Vitamin C

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 supplementing vitamin C, on the other hand, 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.

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.

The Human Effect Matrix looks at human studies (it excludes animal and in vitro studies) to tell you what effects vitamin c has on your body, and how strong these effects are.

Grade	Level of Evidence [show legend]
	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
	Plasma Vitamin C	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).
	Blood Flow	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
	Blood Glucose	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.
	Cortisol	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.
	General Oxidation	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)
	Lipid Peroxidation	Minor	Low See all 8 studies	Mixed and weak influences on lipid peroxidation, but a possible reduction exists
	Muscle Damage	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
	Exercise-Induced Immune Suppression	-	High See all 6 studies	More evidence suggests no significant effect than a possible protective effect, although the latter is possible
	HDL-C	-	Very High See all 10 studies	No significant influence on HDL cholesterol
	HbA1c	-	Very High See all 10 studies	No significant influence of Vitamin C supplementation on HbA1c levels
	Inflammation	-	Very High See all 5 studies	No significant alterations seen in inflammatory cytokines associated with Vitamin C supplementation
	Insulin	-	- See all 11 studies	Mixed evidence across studies. Overall, there may be a modest reduction in fasting insulin levels, though more evidence is needed.
	LDL-C	-	Very High See all 9 studies	No significant alterations in LDL cholesterol seen with Vitamin C supplementation
	Protection from Smoking	-	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
	Total Cholesterol	-	Very High See all 13 studies	No significant influence on total cholesterol seems apparent with vitamin C supplementation
	Triglycerides	-	Very High See all 12 studies	No significant influence on fasting or postprandial triglycerides seems apparent with Vitamin C
	Adaptations to Exercise	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
	Anti-Oxidant Enzyme Profile	Minor	Very High See 2 studies	An increase in antioxidant enzymes have been noted in elderly persons
	Blood Pressure	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.
	Bone Mineral Density	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
	C-Reactive Protein	Minor	Very High See all 5 studies	A possible reduction in C-Reactive Protein exists with Vitamin C supplementation
	Fatigue	Minor	- See study	A decrease in fatigue has been noted in obese adults given Vitamin C in conjunction with exercise
	Frequency of Intercourse	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.
	Heart Rate	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.
	Microcirculation	Minor	- See study	An increase in microcirculation has been noted secondary to increased blood flow, thought to be a general property of antioxidants
	Muscle Soreness	Minor	- See study	A possible reduction in muscle soreness the day after exercise may result when preloading exercise with Vitamin C
	Rate of Perceived Exertion	Minor	- See study	The rate of percieved exertion in obese adults appears to be attenuated with Vitamin C supplementation
	Subjective Well-Being	Minor	- See study	An improvement in mood has been noted in hospitalized persons
	Symptoms of Bacterial Vaginosis	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
	Aerobic Exercise	-	Very High See all 5 studies	No significant influence on aerobic exercise performance
	DNA Damage	-	Very High See 2 studies	No significant influences on DNA damage
	Depression	-	- See study
	Exercise-Induced Oxidation	-	- 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
	Fat Mass	-	- See study	Does not appear to significantly influence fat mass
	Fat Oxidation	-	Very High See 2 studies	No significant influence on fat oxidation
	Glycemic Control	-	- See study	No significant influence on glycemic control in diabetics with Vitamin C supplementation
	Heart Palpitations	-	- See study	No significant influence on heart palpitations
	Insulin Sensitivity	-	- See study	No significant influence on insulin sensitivity
	Interleukin 6	-	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.
	Length of Sickness	-	Moderate See 2 studies	Mixed influences on the length one is sick for, with possibly no effect
	Mineral Bioaccumulation	-	- See study
	Oxidation of LDL	-	Very High See all 3 studies	No significant influence on the oxidation rates of LDL cholesterol
	Oxygenation Cost of Exercise	-	- See study
	Pre-Eclampsia Risk	-	- See study	No significant influence on pre-eclampsia risk
	Risk of Cataracts	-	- See study	No significant influence on the risk of cataracts
	Seminal Motility	-	- See study
	Sperm Quality	-	- See study
	Spontaneous Birth Risk	-	- See study	No significant influence on spontaneous birthing
	Symptoms of Charcot-Marie-Tooth Disease	-	- See study	Insufficient evidence to support a role
	Upper Respiratory Tract Infection Risk	-	Very High See 2 studies	No significant influence on the rate of acquiring sickness
	VO₂ Max	-	- See study	Does not appear to have a role in altering VO2 max
	Weight	-	Very High See all 4 studies	Vitamin C does not appear to have a weight reducing effect
	Resistin	Notable	- See study	There was a notable decrease in one study, but more research is needed to tell how reliable this effect is.
	Endothelial Function	Minor	High See all 3 studies	Attenuation of impairment due to either acute hyperglycemia or elevated free fatty acid levels
	Vascular Function	Minor	- See study	Seemed to attenuate the impairment due to acute hyperglycemia, but more research is needed.
	Adiponectin	-	- See study	No statistically significant effect of 200, 500 and 1000 mg/d for 14 days on postprandial levels.
	Adrenocorticotropic Hormone	-	- See study	Higher than control group at some points post-surgery, but not statistically significant.
	Asymmetric dimethylarginine (aka ADMA)	-	- See study
	Body Fat	-	Very High See 2 studies	No apparent effect on body weight over the course of 4 months.
	Creatinine	-	- See study	No difference from 500 mg/d after 3 months of supplementation in type 2 diabetes patients (within-group).
	Fructosamine	-	- See study	Unclear effect after taking 1 g/d for 4 months.
	Glomerular Filtration Rate	-	- See study	No difference compared with placebo after taking 6 g/d for 4 weeks.
	Glycemic Control / Insulin Sensitivity	-	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.
	IL-1Ra	-	- See study	Unclear if there was an effect of vitamin C compared with placebo in ultramarathon runners.
	Interferon Gamma	-	- See study	No apparent difference compared with placebo in response to ultramarathon running.
	Interleukin 10	-	- See study	Nonsignificant increase compared with placebo in response to ultramarathon running.
	Interleukin 2	-	- See study	Possible large reduction due to 7 days of taking 1.5 g/d, however, pre-supplementation measurements weren't taken.
	Interleukin 8	-	- See study	1.5 g/d didn't lead to a significant difference compared with placebo in response to ultramarathon running.
	Kidney Function	-	- See study	No notable change in estimated glomerular filtration rate (eGFR) in type 2 diabetics over the course of 4 months.
	Lean Mass	-	- See study	1 g/d for 4 months showed no effect in type 2 diabetic patients.
	Liver Enzymes	-	- See study	No notable change in AST, ALT, GGT, or ALP in type 2 diabetics over the course of 4 months.
	Lymphocyte Count	-	- See study	1.5 g/d didn't lead to a significant difference compared with placebo in response to ultramarathon running.
	Monocyte Count		- See study	1.5 g/d didn't lead to a significant difference compared with placebo in response to ultramarathon running.
	Natural Killer Cell Content	-	- See study	1.5 g/d didn't lead to a significant difference compared with placebo in response to ultramarathon running.
	Neutrophil Count	-	- See study	Supplementation of 1.5 g/d didn't alter the increase in response to ultramarathon running compared with placebo.
	Osteocalcin	-	- See study	No notable change in response to surgery-induced cortisol increases.
	Plasma Vitamin E	-	- See study	Two weeks of 2 g per day didn't alter plasma vitamin E levels.
	Power Output	-	- See study	No apparent effect when used acutely before exercise with a cycle ergometer.
	Rate of Perceived Exertion	-	Very High See 2 studies	High dose vitamin C didn't have an effect in two exercise studies.
	Symptoms of Osteoarthritis	-	- See study	No significant influence on the symptoms of osteoarthritis
	TNF-Alpha	-	- See study
	Urea	-	- See study	It's simply not clear from one study and a small, non-significant reduction.
	Uric Acid	-	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

• Confounded with Vitamin E^[1][2][3]

• Confounded with zinc^[4]