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glutamyl folate tracer. The authors concluded that naturally occurring polyglutamyl folates in orange juice are approximately 67 percent available—slightly more available than the food folate bioavailability estimate of Sauberlich. Related issues have been discussed in several reviews on this subject (Gregory, 1989, 1995, 1997).

Bioavailability Estimates and Assumptions

Many controlled studies on folate requirements have used a defined diet (food folate) supplemented with folic acid. Because folic acid taken with food is 85 percent bioavailable but food folate is only about 50 percent bioavailable, folic acid taken with food is 85/ 50 (i.e., 1.7) times more available. Thus, if a mixture of folic acid plus food folate has been fed, dietary folate equivalents (DFEs) are calculated as follows to determine the Estimated Average Requirement (EAR):

µg of DFEs provided = µg of food folate + (1.7 × µg of folic acid)

Expressed differently, to be comparable with food folate, only half as much folic acid is needed if taken on an empty stomach, or

1 µg of DFEs = 1 µg of food folate = 0.5 µg of folic acid taken on an empty stomach = 0.6 µg of folic acid with meals.

When food folate was the sole source of folate in studies used to determine requirements, no corrections were applied to convert to DFEs. Adjustments made for DFEs are indicated, if applicable, where folic acid was a source of folate. Adjustments cannot be made for epidemiological studies if data are lacking on the folate sources. If future research indicates that food folate is more than 50 percent bioavailable, this could lower the estimated requirements that appear later in the chapter.

Nutrient-Nutrient Interactions

No reports were found that demonstrate that the intake of other nutrients increases or decreases the requirement for folate. However, coexisting iron or vitamin B12 deficiency may interfere with the diagnosis of folate deficiency. In contrast to folate deficiency, iron deficiency leads to a decrease in mean cell volume. In the combined deficiency, interpretation of hematological changes may be unclear (Herbert, 1962a). A vitamin B12 deficiency results in the

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