Plasma and Leukocyte Concentrations of Ascorbate

The plasma concentration of vitamin C falls relatively rapidly during depletion studies, to undetectably low levels within 4 weeks of initiating a vitamin C-free diet, although clinical signs of scurvy may not develop for a further 3 to

Table 13.2 Plasma and Leukocyte Ascorbate Concentrations as Criteria of Vitamin C Nutritional Status

Deficient

Marginal

Adequate

Whole blood

mmol/L

<17

17-28

>28

mg/L

<3.0

3.0-5.0

>5.0

Plasma

mmol/L

<11

11-17

>17

mg/L

<2.0

2.0-3.0

>3.0

Leukocytes

pmol/106 cells

<1.1

1.1-2.8

>2.8

|xg/106 cells

<0.2

0.2-0.5

>0.5

4 months and tissue concentrations of the vitamin may be as high as 50% of saturation. In field studies and surveys, subjects with plasma ascorbate below 11 ^mol per L are considered to be at risk of developing scurvy (see Table 13.2), whereas anyone with a plasma concentration below 6 ^mol per L would be expected to show clinical signs. At intakes above about 100 mg per day, the plasma concentration of ascorbate reaches a plateau around 70 to 80 ^mol per L, because of quantitative excretion of the vitamin as the renal threshold is exceeded (Section 13.2.4).

The concentration of ascorbate in leukocytes is well correlated with the concentrations in other tissues and falls more slowly than plasma concentration in depletion studies. The reference range of leukocyte ascorbate is 1.1 to 2.8 pmol per 106 cells; a significant loss of leukocyte ascorbate coincides with the development of clear clinical signs of scurvy. Predictably, at high levels of ascorbate intake, although the plasma concentration continues to increase with intake, the leukocyte content does not, because the cells, like othertissues, are saturated.

There is a problem in the interpretation of leukocyte concentrations of ascorbate as an index of vitamin C nutritional status. As discussed in Section 13.2.2, the different types of leukocytes have different capacities to accumulate ascorbate. This means that a change in the proportion of granulocytes, platelets, and mononuclear leukocytes will result in a change in the total concentration of ascorbate per 106 cells, although there may well be no change in vitamin nutritional status. Stress, myocardial infarction, infection, burns, and surgical trauma all result in changes in leukocyte distribution, with an increase in the proportion of granulocytes that achieve saturation at a lower concentration of ascorbate than other leukocytes, and thus an apparent change in leukocyte ascorbate. This has been misinterpreted to indicate an increased requirement for vitamin C in these conditions (Schorah et al., 1986). Without a differential white cell count, leukocyte ascorbate concentration cannot be considered to give a meaningful reflection of vitamin C status.

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Responses

  • Bucca
    Is leukocyte concentration a better indicator of vitamin c stores than plasma level?
    14 days ago

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