Vitamin C, or L-ascorbic acid, is a water-soluble essential vitamin. It is a very popular dietary supplement due to its antioxidant properties, safety, and low price.
Vitamin C is often supplemented to reduce the symptoms of the common cold. However, vitamin C is unable to reduce the frequency of colds in a healthy population. An athlete who frequently undergoes intense physical activity can expect to cut the risk of getting a cold in half. Supplemental vitamin C is able to reduce the duration of a cold by 8-14% in any population, when it is taken as a daily preventative measure, or at the beginning of a cold. Though superloading vitamin C (5-10g daily) is said to be more effective, further research is needed to determine the accuracy of this claim.
Vitamin C is capable of being both an antioxidant and pro-oxidant, depending on what the body needs. Vitamin C also required for various biological processes, including the synthesis of collagen, L-carnitine, and norepinephrine (aka noradrenaline). These functions mean vitamin C may have beneficial effects on brain health, depression, cortisol levels, pancreatic health, and blood flow. By protecting the testes from oxidative stress, vitamin C may also preserve testosterone levels.
Vitamin C is often supplemented as a way of reducing the risk of cardiovascular disease, however existing evidence suggests it is generally ineffective for this purpose.
According to the WHO, most adults need only 45 mg of vitamin C per day, but more recent research has shown this number to be too low. The minimum has been set at 75 mg for women and 90 mg for men in the U.S., and 95 mg for women and 110 mg for men in the E.U.
Moreover, a 2022 study using 110 mg as its starting number recommended adding 10 mg per 10 kg (22 lb) above 60 kg (132 lb) of body weight.
Any of these numbers are easily attained through the diet, so supplementation of such low doses is usually unnecessary. Higher doses of vitamin C, up to 2,000 mg, are used to support the immune system (for athletes) or reduce the duration of the common cold.
Most studies on vitamin C prescribe one dose per day. The claim that taking 2,000 mg up to five times a day to optimally reduce cold symptoms is not sufficiently tested and requires more evidence.