When to take vitamin C

Short answer

No measured advantage of any timing for vitamin C, such as morning, evening, fasting or with food, in changing absorption has been shown. What has been measured is that the percentage absorbed falls as the amount at one time rises: according to the NIH, absorption is about 70–90% at 30–180 mg per day and below 50% above 1 g per day. ^[1] This page does not recommend personal doses or schedules.

Is vitamin C taken in the morning or evening, fasting or with food?

Vitamin C is a water-soluble vitamin. No study directly comparing morning and evening or fasting and fed intake appears in the NIH review; this means “no difference has been shown”, not “there is no difference”. The measured information given by the NIH is as follows: the bioavailability of ascorbic acid in supplements is equivalent to that of ascorbic acid in foods such as orange juice and broccoli, and absorption in the intestine is regulated by at least one dose-dependent active transporter. ^[1] The NIH also notes that dehydroascorbic acid, the oxidized form, enters cells in laboratory studies through some glucose transporters, but that its contribution to vitamin C balance in the body is unknown. ^[1]

Why does absorption fall as the dose rises, and does splitting make sense?

Absorption is saturable. In the hospital study by Levine and colleagues (seven healthy volunteers, 4–6 months, seven daily doses between 30–2,500 mg), the bioavailability of a single 200 mg dose was complete; at single doses of 500 mg and above, bioavailability decreased and the absorbed excess was excreted in urine. Plasma level drew a sigmoid curve with dose: the curve flattened beyond 200 mg/day, and full plasma saturation was seen at 1,000 mg/day. Urinary oxalate and urate excretion increased at 1,000 mg/day compared with lower doses. ^[2]

In the same study, in six of seven volunteers no vitamin C was seen in urine until the daily dose reached 100 mg; neutrophils, monocytes and lymphocytes reached saturation at 100 mg/day and carried at least 14-fold higher concentration than plasma. That is, the kidneys begin to excrete the excess as the body pool approaches filling. Because it was a seven-person, hospital-controlled physiology study, these thresholds may vary from person to person. ^[2]

The NIH reports that oral 1.25 g/day of ascorbic acid produces a mean peak plasma level of about 135 µmol/L (about twice what is seen with 200–300 mg per day from food), and that very high oral doses such as 3 g every 4 hours would raise the peak level only to about 220 µmol/L. ^[1] These data show that a large single amount is not absorbed proportionally more; data comparing health outcomes of split and single doses do not appear in the NIH review; this page does not recommend a splitting schedule. The upper limit (UL) for adults is 2,000 mg/day in total from food and supplements. ^[1]

Is it taken together with iron?

The NIH reports that vitamin C increases the absorption of non-heme iron found in plant foods. ^[1] This is valid physiological information for food combinations (a plant iron source with a vitamin C source). For iron medication (oral iron therapy), the situation is different: the 2021 guideline of the British Society of Gastroenterology (BSG) does not recommend adding vitamin C to iron therapy, noting that although it was previously thought to be beneficial, a large randomized trial showed that vitamin C did not increase the hematologic response or the rate of replenishing stores and did not reduce side effects. ^[6] In people with low iron levels, the treatment plan is made with a physician. In people with hereditary hemochromatosis, chronic high-dose vitamin C can worsen iron overload. ^[1]

Which forms exist, and do they make a difference?

According to the NIH, the most common form in supplements is ascorbic acid; sodium ascorbate, calcium ascorbate and other mineral ascorbates, ascorbic acid with bioflavonoids, and combination products such as Ester-C also exist. In one of the two small human studies cited by the NIH, Ester-C and ascorbic acid gave the same plasma level while Ester-C provided a significantly higher level in leukocytes after 24 hours; in the other study, no difference was found among ascorbic acid, Ester-C and ascorbic acid with bioflavonoids in plasma level or urinary excretion. The authors of the study considered plain ascorbic acid the preferred source because of these findings and its low cost. ^[1]

Liposomal vitamin C. In the study by Davis and colleagues, 11 women and men received placebo or 4 g vitamin C (oral, oral liposomal or intravenous) in four separate random orders. The liposomal oral form gave a higher circulating level than the unencapsulated oral form but remained lower than intravenous administration; the three active administrations were similar in protection against oxidative stress caused by ischemia-reperfusion. The study was funded by the manufacturer supplying the liposomes; one of the authors owns a company that produces and sells liposomal vitamin C (declared in the article). ^[4] This is a very small, short study; the 4 g dose is twice the 2,000 mg/day UL and is not a recommendation for daily use. Liposomal form has not been shown to improve health outcomes with daily use.

Does taking it once a cold starts work?

In the Cochrane review, no consistent effect on illness duration or severity was seen with vitamin C started after cold symptoms began (7 comparisons, 3,249 illness episodes). With regular use, duration was shortened by 8% in adults and 14% in children. ^[3] This is the only example of the “timing” question measured in studies: prior regular use and starting once illness began gave different results.

What should be kept in mind with high-dose use?

For high-dose supplements, digestive complaints and, in observational studies, kidney stone risk in men come up. In the analysis by Ferraro and colleagues, total vitamin C intake of ≥1,000 mg per day in men was associated with an HR of 1.43 (95% CI 1.15–1.79) compared with <90 mg; there was no significant association in women. ^[5] Details are on the excess page.

Detailed guideWhat is vitamin C? Main guide→ Detailed guideVitamin C excess: digestive complaints, kidney stones and iron→ Detailed guideVitamin C benefits: which claims are supported?→ Detailed guideWhich foods contain vitamin C?→ Detailed guideWhen to take iron medication: vitamin C, tea and medication intervals→

This guide does not recommend personal doses.