Vitamin C
Also called: ascorbic acid, sodium ascorbate, calcium ascorbate, buffered vitamin C.
A water-soluble vitamin whose absorption caps out as the dose rises, and whose most concrete effect is on non-heme iron.
What it is
Vitamin C serves as a cofactor in the synthesis of collagen, carnitine and certain neurotransmitters, and acts as an antioxidant. The human body does not make it and does not store large amounts of it. People buy it mostly for immunity, skin and the cold season.
What the label does not say
- Absorption saturates: 70 to 90% between 30 and 180 mg per day, but less than 50% above 1 g per day, with the absorbed excess excreted unchanged in urine. Going from 500 mg to 2,000 mg does not proportionally increase what gets in.
- The plasma ceiling is physiological. Even 3 g every 4 hours by mouth reaches only about 220 micromoles/L, whereas the intravenous route reaches up to 26,000 micromoles/L. That is what separates the oral megadoses of the Pauling era from the trials that followed them.
- The digestive effects of high doses (diarrhea, nausea, cramps) come from the osmotic effect of unabsorbed vitamin C, and so directly from that saturation.
- Vitamin C improves non-heme iron absorption. That is an asset at a plant-based meal, but the NIH indicates that chronic consumption of high doses could worsen iron overload in someone with hereditary hemochromatosis.
The forms on the shelf
- Ascorbic acid: the most common form. The NIH concludes that plain ascorbic acid is the source to favour.
- Mineral ascorbates, called buffered (sodium, calcium): chosen for digestive tolerance. A study cited by the NIH found no difference in plasma concentration or urinary excretion between three sources of vitamin C.
- Ascorbic acid with bioflavonoids and combination products: the NIH reports that one combination product gave the same plasma concentrations as ascorbic acid, with higher leukocyte concentrations 24 hours after the dose.
- Liposomal forms: they are not among the forms evaluated in the NIH fact sheet, and the comparative data could not be verified in a source acceptable here.
Published reference values
- Recommended intake for adults: 90 mg per day for men, 75 mg for women. The NIH adds 35 mg per day for people who smoke.
- Upper limit: 2,000 mg per day for adults.
- On the common cold, a Cochrane review cited by the NIH reports a modest reduction in duration of 8% in adults and 14% in children with regular use, no reduction in incidence in the general population, and no benefit when use starts after symptoms appear.
- High doses increase urinary excretion of oxalate and uric acid, which could contribute to kidney stones, especially with pre-existing hyperoxaluria. The studies are contradictory.
When
At the meal containing plant iron if the goal is iron absorption. Splitting into two doses is more consistent with absorption saturation than a single high dose.
Keep apart from
- Non-heme iron: not to be spaced apart, quite the opposite. Vitamin C at the same meal increases its absorption.
To mention to a professional
- Hemochromatosis or iron overload, history of kidney stones, kidney disease, pregnancy: check with a health care professional.
From the plate first
According to the NIH: 95 mg per half cup of raw red pepper, 93 mg per three-quarter cup of orange juice, 70 mg per medium orange, 64 mg per kiwi, 51 mg per half cup of cooked broccoli.
Sources
- NIH Office of Dietary Supplements, Vitamin C (Health Professional)
- Health Canada, Dietary Reference Intakes, reference values for vitamins
Educational information, drawn from the sources above. It does not replace the advice of a health professional, who remains the reference for your situation, your medications, pregnancy or breastfeeding.