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9.3A: Vitamin C Absorption & Tissue Accumulation

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    1518
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    Vitamin C is found in foods primarily as ascorbic acid (80-90%), but dehydroascorbic acid (10-20%) is also present. The bioavailability of vitamin C is high at lower doses as shown below, but drops to less than 50% at higher doses.

    Table 9.311 Bioavailability of vitamin C1

    Dose (mg)

    % Bioavailability

    200

    112

    500

    73

    1250

    49

    Ascorbic acid is actively absorbed by the sodium vitamin C cotransporter (SVCT) 1. This active transport is driven by the sodium electrochemical gradient created by sodium-potassium ATPase. Ascorbic acid then diffuses into the capillary and ultimately enters general circulation. Vitamin C generally circulates as ascorbic acid.

    Figure 9.311 Ascorbic acid (Asc) absorption

    Accumulation

    Most water-soluble vitamins are not stored in the body. Vitamin C is not stored, but is accumulated in certain tissues in the body where it can be 5-100 times higher than found in the plasma2. The table below shows the concentrations of vitamin C in different tissues and fluids.

    Table 9.312 Human tissue & fluid ascorbic acid concentrations1

    Organ/Tissue

    Vitamin C Concentration*

    Organ/Tissue

    Vitamin C Concentration*

    Pituitary Gland

    40-50

    Lungs

    7

    Adrenal Gland

    30-40

    Skeletal Muscle

    3-4

    Eye Lens

    25-31

    Testes

    3

    Liver

    10-16

    Thyroid

    2

    Brain

    13-15

    Cerebrospinal Fluid

    3.8

    Pancreas

    10-15

    Plasma

    0.4-1

    Spleen

    10-15

    Saliva

    0.1-9.1

    Kidneys

    5-15

     

     

    * mg/100 g wet tissue, mg/100 mL fluids

    How does the body accumulate such high levels of vitamin C? There are 2 primary mechanisms:

    1.  Ascorbic Acid (Ascorbate) uptake using sodium-dependent vitamin C transporter (SVCT) 1 or 2
    2. Ascorbic Acid (Ascorbate) Recycling

     Ascorbic Acid (Ascorbate) transport using sodium-dependent vitamin C transporter (SVCT) 1 or 2

    As shown below, SVCT 1 and SVCT 2 transport ascorbic acid or ascorbate into the cell against the concentration gradient (represented by the orange wedge in the figure below). Like absorption, this uptake is driven by the action of sodium-potassium ATPase. This mechanism is saturable, meaning that at high concentrations it reaches a threshold where it cannot take up ascorbic acid any faster. Thus, there is a limit to how much can be taken up through this mechanism3.


    Figure 9.312 Ascorbic acid (Asc) uptake using SVCT 1 and 2 against the ascorbic acid concentration gradient3

    Ascorbic Acid (Ascorbate) Recycling

    In ascorbic acid recycling, ascorbic acid is oxidized to dehydroascorbic acid (DHA). DHA is then transported into the cell moving with its concentration gradient using GLUT1 or 3. Once inside the cell, DHA is reduced back to ascorbic acid, thus maintaining the DHA gradient. As a result, the cell is able to accumulate high levels of ascorbic acid3. The figure below depicts ascorbic acid recycling. 

    Figure 9.313 Ascorbic acid recycling3

    References & Links

    1.  Shils ME, Shike M, Ross AC, Caballero B, Cousins RJ, editors. (2006) Modern nutrition in health and disease. Baltimore, MD: Lippincott Williams & Wilkins.
    2.  Stipanuk MH. (2006) Biochemical, physiological, & molecular aspects of human nutrition. St. Louis, MO: Saunders Elsevier.
    3.  Li Y, Schellhorn H. (2007) New developments and novel therapeutic perspectives for vitamin C. J Nutr 137(10): 2171-2184.