Vitamin C

What it is

Most animals make their own vitamin C from glucose. Humans and a few other mammals lack the enzyme to do it, which is why the vitamin is essential in the diet.^[1] In the body it does two kinds of work. It is a strong reducing agent, so it acts as a water-soluble antioxidant. And it is a cofactor for enzymes involved in making collagen, carnitine and neurotransmitters.^[1] The collagen role is why the classic deficiency disease, scurvy, shows up in connective tissue such as the gums and skin.^[1]

Vitamin C also affects iron. Added to a meal, it increases absorption of nonheme iron, the form in plants and fortified foods. Long-term studies of adding it to meals, though, have not shown a meaningful improvement in body iron status.^[1]

Almost 90 percent of the vitamin C in a typical diet comes from fruits and vegetables. Citrus fruits, tomatoes and potatoes are major contributors, and broccoli, Brussels sprouts, cauliflower, strawberries, cabbage and spinach add more.^[1] Ascorbic acid is vitamin C itself, and the vitamin from supplements and from foods has not been shown to be absorbed differently.^[1] Replenish uses ascorbic acid.

Absorption depends on the dose. About 70 to 90 percent of usual intakes, 30 to 180 mg a day, is absorbed. Above 1 g a day, absorption falls to about 50 percent or less, and the kidneys excrete more of what is absorbed once intake passes about 80 mg a day.^[1]

How much adults need

Source: Institute of Medicine, Dietary Reference Intakes for Vitamin C, Vitamin E, Selenium, and Carotenoids (2000), chapter 5.^[1] The smokers' row adds the report's extra 35 mg a day to the RDA. RDA = Recommended Dietary Allowance; UL = Tolerable Upper Intake Level.

Amount in Replenish: confirmed at launch

The RDA was set at the intake that keeps vitamin C in white blood cells near their maximum with little lost in urine. Smoking increases oxidative stress and the turnover of the vitamin, so smokers need 35 mg a day more.^[1] When the report was written, the median intake from food was about 102 mg a day in the United States.^[1] Because most of it comes from fruit and vegetables, a smaller appetite that crowds out produce takes vitamin C with it. How to keep produce in on small-appetite days is in what to eat when you're eating less.

The UL of 2,000 mg is the highest daily intake unlikely to cause harm in healthy adults. It is based on diarrhea and other gut upset, which the report described as mild and reversible.^[1]

What the evidence says

Colds. The 2013 Cochrane systematic review on vitamin C and the common cold included only placebo-controlled trials using at least 200 mg a day. In general-population trials with 10,708 participants, taking vitamin C every day did not reduce the chance of catching a cold (risk ratio 0.97). In five trials of 598 marathon runners, skiers and soldiers on subarctic exercises, it roughly halved the risk (risk ratio 0.48).^[2]

Regular vitamin C did shorten colds a little: by 8 percent in adults and 14 percent in children. Starting it only once symptoms began showed no consistent effect on how long or how bad colds were, in the few trials that tested it.^[2] The authors concluded that routine supplementation to prevent colds is not justified for the general population.^[2]

Antioxidant effects. Of 13 studies the Institute of Medicine summarized that tested vitamin C supplements alone on markers of lipid oxidation, 7 found a significant decrease, at doses of 500 to 2,000 mg a day.^[1] The report set the RDA on the intake that nearly saturates white blood cells, not on disease outcomes. It noted that many supplement studies in well-nourished people tested doses at or above the point where tissues are saturated, which can hide any link between ordinary intakes and disease, and suggested that future studies focus on people who eat few fruits and vegetables.^[1]

Safety and interactions

Gut upset at high doses. Unabsorbed vitamin C is broken down in the intestine, which is the likely reason very large doses cause diarrhea and stomach discomfort. That is the effect the UL is built on.^[1]

Kidney stones. The body breaks some vitamin C down to oxalate, and kidney stone formation is among the possible effects of high intakes the Institute of Medicine considered.^[1] In three large US cohorts followed for about 11 years, men whose total vitamin C intake was 1,000 mg a day or more had a 43 percent higher risk of a first kidney stone than men taking in under 90 mg; supplemental intake of 1,000 mg or more was linked to a 19 percent higher risk than taking no supplement. No link was seen in women, and vitamin C from food was not linked to stones in either sex.^[3] These are observational associations, not trial results. People who have had stones should ask their doctor before taking high-dose vitamin C.

Iron overload. In iron-replete people, up to 2 g a day taken with meals for 20 months did not raise iron stores. Whether long-term high doses could harm people with hereditary hemochromatosis is not known.^[1]

Medicines. We did not find a well-documented interaction between vitamin C at nutrient amounts and a common drug class that we could cite from a primary source, so we have not listed one. High-dose vitamin C alongside specific treatments, such as cancer therapy, is a question for the treating team.

If you take prescription medicine, ask your pharmacist or prescriber before adding a supplement.

In Replenish

Replenish contains vitamin C as ascorbic acid, one of its six vitamins. It is part of how Replenish provides key vitamins and minerals.^† The amount per serving is published with the Supplement Facts panel on the Replenish page. The other vitamins in the formula have their own pages, including vitamin D and vitamin B12, and the minerals include zinc. Which nutrients tend to run short when meals get smaller is covered in which vitamins matter on a GLP-1.