On The Shelf

Mary Warner

Vitamin C, also known as L-ascorbic acid, is a water-soluble vitamin that is naturally present in some foods, added to others, and available as a dietary supplement. Ascorbic acid is a powerful antioxidant and can decrease inflammation, which may help improve immune function. Vitamin C has long been thought to shorten the duration of the common cold, which may explain the claims that high doses can protect against or shorten the duration of COVID-19. 

Function

Humans, unlike most animals, are unable to synthesize vitamin C endogenously, making the vitamin an essential dietary component. Vitamin C is required for the biosynthesis of collagen, L-carnitine, and some neurotransmitters, and is involved in protein metabolism.

It is also thought that vitamin C’s antioxidant activity may help prevent or delay the development of certain cancers, cardiovascular disease, and other diseases that are at least partially caused by oxidative stress. There is currently no data, either in vitro or in vivo, that vitamin C inhibits SARS-CoV-2 viral replication, prevents viral infection, alleviates clinical symptoms, or alters the disease course of COVID-19.

Daily requirements and dietary sources

The recommended dietary allowance (the average daily level of intake sufficient to meet the nutrient requirements of most healthy individuals) ranges from 40–50 mg for infants and 15–25 mg for children younger than 8 years old to 45–75 mg for teenagers and 75–90 mg for adults older than 19 years. Pregnant or lactating women and smokers require an additional 35 mg per day.

Fruits and vegetables, particularly citrus fruits, tomatoes, and potatoes, are the best sources of dietary vitamin C. While it is not naturally present in grains, vitamin C is added to some fortified breakfast cereals.

Prolonged storage and cooking can reduce the vitamin C content of food because ascorbic acid is both water soluble and destroyed by heat. Fortunately, many of the best food sources of vitamin C, such as fruits and vegetables, are usually consumed raw.

Vitamin C supplements and deficiency

Supplements typically contain vitamin C in the same form as naturally occurring ascorbic acid in foods such as orange juice and broccoli.

Most adults take vitamin C supplements, either as part of a daily multivitamin or as a separate supplement, at levels much higher than the recommended dietary allowance, usually because they have heard that it can stave off colds or other diseases. Because it’s water soluble, any excess is usually excreted in the urine rather than stored in the body, and high doses are generally safe. For adults, the tolerable upper intake level—the highest daily intake likely to pose no risks—is 2,000 mg per day.

 In some people, however, more than 2,000 mg per day can cause diarrhea, nausea, heartburn, gastritis, fatigue, flushing, headache, and insomnia. Patients with chronic liver or kidney conditions, gout, or a history of calcium-oxalate kidney stones should take no more than 1,000 mg a day.

Vitamin C deficiency, and the scurvy it can cause, is rare in developed countries. According to the NIH, overt deficiency symptoms occur only if vitamin C intake falls below approximately 10 mg per day for many weeks and occur mainly in people with limited food variety who don’t take a daily supplement.

Vitamin C and COVID-19

Because vitamin C can enhance immune response and some patients use vitamin C to prevent the common cold, patients may ask about taking vitamin C as a way to prevent COVID-19 disease. Some studies have postulated that high doses of vitamin C, delivered intravenously, can be a useful treatment for COVID-19, perhaps because very high levels of vitamin C in the bloodstream may create free radicals that destroy the virus.

However, to date no research has been done on the effect of vitamin C supplements on the risk of contracting the  coronavirus that causes COVID-19. Pharmacists should caution patients against taking high doses of vitamin C as a way to protect against the disease.

What to tell your patients

Ensure that patients understand that high doses of vitamin C from supplements are not necessary for most people. Even those at risk for vitamin C deficiency, such as smokers (and those exposed to second-hand smoke), individuals who don’t eat a variety of foods, and patients with malabsorption or chronic diseases (cancer or renal disease), need far less than the 500 mg per day in most supplements. Warn patients that taking high doses of vitamin C can cause side effects like diarrhea, headache, and insomnia.

Finally, vitamin C supplements have the potential to interact with several types of medications, including chemotherapeutic agents and statins. Advise patients taking these medications on a regular basis to discuss their vitamin C intakes with their health care providers.