## 4 Discussion  
Dietary folates derived from foods are mostly MTHF including the many reduced and oxidized polyglutamate forms. These are converted to monoglutamate by the conjugase activity in the gut prior to absorption in the upper ilium via PCFT expressed on the villi of the ileal enterocyte [6]. Cooking and processing are likely to oxidize or break down some of the folates. The breakdown products may be a source of carbon units for gut microflora to sustain growth and convert to active forms of folate. This process more than likely meets the daily dietary folate requirement under normal conditions and adequate dietary intake. The increased demand for folates during pregnancy and infancy coupled with genetic and metabolic abnormalities that affect folate pathways, require additional folate supplementation [38; 39; 40]. The recent identification of folate receptor autoantibodies and their significant association with NTD pregnancy, cerebral folate deficiency (CFD) syndrome, and ASD has required the use of pharmacologic doses of foliinic acid to overcome the blocking of folate transport by antibodies and use alternate pathways of folate uptake to restore folate status [16; 25; 26; 41; 42]. Absorption and distribution of folate forms in the rat indicate that high DHFR activity exists in the rat gut with a substantial capacity to rapidly convert them to MTHF [8; 43; 44]. Based on the peak appearance of MTHF in the blood, at the dose administered, approximately 70% of levofoliate and d,l-folinic acid (0.6-0.7 mg) is converted to MTHF in the first hour. This drops by about 50% for folic acid, which may be attributed to the processing of folic acid through DHFR. Nevertheless, the gut appears to have a high capacity to absorb and rapidly convert to MTHF to provide the appropriate form of folate for transport out of the enterocyte via RFC expressed on the basal side. Such high DHFR activity is also present in human jejunum to convert folates to MTHF [3; 8]. It is proposed that the ATP cassette MRP transporter may also transport folate [6; 9]. Folic acid has low affinity for RFC and therefore, is unlikely to be transported via RFC. Since substantial folic acid is  
Figure 8: Distribution of IP-injected anti-rat FR\(\alpha\) antibody (FR\(\alpha\)Ab) in rat placenta and embryo on GD 14; accumulation of FR\(\alpha\)Ab (brown staining) can be observed in the placenta and embryo (**A**); in the yolk sac (**B**) and in the choroid plexus, ventricular lining and in the brain tissue of the embryo (**C**). Hematoxylin (blue/purple staining) was used as a counterstain. transported as such, it is likely that MRP, or some FR\(\alpha\) that is expressed, may be carrying out this transport. However, at low doses of folic acid, most of the compound is likely to be converted to MTHF, and therefore, little or no folic acid should be appearing in circulation. The only probability in this situation is that DHFR slows substantially below the K\({}_{\text{m}}\) for folic acid and therefore, some residual folic acid may remain and appears in circulation, especially with daily supplementation. Even though some recent publications have suggested an association of circulating folic acid with disorders attributed to metabolic folate deficiency, there is no evidence to corroborate an effect on folate pathways by extracellular or intracellular unmetabolized folic acid. While folic acid can be used as a source of folate since it is readily converted to MTHF, at large doses substantial folic acid is likely to be absorbed unconverted. Even though most of the folate will be taken up by tissue cells and converted to MTHF, the excess folate in circulation will be excreted in the following 24 h or recycled via the entero-hepatic circulation [45]. Polic acid is a highly stable compound and therefore has been widely used in vitamin preparations and in food fortification.