Asthma Clin Immunol, 10(1), 58.
10. Jakeman, P., & Maxwell, S. (1993). Effect of antioxidant vitamin supplementation on muscle function after eccentric exercise. Eur J Appl Physiol Occup Physiol, 67(5), 426-430.
11. Thompson, D., Williams, C., McGregor, S. J., Nicholas, C. W., McArdle, F., Jackson, M. J., & Powell, J. R. (2001). Prolonged vitamin C supplementation and recovery from demanding exercise. Int J Sport Nutr Exerc Metab, 11(4), 466-481.
12. Bryer, S. C., & Goldfarb, A. H. (2006). Effect of high dose vitamin C supplementation on muscle soreness, damage, function, and oxidative stress to eccentric exercise. Int J Sport Nutr Exerc Metab, 16(3), 270-280.
13. Connolly, D. A., Lauzon, C., Agnew, J., Dunn, M., & Reed, B. (2006). The effects of vitamin C supplementation on symptoms of delayed onset muscle soreness. J Sports Med Phys Fitness, 46(3), 462-467.
14. Thompson, D., Bailey, D. M., Hill, J., Hurst, T., Powell, J. R., & Williams, C. (2004). Prolonged vitamin C supplementation and recovery from eccentric exercise. Eur J Appl Physiol, 92(1-2), 133-138.
15. Close, G. L., Ashton, T., Cable, T., Doran, D., Holloway, C., McArdle, F., & MacLaren, D. P. (2006). Ascorbic acid supplementation does not attenuate post-exercise muscle soreness following muscle-damaging exercise but may delay the recovery process. Br J Nutr, 95(5), 976-981.
16. Gomez-Cabrera, M., Domenech, E., Romagnoli, M., Arduini, A., Borras, C., Pallardo, F., Sastre, J., Viña, J. (2008). Oral administration of vitamin C decreases muscle mitochondrial biogenesis and hampers training-induced adaptations in endurance performance. Am J Clin Nutr, 87(1), 142-9.
17. Braakhuis AJ, Hopkins WG, Lowe TE. (2014). Effects of dietary antioxidants on training and performance in female runners. Eur J Sport Sci, 14(2), 160-8.
18. Padayatty, S., Katz, A., Wang, Y., Eck, P., Kwon, O., Lee, J.-H., . . . Levine, M. (2003). Vitamin C as an Antioxidant: Evaluation of Its Role in Disease Prevention. J Am Coll Nutr, 22, 18-35.
19. Naidu, K. A. (2003). Vitamin C in human health and disease is still a mystery? An overview. Nutr J, 2, 7.
20. National Health and Medical Research Council (NHMRC), Australian Government Department of Health and Ageing, New Zealand Ministry of Health. Nutrient Reference Values for Australia and New Zealand.  Canberra: National Health and Medical Research Council; 2006.
21. Massey, L. (2005). Safety of vitamin C. Am J Clin Nutr, 82(2).
22. Robitaille, L., Mamer, O. A., Miller, W. H., Jr., Levine, M., Assouline, S., Melnychuk, D., . . . Hoffer, L. J. (2009). Oxalic acid excretion after intravenous ascorbic acid administration. Metabolism, 58(2), 263-269.
23. Traxer, O., Huet, B., Poindexter, J., Pak, C. Y., & Pearle, M. S. (2003). Effect of ascorbic acid consumption on urinary stone risk factors. J Urol, 170 (2 Pt 1), 397-401.
24. Barton, J. C., McDonnell, S. M., Adams, P. C., Brissot, P., Powell, L. W., Edwards, C. Q., . . . Kowdley, K. V. (1998). Management of hemochromatosis. Hemochromatosis Management Working Group. Ann Intern Med, 129(11), 932-939.