Vitamin C Supplements Review
Initial Posting: 3/10/17 Last Update: 3/26/19
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What It Is:
- Does it help? If you get sufficient vitamin C from your diet (which you can get from a cup of tomato or orange juice), taking more from a supplement will generally not be helpful. Nevertheless, taking high-dose vitamin C daily from a supplement during cold season can slightly reduce the risk of getting a cold, particularly if you are deficient in vitamin C — although it won't help once you're sick. Vitamin C supplementation may also slightly reduce blood pressure, although it has not been shown to reduce rates of cardiovascular disease. Taking vitamin C has also been associated with reducing the risk of gout. (See What It Does)
- How much should I take? If you want to be sure you're getting the daily requirement, a supplement providing roughly 50 mg to 100 mg of vitamin C is sufficient for most adults and is quite safe (see What to Consider When Using).
When higher doses are taken in hopes of reducing the risk of a cold or gout, or to slightly reduce blood pressure, a typical dose is 500 mg taken twice daily or up to 2,000 mg per day. Be aware, however, that taking more than 500 mg of vitamin C per day on a regular basis (which will saturate your blood with vitamin C) may increase your risk of developing cataracts, and taking more than 1,000 mg per day may also increase your risk of kidney stones. Diarrhea can result from a single dose of more than 2,000 mg for an adult and lower amounts for children (see Concerns and Cautions). In short, there is a risk/benefit trade-off with taking high-dose vitamin C.
- Which form is best? There are many forms of vitamin C available (ascorbic acid, sodium ascorbate, calcium ascorbate, liposomal vitamin C, etc.) but there is no compelling evidence that one is much better than another. The ascorbate forms may be easier on your stomach, but you will still run a risk of developing loose stools at a very high dose. Whole foods will provide additional bioflavonoid compounds, which may be of some benefit, but are not necessary to meet your nutritional needs, and natural vitamin C, such as from rose hips, is in the exact chemical form as in most synthetic vitamin C supplements -- L-ascorbic acid. (See Forms of Vitamin C).
- Which product is best? Among the products which contained the amounts of vitamin C listed and were "Approved" by ConsumerLab.com, five were chosen as CL's Top Picks. Unfortunately, we also discovered three products which contained roughly 50% more vitamin C than listed. Depending on the dose you take, this could increase your risk of adverse effects from vitamin C.
Vitamin C (ascorbic acid or dehydroascorbic acid) is an essential water-soluble vitamin that the human body can't manufacture. It must, therefore, come from foods or supplements. Fruits and vegetables are the richest food sources of vitamin C. Dietary supplements are typically sold as ascorbic acid, calcium ascorbate (Ester-C®), sodium ascorbate, ascorbyl palmitate, or a combination of these forms. Supplements also commonly contain natural sources of vitamin C such as rose hips (the pear-shaped fruit of the rose, without the flower's petals) and/or acerola (a cherry-like fruit). (See ConsumerTips™: What to Consider When Buying for more information about types of vitamin C).
What It Does:
Vitamin C helps the body produce collagen, a basic component of connective tissue. Collagen is an important structural element in blood vessel walls, gums, skin, and bones, making it particularly important to anyone recovering from wounds or surgery.
Vitamin C also acts as an antioxidant, scavenging potentially harmful molecules called free radicals. Although not established by clinical trials, this antioxidant activity is theorized by some to help protect against age-related macular degeneration of the retina, age-related skin changes, osteoarthritis, and other chronic diseases. It was also theorized that vitamin C might prevent cancer, but a study found that vitamin C (500 mg per day) did not reduce the risk of prostate or total cancer in healthy men aged 50 and older, nor did vitamin E (400 IU per day) (Gaziano, JAMA 2009). (Similarly, a large study of older men in Canada failed to find any association between intake of vitamin C (from food and/or supplements) and subsequent diagnosis of prostate cancer or the severity of the disease at the time of diagnosis (Parent, Front Physiol 2018)). The same study found that vitamin C (500 mg), vitamin E and the combination of the two failed to prevent cataracts in healthy men (Christen, Arch Opthalmol 2010).
It is clear that some amount of vitamin C is important to reducing the risk and progression of cataracts. A study in the UK showed that, in comparison to people with the lowest intakes of vitamin C from their foods, those with the highest intakes were 19% less likely to have nuclear cataracts and were 33% less likely to experience progression of the cataracts over the next 10 years. People who had taken supplements (including multivitamins) were also less likely to have cataracts, but this did not reduce the risk of subsequent progression (Yonova-Doing, Ophthalmology 2016). Other long-term studies (more than 10 years) suggest that vitamin C supplementation, at modest dosage, may reduce the risk of developing cataracts and that the daily intake of vitamin C needed to saturate the eye is only about 150 mg to 250 mg per day (Jacques, Am J Clin Nutr 1997). A long-term study of a modest-dose multivitamin (containing 60 mg of vitamin C) showed that the multi reduced the risk of developing cataracts except when a separate, daily, high-dose vitamin C supplement (500 mg -- about 6 times the RDA) was added to the regimen. In summary, over the long-term (10 years or more), it seems that low-dose vitamin C (60 mg to 250 mg) may help prevent cataracts, while high-dose does not and may have a negative effect.
Vitamin C intake may be particularly helpful to smokers, who suffer from oxidative stress and cell damage, which can deplete the body's store of vitamin C.
Starting vitamin C supplements prior to getting a cold may reduce cold duration and severity: An analysis of 44 studies found that when people developed a cold while taking vitamin C (usually 1 gram daily) their cold symptoms were slightly less severe (by 5% in one large study) compared to those who were taking a placebo. The duration of symptoms was also reduced by about 8% in adults and 14% in children. Studies evaluating vitamin C supplementation after cold symptoms develop have not conclusively shown a benefit (Hemila, Cochrane Database Syst Rev 2013). However, an analysis of nine clinical trials published between 1950 and 2001 concluded that, in people who already regularly supplemented with vitamin C (1 gram or less daily), taking a higher dose of vitamin C (3 to 4 grams daily) during a cold may slightly shorten the duration of the cold (by about half a day) and modestly reduce certain symptoms (chest pain and chills) but not nasal congestion or sore throat, compared to placebo. This high-dose vitamin C benefit was not seen in people who weren't already supplementing regularly with vitamin C — again suggesting that some vitamin C may need to be onboard prior to developing a cold to get a benefit (Ran, Biomed Res Int 2018).
Although studies have generally not found that taking vitamin C can prevent a cold, this has been shown to occur in certain situations: The Cochrane Database analysis noted above found that people undergoing short-term, severe physical stress, such as running a marathon, had a 52% lower risk of developing a cold if they took vitamin C (as opposed placebo) for several days before the event (Hemila, Cochrane Database Syst Rev 2013). A small study of men ages 18 to 35 with low to adequate levels of vitamin C found that those taking 1 gram of vitamin C daily (500 mg of crystalline vitamin C taken as a capsule (TwinLab C-500 Caps) in the morning and, again, in the evening) for two months in the winter reported fewer colds compared to those taking a placebo (7 vs. 11) (Johnston, Nutrients 2014).
Vitamin C may also help maintain the effectiveness of nitrate drugs, such as nitroglycerin, taken for chest pain and coronary artery disease.
Blood pressure appears to be modestly reduced with vitamin C supplements, at least in short-term studies. A review of 29 such studies, most of which used a daily dose of 500 mg or 1,000 mg of vitamin C, found average decreases of 3.84 mm Hg and 1.48 mm Hg, respectively, in systolic and diastolic blood pressures (Juraschek, Am J Clin Nutr 2012). Among patients with high blood pressure, the decreases were greater: 4.85 mm Hg and 1.67 mm Hg. Vitamin C was not found to directly prevent or reduces rates of cardiovascular disease (CVD), although the trials may have been too short (2 weeks to 6 months) to detect an effect. The blood pressure reduction with vitamin C supplementation is minor compared to that with ACE inhibitors and diuretics, which reduce pressures by about 10 mm Hg.
Vitamin C's modest effect on lowering blood pressure may result from lowering the activity of the endothelin-1 system, relaxing blood vessels. A small, 3-month study in the U.S. found that giving 500 mg per day of timed-release vitamin C each morning to overweight and obese individuals (who tend to have elevated endothelin-1 activity) reduced endothelin-1 activity as much as an exercise regime of daily aerobic walking for 45 to 60 minutes. The mean blood level of vitamin C was 1.2 mg/dL (rising in the supplemented group to 1.5 mg/dL), suggesting that the group was not vitamin C deficient. The researchers suggest vitamin C as a strategy for reducing endothelin-1-mediated vasoconstrictor tone in overweight and obese adults (Dow, Am Phys Soc Conf 2015 — abstract).
Another study among overweight or obese adults, all 27 of whom had type 2 diabetes, showed that 500 mg of vitamin C taken twice daily for four months not only lowered blood pressure (by an average 7 mm Hg systolic and 5 mm Hg diastolic) compared to placebo, but also lowered the amount of glucose in the blood after eating by 36% and reduced by 2.8 hours the period during which glucose levels were elevated daily. There was no significant improvement in HbA1c, although this may have been due to the limited size and duration of the study, according to the researchers (Mason, Diab Obes Metab 2019).
Vitamin C can enhance iron absorption from supplements and plant foods. In fact, vitamin C deficiency can also result in iron deficiency anemia due to increased bleeding and decreased nonheme iron absorption (NIH, 2016).
A long-term study of Americans found the risk of dying over the course of the study (about 14 years) was lowest when vitamin C levels in the blood serum were above the lowest levels (the bottom 20% of the population, i.e., above 15.33 millimol/liter) and the risk continued to decrease until vitamin C levels reached 45.99 to 59.62 millimol/liter, with no further decrease at higher levels. The risk of dying specifically from cardiovascular disease followed a similar pattern, although the risk of dying from cancer continued to decrease even into the highest blood levels of vitamin C (at or above 60.19 millimol/liter) (Goyal, Cancer Epidemiol Biomarkers Prev 2013). However, one study suggests that getting vitamin C from supplements may increase breast cancer risk in women who already have a high dietary intake of vitamin C. The ten year observational study among 57,403 women in France found that while those who reported the highest intakes of vitamin C from foods (> 177.6 mg/ day) had a reduced risk of postmenopausal breast cancer compared to women reporting the lowest intake of vitamin C from foods (<101.9 mg/day), women who reported the highest intakes of vitamin C from foods and also took vitamin C supplements had an increased risk of postmenopausal breast cancer (Cadeau, Am J Clin Nutr 2016).
Low blood levels of vitamin C (≤45 µmol/L) have been associated with an increased risk of developing post-herpetic neuralgia -- nerve pain that persists after shingles (Herpes zoster) lesions have healed (Chen, Br J Nutr 2011). Vitamin C given intravenously (2.5 to 15 grams daily or every other day for 3 to 14 days), given along with standard treatment, has been shown to reduce this delayed pain and its severity but not acute shingles pain (Carr J Transl Med 2017; Kim, Ann Dermatol 2016). However, there do not appear to be any clinical studies investigating the effects of oral vitamin C supplementation on treating shingles or reducing acute or post-shingles nerve pain.
In men, intake of vitamin C from supplements is associated with reduced risk of gout. Compared to men who did not use supplemental vitamin C, intakes of 1,000 to 1,499 mg and more than 1,500 mg were, respectively, associated with 34% and 45% reductions in the risk of gout (Choi, Arch Intern Med 2009). This may be due to lower levels of uric acid in the blood with vitamin C intakes of up to 400-500 mg per day — although there is no further decrease with higher vitamin C intake (Gao, J Rhematol 2008).
Vitamin C deficiency has been associated with changes in nails such as koilonychia (spoon nails) and hapalonychia (top of the nail becomes soft and thin, causing it to bend or break) (Cashman, Clin Dermatol 2010). As discussed above, vitamin C can enhance iron absorption, and iron deficiency can result in brittle nails. However, there do not appear to be any clinical studies demonstrating that vitamin C improves nail health or strength in people who are not deficient in vitamin C or iron (Scheinfeld, J Drugs Dermatol 2007).
Good sources of vitamin C include many vegetables, such as broccoli and Brussels sprouts, as well citrus and other fruits. A healthful diet should provide the Recommended Dietary Allowance (RDA) of vitamin C (see ConsumerTips™ for RDAs by age, gender, and life stage).
Additional information about potential uses of vitamin C is found in the Encyclopedia article on this site. Be aware that vitamin C can also affect other drugs you may be taking (see ConsumerTips™: Concerns and Cautions)
Quality Concerns and What CL Tested for:
Regulations from the U.S. Food and Drug Administration (FDA) require that any vitamin C sold as a dietary supplement in the United States contain at least 100% of its labeled amount, but neither the FDA nor any other federal or state agency routinely tests vitamin C supplements for quality prior to sale.
ConsumerLab.com, as part of its mission to independently evaluate products that affect health, wellness, and nutrition, purchased several popular vitamin C supplements sold in the U.S. and Canada, and tested them to determine whether they contained their labeled amounts of vitamin C and whether tablets properly disintegrated (break apart) to enable their absorption. All products with botanical ingredients or large amounts of minerals were also tested for heavy metals (lead, cadmium, and arsenic). See How Products Were Evaluated for more details about the testing.