Is vitamin C all it’s cracked up to be?

Neither seafarers nor landlubbers are likely to get scurvy these days, thanks to our regular intake of vitamin C in fruits, vegetables, and fortified cereals. But that doesn’t mean that everyone in the modern world gets adequate vitamin C. Even in the United States, vitamin C deficiency is the fifth leading nutrient deficiency, representing roughly 6% of the population, according to the CDC.

Humans can’t produce vitamin C endogenously, yet most of us are able to get an adequate supply from our diet. The recommended dietary allowance (RDA) of vitamin C is 90 mg for adult males and 75 mg for adult females. However, certain populations—including smokers, the elderly, breastfeeding mothers, people with a limited-variety diet, and those with chronic diseases or absorption difficulties—are at risk of not getting enough vitamin C, so supplementary vitamin C may be needed. Smokers, for instance, should get an additional 35 mg of vitamin C per day.

Even though most Americans get enough vitamin C from the foods they eat, millions still take extra vitamin C through multivitamins or a vitamin C supplement. Is extra vitamin C necessary? Does extra vitamin C provide any benefit? Does it even do anything?

Matter of fact, it does. Taking extra vitamin C can produce both beneficial and harmful effects. Let’s take a look at the pros and cons of vitamin C.

Pro: Helps fight the common cold

Vitamin C won’t cure the common cold—but taking it preventively may shorten a cold’s duration and reduce the severity of symptoms. (Researchers point to vitamin C’s antihistamine effect as a possible mechanism for these benefits.) A meta-analysis of randomized controlled trials in children found that vitamin C reduced the duration of colds (upper respiratory tract infections) by 1.6 days on average. Additionally, a Cochrane review of placebo-controlled trials found that regular use of vitamin C (at least 200 mg per day) reduced the duration of colds by 8% in adults and 14% in children. Higher doses (1-2 g per day) shortened colds by 18% in children. Regular vitamin C use also reduced the severity of cold symptoms, the researchers noted.

However, taking vitamin C after the onset of a cold doesn’t appear to help to reduce either symptoms or duration.

Con: May increase formation of kidney stones

High intake of vitamin C has been associated with increased likelihood of developing kidney stones, at least in men, researchers have found. Men who took up to 250 mg of vitamin C daily had a 22% greater risk of kidney stones compared with men who consumed less than the RDA of 90 mg per day. The likelihood for kidney stones was even higher—41%—in men who took very high daily doses (1 g or more) of vitamin C. On the other hand, vitamin C didn’t raise the same risk of kidney stones in women, researchers found.

Pro: Has potent antioxidant activity

Vitamin C is a powerful antioxidant that directly scavenges free radicals. It even helps regenerate other antioxidants in the body, notably vitamin E and tetrahydrobiopterin. Vitamin C also augments the antioxidant effect of flavonoids and other polyphenols. It protects the lungs and host cells against oxidative stress caused by inflammation and infections while it inhibits viruses from replicating.

One recent meta-analysis found that vitamin C may shorten patients’ length of stay in the ICU as well as reduce the duration that critically ill patients spend on mechanical ventilation. These and other results suggest that vitamin C might even help combat COVID-19.

Con: Doesn’t help heart disease

Researchers have investigated whether vitamin C—with its potent antioxidant activity—could help delay or prevent the development of diseases linked to oxidative stress, such as cardiovascular disease. Some prospective studies have linked vitamin C intake with reduced heart disease risk. However, a number of experimental studies failed to show significant evidence that vitamin C supplementation protects against cardiovascular disease or reduces morbidity or mortality. 

However, clinical trials have shown that supplemental vitamin C moderately reduced blood pressure levels in hypertensive patients, although this effect didn’t reduce cardiovascular events or mortality.

Pro: Gives a boost to the immune system

Vitamin C can provide positive effects on cellular functions of both the innate and adaptive immune system, particularly in those with a vitamin C deficiency. “Vitamin C stimulates neutrophil migration to the site of infection, enhances phagocytosis and oxidant generation, and microbial killing. At the same time, it protects host tissue from excessive damage by enhancing neutrophil apoptosis and clearance by macrophages…Thus, it is apparent that vitamin C is necessary for the immune system to mount and sustain an adequate response against pathogens, whilst avoiding excessive damage to the host,” wrote the authors of a review article in the journal Nutrients about vitamin C and immune function.

Con: May cause side effects and drug-drug interactions

Vitamin C is a water-soluble vitamin that’s generally safe because any excess unmetabolized amount is excreted in the urine. However, continued intake of high doses can cause side effects of nausea, heartburn, vomiting, diarrhea, or abdominal cramps (as well as kidney stones, as noted above).

High doses of vitamin C may interact with other drugs due to its antioxidant activity. These include:  

• Statins. Vitamin C may potentially reduce the effectiveness of these lipid-lowering medications, particularly in combination with niacin or other antioxidants.

• Warfarin. Several case studies have reported that high doses of supplemental vitamin C reduce the response to this anticoagulant.

• Hormone replacement therapy. Taking high-dose supplemental vitamin C has been shown to increase estrogen levels in postmenopausal women on hormone replacement therapy.

High levels of vitamin C may also interfere with certain types of lab work, such as stool tests for occult blood or glucose screening tests.

Pro: May protect against cancer

Cancer patients often have vitamin C deficiency due to disease processes, decreased oral intake, infection, inflammation, and treatment (radiation, chemotherapy, and surgery). Accordingly, researchers have investigated whether eliminating vitamin C deficiency could improve cancer outcomes. And, indeed, an increase in vitamin C has been associated with reduced risk of certain cancers, including pancreatic cancer, esophageal cancer, prostate cancer, cervical neoplasia, renal cell carcinoma, and others.

Unfortunately, experimental and epidemiologic studies of dietary and supplemental forms of vitamin C have shown inconclusive results.

However, vitamin C administered intravenously has shown promise as an adjuvant cancer treatment. Researchers have found that intravenous vitamin C (alone or in combination with oral vitamin C) can slow disease progression, shrink tumors, decrease the toxicity of chemotherapy, and possibly improve quality of life. This research provides “evidence that high dose vitamin C shows anti-tumorigenic activity by elevating the amount of reactive oxygen species (ROS) in cancer cells without meaningful toxicities,” wrote the authors of a recent review article.

This research isn’t definitive though, so large controlled clinical trials need to be conducted before this therapy can be uniformly recommended, the researchers noted.