Ingredient review

Vitamin C

Description

What is it?

Vitamin C (ascorbic acid) is an essential, water-soluble vitamin. It is well-known for its antioxidative properties and ability to reduce free radicals by donating electrons through reduction reactions. (20) The discovery of vitamin C has been attributed to the treatment of scurvy in sailors and the potato famine in Ireland. (4)

Not to be confused with: L-threonate

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Main uses

Cardiovascular disorders
Chronic pain & inflammation
Immune function
Metabolic disorders

Formulations

Form
Bioavailability
Natural/food
Equal bioavailability to supplemental sources in humans (5)(22)
Ascorbic acid - liposomal
Relative bioavailability of liposomal-encapsulated ascorbic acid was increased by ~74% higher than unformulated ascorbic acid (7)
Ascorbic acid - unformulated
200 mg doses produces a plateau in plasma concentration, with full plasma saturation at 1000 mg (18)
Ascorbic acid - sustained-release
Conflicting evidence on bioavailability: Within 4-12 hours, single admin of fast release formulation increased relative plasma ascorbate by 94% (non-stat. signi.) than slow release. After 2-months, slow-release was 22% higher (non-stat. signi.) (24)
Single admin of slow-release formulations increased relative bioavailability of plasma ascorbate by 23-51% (stat. signi.) than fast-release tablets or solution (2)
Ascorbyl palmitate
Increased bioavailability compared with ascorbic acid in horses (28)
Calcium ascorbate
No difference in relative bioavailability in plasma ascorbate between Ester C® and standard ascorbic acid in humans, but more tolerable (10)(14)(23)
Single admin of CA increased plasma ascorbate up to 180% more than ascorbic acid in rats (3)(17)
Magnesium ascorbate
No information on bioavailability currently available
Potassium ascorbate
No information on bioavailability currently available
Sodium ascorbate
No information on bioavailability currently available
Vitamin C with bioflavonoids
No difference in bioavailability with AA or Ester C (14)

Dosing & administration

Adverse effects

Vitamin C is considered to be safe and tolerable. Exceeding the Tolerable Upper Intake Level (UL) of 2000 mg per day for adults can lead to gastrointestinal disturbances and osmotic diarrhea. The daily UL for children aged one to three is 400 mg, aged four to eight is 650 mg, and aged nine to 13 is 1800 mg. (9)

Pharmacokinetics

Absorption

  • Vitamin C predominantly exists in its reduced form, ascorbate, though it can be found in its oxidized form as dehydroascorbic acid (DHA). (21)
  • Ascorbate and DHA are efficiently interconvertible, but may be absorbed via different mechanisms. (21)
  • Ascorbate is absorbed via active transport in the small intestine through sodium-dependent vitamin C transporter 1 (SVCT1). (19)(21)(27)
  • DHA has a similar structure to glucose and is absorbed into the intestinal epithelium by glucose transporters GLUT1 and GLUT3, and into the blood by GLUT1 and GLUT2. (21)(26)

Distribution

  • Vitamin C can be primarily found circulating in plasma and erythrocytes (50-70µM), but can be found in the adrenals (4-10 µM), brain (2-10 µM), liver (~1 µM), lung (~1 µM), kidney (0.3-0.5 µM), heart (0.2-0.4 µM), and muscle (0.2-0.4 µM). (21)

Metabolism

  • Ascorbate is an electron donor, acting as an antioxidant. Ascorbate is oxidized to the stable ascorbyl free radical.
  • Intracellularly, DHA is efficiently reduced to ascorbate, recycling ascorbate pools. (21)

Excretion

  • If not absorbed back into plasma (via GLUT2 for DHA, and by an unknown mechanism for ascorbate), and not reabsorbed back into the renal epithelium (via SVCT1), vitamin C is excreted in the urine. (8)(21)

References