Vitamin C: a concentration-function approach yields pharmacology and therapeutic discoveries

Abstract

A concentration-function approach to vitamin C (ascorbate) has yielded new physiology and pharmacology discoveries. To determine the range of vitamin C concentrations possible in humans, pharmacokinetics studies were conducted. They showed that when vitamin C is ingested by mouth, plasma and tissue concentrations are tightly controlled by at least 3 mechanisms in healthy humans: absorption, tissue accumulation, and renal reabsorption. A 4th mechanism, rate of utilization, may be important in disease. With ingested amounts found in foods, vitamin C plasma concentrations do not exceed 100 μmol/L. Even with supplementation approaching maximally tolerated doses, ascorbate plasma concentrations are always <250 μmol/L and frequently <150 μmol/L. By contrast, when ascorbate is i.v. injected, tight control is bypassed until excess ascorbate is eliminated by glomerular filtration and renal excretion. With i.v. infusion, pharmacologic ascorbate concentrations of 25-30 mmol/L are safely achieved. Pharmacologic ascorbate can act as a pro-drug for hydrogen peroxide (H(2)O(2)) formation, which can lead to extracellular fluid at concentrations as high as 200 μmol/L. Pharmacologic ascorbate can elicit cytotoxicity toward cancer cells and slow the growth of tumors in experimental murine models. The effects of pharmacologic ascorbate should be further studied in diseases, such as cancer and infections, which may respond to generation of reactive oxygen species via H(2)O(2).

Conflict of interest statement

Author disclosures: M. Levine, S. J. Padayatty, and M. G. Espey, no conflicts of interest.

Figures

Figure 1

(A) Plasma vitamin C concentrations as a function of dose. The relationship between oral doses of vitamin C and the mean fasting steady-state plasma vitamin C concentration in 7 healthy men and 15 healthy women are shown. The daily doses of vitamin C were: 30, 60, 100, 200, 400, 1000, and 2500 mg. The dose concentration curve is sigmoidal with its steep portion between 30 and 100 mg of vitamin C daily. Plasma vitamin C concentrations likely to be attained by the consumption of 5 servings of fruits and vegetables per day (containing ∼200 mg of vitamin C) are also shown. It is possible that vitamin C bioavailability from fruits and vegetable is less than that from oral doses of pure vitamin C in solution. Adapted from (18) and (21). (B) Intracellular vitamin C concentrations in circulating cells as a function of dose in healthy women. Cells were isolated when steady state was achieved for each dose. Reproduced from (21) with permission of The National Academy of Sciences, Washington DC. (C) Urinary vitamin C excretion as a function of single vitamin C doses at steady state in 7 healthy men. Vitamin C excretion over 24 h was determined after administration of single doses given either orally or i.v. (Inset A) Vitamin C excretion for single oral or intravenous doses of 15–100 mg. x-axis indicates dose, y-axis indicates amount (mg) excreted in urine. (Inset B) Fractional excretion (the fraction of the dose excreted) after i.v. administration of single doses of vitamin C. x-axis indicates dose, y-axis indicates fractional excretion (vitamin C excreted in urine in milligrams divided by the vitamin C dose in milligrams). Reproduced from (18) with permission of The National Academy of Sciences, Washington DC.

Figure 2

(A) Determination of ascorbic acid bioavailability in a single participant using area under curve (AUC) methodology. Bioavailability was determined by calculating the ratio of the AUC of vitamin C plasma concentrations, following oral and i.v. administration of the same dose of vitamin C on successive days. AUC was calculated by the linear trapezoidal method. In the example shown, a healthy male was given 200-mg doses of vitamin C after he had attained steady state for that dose. (B) Plasma vitamin C concentrations in healthy volunteers as a function of time. Twelve participants (3 men, 9 women) were administered vitamin C 1.25 g in the fasting state after they had attained steady state for that dose. Plasma vitamin C concentrations following i.v. or oral administration are shown. Following i.v. administration, blood samples were collected at 0, 2.5, 5, 10, 15, and 30 min, and at 1, 1.5, 2, 2.5, 3, 3.5, 4, 5, 6, 7, 8, 9, and 10 h. Following oral administration, blood samples were collected at 0, 15, and 30 min, and at 1, 1.5, 2, 2.5, 3, 3.5, 4, 5, 6, 7, 8, 9,10, 11, 12, 13, 14, 15, 16, 19, 22, and 24 h. Adapted from (58) with permission of the Annals of Internal Medicine.

Figure 3

(A) In reaction 1, ascorbate (AscH−) reacts with ferric iron (Fe3+) to produce ferrous iron (Fe2+) and ascorbate radical (Asc·−). In reaction 2, the classic Fenton reaction generates the hydroxyl radical (OH·) species from H2O2. (B) Proposed mechanism of formation of ascorbate radical and H2O2 in extracellular fluid compared with blood. After oral and parenteral administration, ascorbic acid achieves equivalent pharmacological concentrations in blood (left side of the diagram) and extracellular fluid (right side). In extracellular fluid, a molecule of ascorbic acid loses 1 electron and forms ascorbate radical. This electron subsequently reduces a protein-centered metal, shown as the reduction of Fe3+ to Fe2+. This complex donates an electron to molecular oxygen, forming superoxide anion (O2·−) with ensuing dismutation to H2O2. In blood (left side), these reactions are damped or inhibited (dashed lines). The appearance of ascorbate radical is inhibited by RBC membrane-bound reducing proteins and/or by large plasma proteins that do not distribute to the extracellular space. RBC enzymes glutathione peroxidase and catalase destroy H2O2 so that none is detectable in blood. The identities of the metal-centered proteins are unknown. Reproduced from (59) with permission of The National Academy of Sciences, Washington DC.