Vitamin C · Evidence
Each point below summarizes specific sources, linked by number to the reference list at the bottom. Where the research is thin or mixed, we say so.
The NIH Health Professional fact sheet says vitamin C is needed to make collagen, L-carnitine and some neurotransmitters. It is an important antioxidant, plays a role in immune function, and improves absorption of nonheme iron, the kind in plant foods.[1]
Supplements usually contain ascorbic acid. Some use sodium ascorbate, calcium ascorbate, other mineral ascorbates, ascorbic acid with bioflavonoids, or Ester-C. The Health Professional fact sheet says ascorbic acid has equivalent bioavailability to vitamin C in foods, and that the authors of a review comparing forms concluded simple ascorbic acid is the preferred source. The fact sheet doesn’t discuss liposomal vitamin C.[2][1]
Grouped by how much research backs each outcome. A stronger group means more consistent results across more studies, not a bigger effect.
In trials of marathon runners, skiers and soldiers doing exercises in subarctic conditions, regular vitamin C at 250 mg to 1 g a day cut the number of colds by 50%.[1]
The Health Professional fact sheet, describing a Cochrane review, says taking vitamin C regularly did not significantly lower the risk of catching a cold in the general population.[1][2]
The Health Professional fact sheet says most randomized trials found vitamin C, alone or with other nutrients, does not affect cancer risk. For example, 500 mg with vitamin E in the Physicians’ Health Study II (about 8 years, men) didn’t lower prostate or total cancer, and 500 mg a day in the Women’s Antioxidant Cardiovascular Study (9.4 years) had no significant effect on cancer rates or cancer deaths.[1]
Some observational studies link more vitamin C with lower heart disease risk, and a 2008 meta-analysis of 14 cohorts found this for vitamin C from food but not from supplements. In randomized trials, 500 mg a day had no overall effect on cardiovascular events in women (9.4 years) or major cardiovascular events in men (about 8 years). The fact sheet concludes that most trials don’t give convincing evidence that vitamin C supplements protect against cardiovascular disease.[1]
The NIH fact sheets list a Recommended Dietary Allowance of 90 mg a day for adult men and 75 mg for adult women, from all sources; 85 mg in pregnancy and 120 mg while breastfeeding. People who smoke need 35 mg more a day.[2][1]
The upper limit for adults is 2,000 mg a day from food, drinks and supplements combined.[2][1]
According to the NIH fact sheets, high doses can cause diarrhea, nausea and stomach cramps. In people with hemochromatosis, long-term high doses could worsen iron overload and damage tissue.[2][1]
The Health Professional fact sheet says high intakes can raise oxalate and uric acid in urine, but studies of 30 mg to 10 g a day have had conflicting results. The best evidence that vitamin C adds to kidney stone formation is in people who already have hyperoxaluria (high urine oxalate).[1]
The fact sheets say the safety of vitamin C supplements during chemotherapy and radiation is controversial, so patients should ask their oncologist first. Vitamin C with other antioxidants may blunt the rise in HDL cholesterol from niacin taken with simvastatin.[2][1]