• People who are deficient in vitamin C could be more at risk of SARS-CoV-2, or of developing severe COVID-19 because their immunity is impaired
• Vitamin C deficiency is associated with increased susceptibility to infections, a less robust immune response, poor wound healing, and an increased risk of pneumonia
• Research has suggested up to 7% of the U.S. population may be deficient in vitamin C. People with certain conditions, such as cancer, diabetes, inflammatory conditions, immunosuppression, or who smoke are more likely to be deficient
• Fruit and vegetables provide the best source of vitamin C. Supplementation should be considered if vitamin C requirements are not being met through a person’s diet
• Vitamin C is an antioxidant that is important for the production of collagen and carnitine and plays a crucial role in our immune response.

Why is vitamin C important?

Humans, unlike most animals, cannot make vitamin C, so we must obtain it from our diet.

It is vital for many different processes within our bodies. Vitamin C has the following key activities:

• It is a highly efficient antioxidant. Antioxidants protect important biomolecules in our body (such as carbohydrates, lipids, proteins, and genetic material) from being broken down by oxidants, which are generated during normal cell metabolism or acquired from environmental exposure, such as air pollution or cigarette smoke
• It is vital for our immune response. Research has shown certain cells in our immune system, such as phagocytes and T-cells, accumulate vitamin C and rely on the vitamin to perform their task. A deficiency in vitamin C means we have less resistance against certain pathogens
• It is a cofactor for many enzymes, most notably, those involved with collagen production (the protein that holds our body together, found in our bones, muscles, skin, tendons, and ligaments) and carnitine (the molecule that transports fatty acids into mitochondria to generate metabolic energy)
• It is involved in the production of hormones, such as norepinephrine and vasopressin which have a key role in how our cardiovascular system responds to infection
• Other roles include regulating how certain genes express themselves, iron absorption, and many others.

What evidence is there to support taking vitamin C for COVID-19?

There have been hundreds of studies investigating vitamin C, many of which have conflicting results. One of the biggest problems with vitamin C studies is that most do not measure vitamin C concentrations before or after supplementation. Supplementation is unlikely to show an effect in people whose vitamin C levels are already high.

COVID-19 is a new disease, and we are still learning about it. But several studies in other infections or conditions suggest it may be beneficial in certain groups of people, particularly those already deficient in vitamin C who develop COVID-19. Currently, at least two trials are underway specifically investigating the use of vitamin C to treat severe COVID-19, one in New York and one in China.

Beneficial effects of vitamin C supplementation have been reported for

• Elderly people with acute respiratory infections
• Recurrent acute respiratory distress syndrome
• Reducing the severity and duration of the common cold
• Reducing the length of hospital stay and symptoms in elderly patients with pneumonia
• Reducing the duration of mechanical ventilation in people in ICU
• Preventing the common cold in people who are vitamin C deficient
• Preventing the incidence of pneumonia in people who are vitamin C deficient.
• Studies have shown that excretion of vitamin C is decreased during infections, such as the common cold, suggesting that more is utilized during times of need.

These are just some of the studies that cite a beneficial response. A full analysis of all the research that involves vitamin C is beyond the scope of this answer.

What are the symptoms of vitamin C deficiency?

Vitamin C is a water-soluble vitamin, which means little is stored in the body and a deficiency can develop in as little as four to 12 weeks.

Studies have indicated that hypovitaminosis C (low vitamin C levels indicated by a plasma vitamin C of <23 umol/L) is the fourth leading vitamin deficiency in the United States.

Symptoms of low vitamin C include:

• Fatigue
• Irritability
• Lethargy
• Loss of appetite.

If left untreated, severe vitamin C deficiency (also known as Scurvy) can develop. Symptoms include:

• Bulging eyes
• Dislocated joints, bone fractures, and a weakening of collagenous structures
• Dry hair that breaks off close to the scalp
• Impaired immunity and susceptibility to respiratory conditions, such as pneumonia
• Skin sores and bleeding, poor wound healing, the appearance of bruises
• Swollen, spongy, and purplish gums that bleed
• Teeth that start to detach from the gums.

More than two million sailors died from scurvy during the great voyages of discovery until Dr. James Lind, a Scottish surgeon in the Royal Navy, proved in 1753 that scurvy could be successfully treated with citrus fruit.

How much vitamin C should I be taking?

Recommended intakes of vitamin C (called Dietary Reference Intakes [DRIs]) have been developed by the Food and Nutrition Board. These are much higher than the amount required to prevent deficiency:

• Infants, 0-6 months: 40mg (Adequate intake)
• Infants, 7-12 months: 50mg (Adequate intake)
• Children, 1-3 years: 15mg
• Children, 4-8 years: 25mg
• Children, 9-13 years: 45mg
• Teenagers, 14-18 years: 75mg (boys), 65mg (girls)
• Adults: 90mg (men), 75mg (women)
• Pregnant women: 80mg (<18 years), 85mg (>18 years)
• Breastfeeding women: 115mg (<18 years), 120mg (>18 years).

Whenever we have an infection or inflammation, our bodies require more vitamin C so your general intake should increase to cope with the extra demand.

Many factors can impair the absorption of vitamin C or increase a person’s requirement for it. The following people should include an extra 50-100mg of vitamin C per day in their diet or in the form of supplements:

• People with cancer or who have a compromised immune system
• People who drink alcohol daily or excessively
• People with diabetes
• People with lung conditions
• Pregnant or breastfeeding women
• Smokers.

Rates of vitamin C deficiency vary around the world with rates as low as 7.1% in the United States to nearly 74% in India. Total vitamin C deficiency manifesting as scurvy is rare.

Can taking too much vitamin C cause side effects?

Yes. Taking more than 2000mg of vitamin C a day may cause stomach upsets, including abdominal pain, diarrhea or nausea.

Because vitamin C is a water-soluble vitamin, excessive vitamin C is excreted in the urine. Rarely, over-supplementation with vitamin C causes kidney stones.

What foods contain vitamin C?

In general, fruits contain more vitamin C than vegetables. Examples of foods that are high in vitamin C and the amount of vitamin C they contain include:

• Red capsicum: 240mg per capsicum
• Grapefruit: 94mg per grapefruit
• Orange juice: 93mg per ¾ cup
• Kiwifruit: 64-85mg per kiwifruit
• Broccoli: 51mg per ½ cup cooked
• Strawberries: 50mg per 10 strawberries
• Orange: 46-70mg per orange
• Sweet potato: 30mg per sweet potato
• Tomato: 30mg per tomato
• Cantaloupe: 29mg per ½ cup
• Cauliflower: 26 per ½ cup (raw)
• Silverbeet: 27mg per cup (boiled).

In general, raw fruits and vegetables contain higher quantities of vitamin C than boiled, although steaming and microwaving may reduce vitamin C losses. Prolonged storage can also reduce vitamin C levels.

Experts consider a diet that supplies 100–200 mg/day of vitamin C provides enough vitamin C to cover a healthy individual’s general requirements. People with medical conditions, such as cancer, diabetes, lung disease, or who smoke or exercise a lot should consider increasing their dietary intake of vitamin C.

Consuming five varied servings of fruits and vegetables a day can provide more than 200mg of vitamin C. Breastmilk is considered an adequate source of vitamin C in breastfeeding infants. Infant formula is already fortified with vitamin C.

When should people consider vitamin C supplements?

Vitamin C supplements are relatively cheap and should be considered in the following people whose diets may be deficient in vitamin C, or their absorption of vitamin C may be impaired, such as:

• Anybody who does not eat fruit or vegetables on most days of the week
• Babies fed only cow’s milk
• Individuals with eating disorders
• Older people with a poor diet (such as just tea and toast)
• People undertaking hemodialysis
• People with food allergies who restrict their intakes of vitamin C rich foods
• People who cannot afford to buy fruit or vegetables
• People who have too much iron in their blood (iron overload or hemochromatosis) which leads to wasting of vitamin C by the kidneys
• People with disorders of the GI tract like Crohn’s disease or Ulcerative colitis.

Men are also more likely than women to be deficient, probably because of a lower intake in general of fruits and vegetables.

Are some vitamin C supplements better than others?

Most vitamin C supplements contain synthetic vitamin C, which is made in a laboratory using a variety of different processes. This is the cheapest way of making vitamin C and there is no evidence to suggest that synthetic supplements are less effective than other forms of vitamin C at correcting a vitamin C deficiency.

More expensive vitamin C products often use whole foods; however, there may be some degradation in the vitamin C content during the production process, because vitamin C is not heat stable and degrades with time. Whole food supplements have the advantage that they include other factors, naturally present in foods, such as rutin, bioflavonoids, and other factors that potentially could aid absorption and utilization of vitamin C within our bodies.

Studies have been conflicting that have investigated if there is actually any difference in absorption between different forms of vitamin C, for example, synthetic vitamin C, Ester-C®, or vitamin C with bioflavonoids. Although one study showed higher vitamin C concentrations in leukocytes (white blood cells) with Ester C®, others did not.

Most organizations conclude that simple ascorbic acid (vitamin C) is the preferred source of supplemental vitamin C.