Are there any concerns or considerations?

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.