How we rate
“Strong” = several concordant meta-analyses or systematic reviews. “Moderate” = one synthesis, or several controlled trials. “Limited” = early trials only. The rating is about the quality of the studies, not about how useful the supplement is.
Vitamin C
An antioxidant that the human body cannot produce, involved in immunity, fatigue and iron absorption.
Benefits
- Halves respiratory infections after a very long effort: 33% of supplemented ultra-marathoners versus 68% on placebo
- Lowers blood uric acid, a marker linked to gout (13 trials, 556 participants)
- Reduces muscle soreness within 24 hours following a highly damaging training session, at a high dose
- Shortens respiratory infections in hospitalized elderly people
Precautions
At high doses (over 1,000 mg per day): digestive issues, and increased risk of kidney stones in predisposed individuals. May skew certain capillary blood glucose tests. Also good to know: most Westerners are already saturated in vitamin C through their diet, and in them trials show no benefit — the gain mainly concerns truly deficient individuals, smokers and extreme endurance efforts.
Recommended dosage
200 to 500 mg per day in most trials; for most Westerners, diet already provides enough.
How to take it
- You probably get enough: for most Westerners, diet already saturates the body with vitamin C, and a supplement may have no effect.
- Trial doses: 200 to 500 mg per day most often. 200 mg helped elderly patients hospitalized for a respiratory infection, especially the most deficient ones.
- For a very long effort: 600 mg per day halved respiratory infections after an ultramarathon (33% versus 68%). With 1,500 mg for 7 days, no difference on immunity in another trial.
- How long: on blood glucose, the effect mainly appears beyond 30 days of intake, in type 2 diabetics.
- On blood pressure: a decrease in short-term trials, but no effect over 5 years at 500 mg per day.
What the studies say
This systematic review and meta-analysis of 28 randomized controlled trials evaluated the efficacy of oral vitamin C supplementation on glycemic control and cardiovascular risk factors in individuals with type 2 diabetes. Vitamin C supplementation showed statistically and clinically significant changes in systolic blood pressure and HbA1c. Although evidence suggests improvements in glycemic control and blood pressure, larger, long-term, high-quality trials are needed before recommending vitamin C as a therapy.
Mason SA, Keske MA, Wadley GD. — Diabetes care (2021) — View the source study
For information only: not a substitute for medical advice. Dietary supplements are not a replacement for a varied, balanced diet. Learn more