Plasma and Erythrocyte Concentrations of Folate and Vitamin B2

Measurement of plasma concentrations of the two vitamins is probably the method of choice, and a number of simple and reliable radioligand binding assays have been developed, some of which permit simultaneous determination of both vitamins. Nevertheless, there are a number of problems involved in radioligand binding assays, especially for folate. In some centers, microbiological determination of plasma or whole blood folates is the preferred technique. The ligand binding assays that are normally used to determine erythrocyte folate are specific for 5-methyl-folate. They may not detect formyl-folates that are present in significant amounts in erythrocytes from people who are homozygous for the thermolabile variant of methylene-tetrahydrofolate reductase (Bagley and Selhub, 1998). Radioligand binding assays for vitamin B12 may give falsely high values if the binding protein is cobalophilin, which binds a number of metabolically inactive corrinoids, as well as cobalamins. More precise determination of true vitamin B12 comes from assays in which the binding protein is purified intrinsic factor, although this may still detect some corrinoids without vitamin activity.

Erythrocytes accumulate a considerably higher concentration of folate than is present in plasma (Section 10.2.2). Folate is incorporated into erythrocytes during erythropoeisis and does not enter the cells in the circulation to any significant extent. Therefore, erythrocyte folate is generally considered to give an indication of folate status over 1 to 3 months (the life span of erythrocytes in the circulation is 120 days) and not to be subject to variations in recent intake, as is the plasma concentration of most vitamins. However, folate binds to deoxyhemoglobin considerably more tightly than to oxyhemoglobin, and the degree of oxygenation of hemoglobin in vitro, in the sample being used for assay, will affect the amount of folate that is free in solution and accessible for determination. This means that, without standardization of assay conditions, it is difficult to compare the results of erythrocyte folate determinations from different laboratories (Wright et al., 1998).

A serum concentration of vitamin B12 below 110 pmol per L is associated with megaloblastic bone marrow, incipient anemia, and myelin damage. Below

Table 10.2 Indices of Folate and Vitamin Bi2 Nutritional Status

Reference Range Deficiency nmol/L ^g/L nmol/L ^g/L

Serum folate

9.8-

-16.2

4.4

-7.2

<6.8 <3

Erythrocyte folate

420-

620

185-

-270

<320 <140

Whole blood vitamin B12

0.22-

-0.65

0.29-

-0.87

— —

Serum vitamin B12

0.14-

-0.52

0.19-

0.69

<0.075 <0.10

Erythrocyte vitamin B12

0.06-

0.21

0.08-

0.28

——

Transcorrin II bound vitamin B12

<0.15 <0.22

Mean cell volume

> 100 fL

Serum methylmalonic acid

> 1 |xmol/L

Serum homocysteine

>20 |imol/L

Urine FIGLU more than 8 h after

>50 |g /mL

histidine load

Excretion of radiolabeled vitamin B12

16

-45%

<5%

(Schilling test)

Sources: From data reported by Herbert, 1987a,1987b; van den Berg, 1993; Bailey and Gregory, 1999.

150 pmol per L, there are early bone marrow changes, abnormalities of the dUMP suppression test (Section 10.10.5), and methylmalonic aciduria alter a valine load (Section 10.10.3). As shown in Table 10.2, this is considered to be the lower limit of adequacy.

Serum folate below 7 nmol per L or erythrocyte folate below 320 nmol per L indicates negative folate balance and early depletion of body reserves. At this stage, the first bone marrow changes are detectable.

About30% ofvitamin B12-deficient subjects have elevated serumfolate. This is mainly methyl-tetrahydrofolate, the result of the methyl folate trap (Section 10.3.4.1). About one-third of folate-deficient subjects have low serum vitamin B12; the reason for this is not clear, but it responds to the administration of folate supplements.

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