Vitamin C

What It Does

Scurvy appears when intake falls below about 10 mg a day for a month or more: bleeding gums, bruising, corkscrew hairs, and wounds that will not heal. It still occurs in heavy alcohol users, older adults living alone, people on very restricted diets, and after bariatric surgery. Ascorbate also regenerates vitamin E and increases non-heme iron absorption by reducing ferric iron to the ferrous form.

The pharmacokinetics are the key fact. In an inpatient depletion-repletion study, plasma ascorbate was near saturation at 200 mg a day and reached its ceiling around 1,000 mg; single doses of 500 mg or more were absorbed less efficiently, and the excess was excreted in urine (Levine 1996). Oral dosing cannot push plasma ascorbate above roughly 220 micromol/L; intravenous dosing reaches levels many times higher (Padayatty 2004). That gap is why IV and oral vitamin C should not be discussed as the same treatment.

Most of the popular uses fail on evidence. A Cochrane review of regular supplementation found no reduction in how often people catch colds, except among marathon runners, soldiers, and others under extreme physical stress, where risk was halved. Regular use shortened colds by about 8% in adults; starting vitamin C after symptoms began had no consistent effect (Hemilä 2013).

The harm is kidney stones. Ascorbate is partly metabolized to oxalate, and in a Swedish cohort men taking vitamin C supplements had about twice the risk of kidney stones (Thomas 2013). In hemochromatosis, high doses increase iron absorption.

Smokers need 35 mg a day more than nonsmokers because oxidative stress consumes ascorbate. Most people who eat fruit and vegetables reach saturation from food.

Forms and Bioavailability

Absorption ratings are broad tiers. Head-to-head human trials rarely support a finer ranking among well-absorbed forms.

Labels column: number of labels named as vitamin C products that list this form. A product can list more than one form, and some labels do not state a form at all.

Ascorbic acid is the reference form; natural and synthetic ascorbic acid are identical. Sodium and calcium ascorbate are buffered salts that are less acidic in the stomach and absorbed the same way; they carry meaningful sodium or calcium. Liposomal vitamin C produced higher plasma levels than an equal 4 g dose of plain ascorbic acid in an 11-person study, but stayed far below IV levels (Davis 2016). Ascorbyl palmitate is mainly a fat-soluble preservative and less than half vitamin C by weight.

Typical Dosing

Absorption is 70-90% at intakes of 30-180 mg per day and falls below 50% above 1 g per day.

Food sources: red bell pepper, orange juice, oranges, grapefruit juice, kiwifruit, green bell pepper, broccoli, strawberries.

What to Look For

For most people, 100-250 mg a day reaches the plateau. A 1,000 mg tablet mostly ends up in urine, and larger amounts are better absorbed as two smaller doses than one big one.

Buffered forms are a stomach-comfort choice, not an absorption choice. One gram of sodium ascorbate carries about 110 mg of sodium; one gram of calcium ascorbate carries roughly 90-100 mg of calcium.

Red flags: liposomal products marketed as an oral substitute for IV vitamin C, multi-gram daily doses in anyone with a history of kidney stones, and "immune" products that pair megadoses of vitamin C with unclear amounts of other ingredients.

Products on Titrate

38,122 of the 214,745 labels in the NIH Dietary Supplement Label Database list vitamin C as an active ingredient (19,477 on market). 3,052 are sold as vitamin C products by name.

Top on-market labels, ranked by label completeness and simplicity:

1. Vitamin C Shield Body Kitchen · 1 g as liposomal
2. Vitamin C+ Zemvelo · 500 mg as ascorbic acid
3. Dynamic Liposomal Vitamin C Stonehenge Health · 1.5 g as ascorbic acid
4. Ester-C Plus 1000 mg Vitamin C Solgar · 1 g
5. Ester-C Plus 1000 mg Vitamin C Solgar · 1 g

Open supplement search

Vitamin C does not have its own search category yet. Each product above links to its full label in the NIH database.

Related Tools

• Iron Absorption Estimator
• Supplement Timing Optimizer
• Ascorbic Acid vs Calcium Ascorbate
• Ascorbic Acid vs Liposomal Vitamin C
• Ascorbic Acid vs Ascorbyl Palmitate
• Ascorbic Acid vs Sodium Ascorbate
• Ascorbyl Palmitate vs Liposomal Vitamin C
• Calcium Ascorbate vs Sodium Ascorbate
• All vitamin C form comparisons
• All supplement guides

References

NIH Office of Dietary Supplements. Vitamin C: Fact Sheet for Health Professionals.

Levine M, et al. Vitamin C pharmacokinetics in healthy volunteers: evidence for a recommended dietary allowance. Proc Natl Acad Sci U S A. 1996;93(8):3704-3709.

Padayatty SJ, et al. Vitamin C pharmacokinetics: implications for oral and intravenous use. Ann Intern Med. 2004;140(7):533-537.

Hemilä H, Chalker E. Vitamin C for preventing and treating the common cold. Cochrane Database Syst Rev. 2013;(1):CD000980.

Thomas LD, et al. Ascorbic acid supplements and kidney stone incidence among men: a prospective study. JAMA Intern Med. 2013;173(5):386-388.

Davis JL, et al. Liposomal-encapsulated Ascorbic Acid: Influence on Vitamin C Bioavailability and Capacity to Protect Against Ischemia-Reperfusion Injury. Nutr Metab Insights. 2016;9:25-30.