Vitamin C Explained: Benefits, Deficiency, And Sources

Most people think of vitamin C as the thing you take (whether in tablet or orange juice form) when you're feeling under the weather. But vitamin C isn't just for fighting off the common cold. According to the National Institutes of Health (NIH), this water-soluble micronutrient performs a number of important roles in the body. In addition to supporting a robust immune system, it's necessary for the creation of collagen and other proteins. It's also a powerful antioxidant and an important component of iron metabolism.

Vitamin C is a great example of a micronutrient that almost everyone has heard of but most people know very little about. For example, how much vitamin C should you be getting? Do you know what the best food sources of vitamin C are? (Spoiler alert: it's not citrus!). Supplement companies and even some health experts have made huge claims about the powers of vitamin C, but can you separate hype from reality? Here's everything you need to know about this popular, but sometimes misunderstood, vitamin.

Vitamin C is necessary for a strong immune system

When you hear the words "immune booster," the first thing that probably comes to mind is vitamin C. This micronutrient is indeed essential for a well-functioning immune system. According to a 2017 article published in Nutrients, vitamin C helps give skin its structure, creating a first-line defense against invading germs. It also accumulates in phagocytes, a type of white blood cell designed to destroy germs by "eating" them. It also makes these white blood cells more active. Vitamin C is needed for apoptosis, the destruction and recycling of old or damaged cells. It helps lymphocytes, another type of immune cell, differentiate into specialized B- and T-cells, designed to target specific invaders the body has encountered in the past. The article's authors point out that vitamin C deficiency "results in impaired immunity and higher susceptibility to infections."

But chugging glass after glass of orange juice or taking vitamin C in pill form when you're feeling under the weather may be less effective than you think. In an interview with Harvard Health Publishing, Dr. Bruce Bistrian, chief of clinical nutrition at Beth Israel Deaconess Medical Center, debunked the popular belief that vitamin C is a magic bullet against the common cold. He explained: "The data show that vitamin C is only marginally beneficial when it comes to the common cold." Although some research suggests that large doses of supplemental vitamin C may help extremely active people like marathon runners avoid getting sick, it didn't help the general population dodge the virus. Another study, however, found that getting more than 200 mg a day shortened colds by 8% among adults and 14% among children — equivalent to eliminating one sick day.

Vitamin C is essential for making collagen

Your body needs vitamin C to make collagen (via Molecular Cell Biology). While most people know collagen as the thing that keeps your skin firm and young-looking, its function isn't just cosmetic (via Cedars-Sinai). Collagen is actually the most abundant protein in the body and is a vital structural component of skin, ligaments, tendons, cartilage, muscle, bone, blood vessels, and the interior portions of the teeth and eyes. Although there are more than 16 types of collagen, 80–90% of collagen in the body is types I, II, and III (via Molecular Cell Biology).

Vitamin C is a cofactor (a "helper" that assists enzymes in creating biochemical reactions in the body) for the two enzymes responsible for collagen synthesis. The enzyme prolyl hydroxylase stabilizes collagen molecules, while the enzyme lysyl hydroxylase gives them structural strength. Neither enzyme can do its job without vitamin C. Vitamin C also appears to act directly on DNA to stimulate increased production of collagen (via ScienceDirect).

Because bone is made up largely of collagen, vitamin C also plays an important role in bone health. A 2005 paper published in Joint Bone Spine noted that 80% of individuals with vitamin C deficiency have musculoskeletal symptoms. Bone is constantly being broken down and rebuilt, but when you're low on vitamin C, the breaking down occurs at a much faster rate than the rebuilding. Individuals who don't get enough vitamin C may experience osteonecrosis (the death of bone cells) and osteoporosis (decreased bone density).

Vitamin C is a powerful antioxidant

Vitamin C is a powerful antioxidant, but what exactly does that mean? Antioxidants are chemicals that protect the body against damage from free radicals (via Livescience). Free radicals are highly unstable atoms of oxygen that try to "steal" electrons from other molecules in the body. This destabilizes the molecules and creates a chain reaction that leads to damage throughout the cells and tissues of the body, a process known as oxidative damage or oxidative stress. Substances that produce free radicals can be found in food and in our environment, and free radicals are also created as the byproduct of natural chemical processes in the body. Antioxidants are able to give free radicals the electrons they want without becoming destabilized themselves, thus neutralizing free radicals before they can cause damage.

But that doesn't mean you should go nuts with your vitamin C intake to try to protect yourself from free radicals. Interestingly, although this micronutrient is well known for its antioxidant properties, it can also actually encourage free radical damage in certain situations. A 2017 peer-reviewed paper published by Fadime Eryılmaz Pehlivan noted that "vitamin C can ... act as a prooxidant, especially in the presence of transition metals, such as iron and copper, starting different hazardous radical reactions."

Your body needs vitamin C to make use of iron

If you've ever been diagnosed with anemia, your doctor likely told you to consume vitamin C at the same time you consume iron, whether in food or supplement form (via Harvard School of Public Health). There's good reason for that advice. Vitamin C dramatically improves absorption of iron from plant-based sources. As the Harvard School of Public Health explains, there are two types of iron: heme (from animals) and non-heme (from plants). While heme iron is easily absorbed by the body, non-heme iron is much less bioavailable without the presence of vitamin C. According to a 2014 paper published in Free Radical Biology and Medicine, In addition to improving iron absorption in the digestive tract, vitamin C plays a number of other roles when it comes to iron, both heme and non-heme. These include influencing cells' uptake of iron from the bloodstream and regulating how much iron is stored in the body.

Iron deficiency is extremely common, affecting approximately 20% of nonpregnant women, 50% of pregnant women, and 3% of men (via WebMD). In fact, according to the Iron Disorders Institute, iron deficiency is the most prevalent nutritional deficiency in the world. It can lead to anemia (low red blood cell count), which can cause fatigue, shortness of breath, dizziness, headaches, and weakness.

Vitamin C requirements

Vitamin C is essential for health and proper functioning but, according to a 2011 paper published in Current Genomics, humans are among a small group of vertebrate animals who can't make this critical micronutrient themselves. This means we must get it from food or dietary supplements. According to the National Institutes of Health (NIH), the recommended dietary allowance (RDA) for vitamin C is 90 mg for adult men and 75 mg for adult nonpregnant women. Pregnant adults should aim for 85 mg, while breastfeeding mothers should consume 120 mg daily. The NIH notes that average vitamin C intake for adult men is 105.2 mg per day, while adult women average 83.6 mg.

But this statistic is misleading. According to Oregon State University, 42.9% of adults failed to meet the estimated average requirement of vitamin C. This suggests that about half of American adults are getting a lot of vitamin C (and skewing the average), while the other half aren't getting enough.

Vitamin C deficiency

Even though many people don't meet the estimated average requirement (EAR) for vitamin C, severe vitamin C deficiency, known as scurvy, is rare (per Healthline). That's because you don't need much vitamin C to stave off scurvy – just 10 mg a day is enough, far below the EAR or recommended dietary allowance (RDA). When scurvy does occur, symptoms include fatigue, joint pain, swollen and bleeding gums, tooth loss, easy bruising, slow wound healing, weak bones, anemia, hair and nail changes, depression, and impaired immunity.

If not treated with vitamin C, scurvy is fatal. In fact, until the end of the 18th century, many sailors on long sea voyages died because of a lack of fresh fruits or vegetables (via the National Institutes of Health). In the mid-1700s, Dr. James Lind discovered that citrus fruits and their juices prevented and cured scurvy, though it wasn't until 1932 that scientists identified vitamin C as the reason behind citrus fruits' effectiveness. The symptoms of scurvy can take a while to develop because even though vitamin C is water-soluble, the body is able to store a small amount as an emergency reserve. Today, scurvy is rare in developed countries, although individuals who get less than the EAR for vitamin C may have a milder form of deficiency.

Who's at risk for vitamin C deficiency?

While anyone can become deficient in vitamin C, some groups of people are more at risk than others (via the National Institutes of Health). Smokers and people regularly exposed to secondhand smoke are at particularly heightened risk for deficiency because of the oxidative stress cigarette smoke causes. In fact, people who smoke or are regularly around people who smoke tend to have lower concentrations of vitamin C in their blood and white blood cells. To offset this, the RDA for smokers is 35 mg higher than the RDA for nonsmokers. (For example, an adult male smoker should get 125 mg rather than 90 mg daily). Another at-risk group are those who have an extremely unvaried diet that doesn't include enough fresh fruits and vegetables. This includes those who are elderly, living in poverty, have eating disorders, or follow extremely restrictive diets. While both breastmilk and commercially available infant formula contain adequate amounts of vitamin C, infants in developing countries who are fed evaporated or boiled cow's milk are at risk of deficiency.

Even with a vitamin C-rich diet, individuals with certain medical conditions may become deficient. This includes those who can't absorb nutrients properly because of gastrointestinal conditions such as celiac disease or Crohn's. Those with cachexia (a type of wasting disease, per Healthline), certain types of cancer, or end-stage kidney failure are also more likely to be vitamin C deficient.

Getting vitamin C from food

Vitamin C is found in a wide variety of fruits and vegetables, so even picky eaters can easily get the recommended amount (via Medical News Today). While orange juice has become the poster child for vitamin C, there are a number of other options that contain as much, if not more, of this micronutrient. Guava and red bell peppers are among the richest sources of vitamin C. Other great choices include tomato juice, hot peppers, strawberries, papaya, mango, cantaloupe, broccoli, potatoes, and Brussels sprouts.

Although plentiful, vitamin C is also a very fragile micronutrient. In fact, according to the Better Health Channel, it's one of the most unstable vitamins. Vitamin C content in food can be reduced through use of fertilizers on growing crops as well as processing techniques such as pasteurization or blanching after fruits and veggies are picked.

You may want to take a "less is more" approach when cooking foods that contain vitamin C. In an interview with the Queensland Department of Health, nutritionist Charlotte Morrison explained that cooking method and time can have a big impact on vitamin C content. This is especially true if you opt for boiling, as water-soluble vitamin C leaches out of food and into cooking water. If you can use that cooking water (for example, as the base of a soup), then you'll still get the vitamin C. Morrison suggests sticking with cooking methods that require little or no water, such as roasting and grilling.

Getting vitamin C from dietary supplements

If you don't get enough vitamin C in your diet, you can always opt to take a supplement. According to WebMD, synthetic vitamin C is considered both safe to take and effective at upping your vitamin C instake. A 2013 paper published in Nutrients noted that vitamin C was first synthesized in the early 1930s and has been in wide use ever since. The authors point out that "although synthetic and food-derived vitamin C is chemically identical, fruit and vegetables are rich in numerous nutrients and phytochemicals which may influence its bioavailability." However, in the research they reviewed, they found that "all ... comparative bioavailability studies in humans have shown no differences between synthetic and natural vitamin C." So while it's probably best to prioritize getting vitamin C (and other nutrients) from food rather than from a pill, supplemental vitamin C is still a useful option.

In addition to synthetic ascorbic acid, there are several other forms of manmade vitamin C, including sodium ascorbate and calcium ascorbate (via the National Institutes of Health). Vitamin C can be found as a standalone supplement, as part of a daily multivitamin, and in targeted blends (particularly those promising boosted immunity or improved skin health). Approximately one-third of American adults regularly take a multivitamin and 12% regularly take a standalone vitamin C supplement.

Can vitamin C prevent or cure cancer?

It's true that vitamin C has many vital functions in the body, but over the years this little micronutrient has been the focus of some very big health claims that aren't always backed by solid science. For instance, you may have heard about the potential of using "mega-doses" of vitamin C as an alternative cancer treatment (via Mayo Clinic). According to Dr. Karthik Giridhar at the Mayo Clinic, however, there's no evidence to suggest that vitamin C can cure cancer. Initial studies conducted in the 1970s appeared promising, as vitamin C appeared to be toxic to cancer cells, but these studies were flawed and later discredited. More recently, researchers discovered that intravenous vitamin C has different properties than ingested vitamin C, rekindling interest in the vitamin's use for cancer treatment. Researchers are now investigating whether vitamin C can boost the effectiveness of first-line cancer treatments such as chemotherapy. It's important to note, however, that even if vitamin C can give cancer patients an extra advantage while undergoing treatment, this is likely only at extremely high doses delivered intravenously. This is very different than simply upping your intake of dietary vitamin C or popping a supplement.

When it comes to preventing cancer in the first place, the National Institutes of Health (NIH) noted that "higher consumption of fruits and vegetables is associated with lower risk of most types of cancer, perhaps, in part, due to their high vitamin C content." In theory, vitamin C may help stave off cancer due to its antioxidant properties, ability to limit the production of carcinogens, and impact on immune response. But while consuming a diet rich in vitamin C is certainly a good idea, there's little research evidence to suggest that vitamin C directly prevents cancer, which can have many causes and risk factors.

The link between vitamin C and heart disease

According to the Centers for Disease Control and Prevention (CDC), heart disease is the leading cause of death in the United States, taking the lives of approximately 659,000 Americans every year. By now, most people know that diet can have a big impact on your risk for cardiovascular issues, but saturated fat, sodium, and sugar tend to get the most attention (via Medline Plus). As it turns out, vitamin C (or a lack thereof) may also play an important role.

As with the link between vitamin C and cancer, the National Institutes of Health (NIH) pointed out that a diet high in vitamin C-rich fruits and vegetables has consistently been linked to decreased risk for heart disease. In addition to being a powerful antioxidant that can prevent free radicals from damaging arteries, vitamin C assists with vasodilation (widening of the blood vessels) and makes it harder for plaques to stick to artery walls.

A review published in 2016 in the International Journal of Molecular Sciences explored the link between vitamin C and cardiovascular disease. The authors noted that although "classical vitamin C deficiency, marked by scurvy, is rare in most parts of the world, some research has shown variable heart disease risks depending on plasma vitamin C concentration, even within the normal range." They found that lower levels of vitamin C were associated with greater risk of dying from cardiovascular disease, and that increasing vitamin C intake may "slightly improve" cholesterol and heart function in those with low vitamin C levels. But the researchers found little evidence to suggest that vitamin C provided additional cardiovascular benefits among those who weren't deficient.

Vitamin C and skin health

There's a reason vitamin C is in so many skincare products. A 2017 paper published in Nutrients explored the link between vitamin C and skin health. The authors noted that vitamin C concentration is particularly high in the skin. Vitamin C is required for the creation of collagen, the protein that gives skin its structure. It's also necessary for maintaining the proper balance between collagen and elastin (a protein that gives skin its stretch) in the dermis. As a powerful antioxidant, vitamin C also protects against cellular skin damage caused by UV radiation. Signs of aging, like wrinkles, can be prevented or postponed with vitamin C. That's why so many serums and lotions contain vitamin C. The researchers concluded, however, that delivering vitamin C topically can be challenging, and that ingesting vitamin C through food or supplements is likely to produce better results for your complexion.

Vitamin C is also needed to prevent unsightly scars after an injury to the skin. A 2013 paper published in the British Journal of Community Nursing outlined how vitamin C plays an important role in three of the four phases of wound healing (as described by Wound Source). During the initial inflammatory phase, vitamin C is required to clear away white blood cells after they've performed their function, preventing inflammation from getting out of control. In the proliferative phase, when new tissue is being created, vitamin C is needed for collagen production. During the maturation phase of healing, when collagen is realigned into an orderly network of fibers and the wound fully closes, insufficient vitamin C may lead to scarring.

Vitamin C protects your eyes

While vitamin A tends to get all the attention when it comes to protecting your peepers, vitamin C is also important. As Healthline explains, the collagen that vitamin C helps produce gives your eye structure and is particularly important in the cornea and sclera. It makes sense, then, that consuming a lot of vitamin C could prevent cataracts, a condition in which the cornea becomes cloudy, leading to vision loss. In fact, research suggests that high vitamin C intake may reduce risk of cataracts by 45–75%.

Vitamin C may also be helpful when it comes to age-related macular degeneration (AMD). The National Institutes of Health (NIH) notes that "although research has not shown that antioxidants play a role in AMD development, some evidence suggests that they might help slow AMD progression." In the Age-Related Eye Disease Study (AREDS), a formulation consisting of 500 mg vitamin C, 400 IU vitamin E, 15 mg beta-carotene, 80 mg zinc, and 2 mg copper was shown to be effective in slowing AMD. But, since there were several other micronutrients involved, it's impossible to say how much effect vitamin C specifically had.

Can you get too much vitamin C?

Even though vitamin C is water-soluble and anything unneeded is flushed out of the body through the urine, it may still be possible to overdo it on this micronutrient, especially if taking supplements. A tolerable upper limit (UL) for vitamin C has been set at 2,000 mg for all adults, regardless of sex, pregnancy, or breastfeeding status (via the National Institutes of Health). Large doses of vitamin C may reduce levels of vitamin B12 and copper, or could cause iron levels to rise to toxic levels, particularly in those with a genetic condition known as hereditary hemochromatosis. Vitamin C could also promote the erosion of tooth enamel. Because vitamin C increases excretion of oxalate and uric acid in urine, it may increase the likelihood of kidney stones in some individuals. In postmenopausal women, intakes of more than 300 mg of supplemental vitamin C a day may increase the risk of death from cardiovascular disease. But these potential negative effects of taking too much vitamin C are largely theoretical (based on how the vitamin functions in the body) and haven't been consistently observed in research studies.

The Mayo Clinic also notes that excessive use of vitamin C supplements can cause gastrointestinal issues such as diarrhea, nausea, vomiting, cramping, and heartburn. Other possible symptoms include headache and insomnia.