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Vitamin C

Vitamin C is an essential vitamin with antioxidant properties. It is frequently supplemented to ward off the common cold, though its efficacy is questionable, and likely low.

Our evidence-based analysis on vitamin c features 374 unique references to scientific papers.

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Summary of Vitamin C

Primary information, health benefits, side effects, usage, and other important details

Vitamin C, or L-ascorbic acid, is a water-soluble essential vitamin. It is a very popular dietary supplement due to its antioxidant properties, safety, and low price.

Vitamin C is often supplemented to reduce the symptoms of the common cold.

However, vitamin C is unable to reduce the frequency of colds in a healthy population. An athlete who frequently undergoes intense physical activity can expect to cut the risk of getting a cold in half. Supplemental vitamin C is able to reduce the duration of a cold by 8-14% in any population, when it is taken as a daily preventative measure, or at the beginning of a cold. Though superloading vitamin C (5-10g daily) is said to be more effective, further research is needed to determine the accuracy of this claim.

Vitamin C is capable of being both an antioxidant and pro-oxidant, depending on what the body needs. This mechanism allows it to serve a variety of functions in the body.

Vitamin C sequesters free radicals in the body. It is replenished by antioxidant enzymes, and is often used as a reference drug in antioxidant research. Vitamin C’s structure allows it to act on neurology and depression, as well as interact with the pancreas and modulate cortisol. Its antioxidant properties mean vitamin C provides neuroprotective effects and benefits for blood flow. By protecting the testes from oxidative stress, vitamin C can also preserve testosterone levels.

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How to Take

Recommended dosage, active amounts, other details

The Recommended Daily Intake (RDI) of vitamin C is 100-200mg. This is easily attained through the diet, so supplementation of such low doses is usually unnecessary. Higher doses of vitamin C, up to 2,000mg, are used to support the immune system (for athletes) or reduce the duration of the common cold.

Most studies on vitamin C prescribe one dose per day. The claim that taking 2,000mg up to five times a day to optimally reduce cold symptoms is not sufficiently tested and requires more evidence.

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Human Effect Matrix

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The Human Effect Matrix summarizes human studies to tell you what effects Vitamin C has on your body, how much evidence there is, and how strong these effects are.

Full details are available to Examine members.
Grade Level of Evidence
Robust research conducted with repeated double-blind clinical trials
Multiple studies where at least two are double-blind and placebo controlled
Single double-blind study or multiple cohort studies
Uncontrolled or observational studies only
Level of Evidence
? The amount of high quality evidence. The more evidence, the more we can trust the results.
Outcome Magnitude of effect
? The direction and size of the supplement's impact on each outcome. Some supplements can have an increasing effect, others have a decreasing effect, and others have no effect.
Consistency of research results
? Scientific research does not always agree. HIGH or VERY HIGH means that most of the scientific research agrees.
Notes
grade-b Strong Very High See all 11 studies
For the purpose of increasing plasma Vitamin C concentrations, orally supplemented Vitamin C appears to be the best decision (second only to intravenous vitamin C).
grade-b Minor Very High See all 7 studies
An increase in blood flow is seen in instances of impaired blood flow (smoking, obesity, etc.) which may be due to preservation of nitric oxide function (via reducing oxidation thereof); this is a phenomena general to antioxidants and not unique to Vitamin C
grade-b Minor - See all 20 studies
Studies are very mixed, however, there appears to be a modest benefit to fasting glucose in type 2 diabetics. More research is needed before having great confidence in this effect. The vast majority of studies don't treat this outcome as their primary outcome and more glucose-specific research is needed.
grade-b
Minor
- See all 7 studies
Vitamin C (500-1,500mg daily) appears to be associated with both increases and decreases in exercise-induced cortisol spikes, depending on whether it acts as a prooxidant or antioxidant (respectively). There is no influence on resting cortisol concentrations.
grade-b Minor Moderate See all 7 studies
Surprisingly mixed influences on biomarkers of oxidation, with either a decrease or no significant influence the majority of the time (with limited evidence to hint at an increase being possible)
grade-b Minor Moderate See all 26 studies
The evidence suggests that a small (roughly half a day) reduction is found when vitamin C is taken consistently, even when not sick, but not if it's only started once already sick. For other infectious diseases, it's unclear if vitamin C helps.
grade-b Minor Low See all 8 studies
Mixed and weak influences on lipid peroxidation, but a possible reduction exists
grade-b Minor Moderate See all 4 studies
Although not acute, a possible reduction in biomarkers of muscle damage is sometimes noted with antioxidative supplementation which applies to Vitamin C; results are unreliable
grade-b - High See all 6 studies
More evidence suggests no significant effect than a possible protective effect, although the latter is possible
grade-b - Very High See all 10 studies
No significant influence on HDL cholesterol
grade-b - Very High See all 10 studies
No significant influence of Vitamin C supplementation on HbA1c levels
grade-b - Very High See all 5 studies
No significant alterations seen in inflammatory cytokines associated with Vitamin C supplementation
grade-b - - See all 11 studies
Mixed evidence across studies. Overall, there may be a modest reduction in fasting insulin levels, though more evidence is needed.
grade-b - Very High See all 9 studies
No significant alterations in LDL cholesterol seen with Vitamin C supplementation
grade-b - High See all 3 studies
There do not appear to be any inherent protective effects of Vitamin C against the oxidative and inflammatory changes associated with cigarette smoking, although the reduction in blood flow may be attenuated somewhat with antioxidants and this applies to Vitamin C
grade-b - Moderate See all 28 studies
When used as a prophylactic, vitamin C's effects are very inconsistent, and overall it doesn't seem to reliably reduce the risk of getting a common cold. It's possible that those undergoing extreme exercise (a known risk factor for developing colds) see a meaningful reduction in risk, however this is based on much less research and requires further study. It's unclear if other infectious diseases are affected by vitamin C supplementation.
grade-b - Very High See all 13 studies
No significant influence on total cholesterol seems apparent with vitamin C supplementation
grade-b - Very High See all 12 studies
No significant influence on fasting or postprandial triglycerides seems apparent with Vitamin C
grade-b - See all 25 studies
When used as a prophylactic, vitamin C's effects are very inconsistent, and overall it doesn't seem to reliably reduce the risk of getting a common cold. It's possible that those undergoing extreme exercise (a known risk factor for developing colds) see a meaningful reduction in risk, however this is based on much less research and requires further study. It's unclear if other infectious diseases are affected by vitamin C supplementation.
grade-c Minor Moderate See 2 studies
It is thought that, secondary to reducing the rate of muscular damage, that adaptations gained from exercise are attenuated; there is mixed evidence to support this, but is seems possible with antioxidants
grade-c Minor Very High See 2 studies
An increase in antioxidant enzymes have been noted in elderly persons
grade-c Minor Moderate See all 5 studies
May attenuate the increase due to acute hyperglycemia or over the course of 4 months in type 2 diabetics, though the evidence is mixed and more research is needed.
grade-c Minor Very High See 2 studies
The rate of bone mineral density loss over time in elder women appears to be reduced with dietary antioxidants, and as such applies to Vitamin C supplementation. The protective effect is not remarkably large
grade-c Minor Very High See all 5 studies
A possible reduction in C-Reactive Protein exists with Vitamin C supplementation
grade-c Minor - See study
A decrease in fatigue has been noted in obese adults given Vitamin C in conjunction with exercise
grade-c Minor - See study
Supplementation of 3,000mg Vitamin C appeared to increase sexual frequency from 4 times monthly to 14 times in non-cohabitating subjects. There was no significant effect on cohabitating subjects. No influence on masturbation frequency.
grade-c Minor Moderate See all 5 studies
A decrease in heart rate has been noted in exercising obese adults, a per se effect of Vitamin C on heart rate (rather than secondary to the rate of percieved exertion) seems unlikely.
grade-c Minor - See study
An increase in microcirculation has been noted secondary to increased blood flow, thought to be a general property of antioxidants
grade-c Minor - See study
A possible reduction in muscle soreness the day after exercise may result when preloading exercise with Vitamin C
grade-c Minor - See study
The rate of percieved exertion in obese adults appears to be attenuated with Vitamin C supplementation
grade-c Minor - See study
An improvement in mood has been noted in hospitalized persons
grade-c Minor - See study
Vaginal bacterial infections are somewhat treatable with directly applied (via silicon coated tablets), as Vitamin C exerts some antioxidant effects against those bacterial strains
grade-c - Very High See all 5 studies
No significant influence on aerobic exercise performance
grade-c - Very High See 2 studies
No significant influences on DNA damage
grade-c - - See study
grade-c - - See 2 studies
Highly mixed interactions with the exercise:oxidation axis with Vitamin C, with both increases and decreases being noted. Unlikely to have a reliable role
grade-c - - See study
Does not appear to significantly influence fat mass
grade-c - Very High See 2 studies
No significant influence on fat oxidation
grade-c - - See study
No significant influence on glycemic control in diabetics with Vitamin C supplementation
grade-c - - See study
No significant influence on heart palpitations
grade-c - - See study
No significant influence on insulin sensitivity
grade-c - Low See all 4 studies
Mixed evidence. One study found a significant reduction compared with placebo after 8 weeks supplementation, one study found a non-significant increase in response to ultramarathon running, and one study found no change.
grade-c - - See study
grade-c - Very High See all 3 studies
No significant influence on the oxidation rates of LDL cholesterol
grade-c - - See study
grade-c - - See study
No significant influence on pre-eclampsia risk
grade-c - - See study
No significant influence on the risk of cataracts
grade-c - - See study
grade-c - Moderate See all 12 studies
While some research suggests that vitamin C could modestly reduce the amount of time people spend indoors during colds, overall symptom severity doesn't seem to be meaningfully affected. More research is needed to determine its effect on the severity of symptoms.
grade-c - - See study
grade-c - - See study
No significant influence on spontaneous birthing
grade-c - - See study
Insufficient evidence to support a role
grade-c - Moderate See all 5 studies
There's generally insufficient evidence to discern which cold symptoms vitamin C may affect.
grade-c - - See study
Does not appear to have a role in altering VO2 max
grade-c - Very High See all 4 studies
Vitamin C does not appear to have a weight reducing effect
grade-d Notable - See study
There was a notable decrease in one study, but more research is needed to tell how reliable this effect is.
grade-d Minor High See all 3 studies
Attenuation of impairment due to either acute hyperglycemia or elevated free fatty acid levels
grade-d Minor - See study
Seemed to attenuate the impairment due to acute hyperglycemia, but more research is needed.
grade-d - - See study
No statistically significant effect of 200, 500 and 1000 mg/d for 14 days on postprandial levels.
grade-d - - See study
Higher than control group at some points post-surgery, but not statistically significant.
grade-d - - See study
grade-d - Very High See 2 studies
No apparent effect on body weight over the course of 4 months.
grade-d - - See study
No difference from 500 mg/d after 3 months of supplementation in type 2 diabetes patients (within-group).
grade-d - - See study
Unclear effect after taking 1 g/d for 4 months.
grade-d - - See study
No difference compared with placebo after taking 6 g/d for 4 weeks.
grade-d - Moderate See 2 studies
Mixed evidence. Significant improvement with 1 g/d in type 2 diabetics, though not accompanied by changes in fasting blood glucose. The other study didn't find a change.
grade-d - - See study
Unclear if there was an effect of vitamin C compared with placebo in ultramarathon runners.
grade-d - - See study
No apparent difference compared with placebo in response to ultramarathon running.
grade-d - - See study
Nonsignificant increase compared with placebo in response to ultramarathon running.
grade-d - - See study
Possible large reduction due to 7 days of taking 1.5 g/d, however, pre-supplementation measurements weren't taken.
grade-d - - See study
1.5 g/d didn't lead to a significant difference compared with placebo in response to ultramarathon running.
grade-d - - See study
No notable change in estimated glomerular filtration rate (eGFR) in type 2 diabetics over the course of 4 months.
grade-d - - See study
1 g/d for 4 months showed no effect in type 2 diabetic patients.
grade-d - - See study
No notable change in AST, ALT, GGT, or ALP in type 2 diabetics over the course of 4 months.
grade-d - - See study
1.5 g/d didn't lead to a significant difference compared with placebo in response to ultramarathon running.
grade-d
- See study
1.5 g/d didn't lead to a significant difference compared with placebo in response to ultramarathon running.
grade-d - - See study
1.5 g/d didn't lead to a significant difference compared with placebo in response to ultramarathon running.
grade-d - - See study
Supplementation of 1.5 g/d didn't alter the increase in response to ultramarathon running compared with placebo.
grade-d - - See study
No notable change in response to surgery-induced cortisol increases.
grade-d - - See study
Two weeks of 2 g per day didn't alter plasma vitamin E levels.
grade-d - - See study
No apparent effect when used acutely before exercise with a cycle ergometer.
grade-d - Very High See 2 studies
High dose vitamin C didn't have an effect in two exercise studies.
grade-d - - See study
No significant influence on the symptoms of osteoarthritis
grade-d - - See study
grade-d - - See study
It's simply not clear from one study and a small, non-significant reduction.
grade-d - Moderate See 2 studies
Possibly attenuates an increase from extreme exercise but it's unclear and especially unclear for other circumstances.

Studies Excluded from Consideration

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Frequently Asked Questions and Articles on Vitamin C

Does vitamin C improve blood sugar control?
We recently added several studies to the vitamin C page regarding blood sugar effects. From existing research, it seems theoretically possible that vitamin C could help improve blood sugar control, but the evidence is quite mixed at this point.
Are vitamin C supplements bad for your teeth?

Things to Note

Primary Function:

Also Known As

Ascorbic Acid, Ascorbate, 2-oxo-L-threo-hexono-1, 4-lactone-2, 3-enediol, L-ascorbic acid

Do Not Confuse With

L-Threonic Acid (a metabolite)

Goes Well With

  • Zinc and Iron (may enhance absorption)

Caution Notice

  • Superloading vitamin C, particularly via intravenous injections, has repeatedly been demonstrated in case studies to cause oxalate nephrotoxicity. This is treatable (potentially lethal if not treated) and may be a reason to avoid injections of vitamin C if not supervised by a medical doctor.

Click here to see all 374 references.