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FIGURE 8-4 Risk of neural tube defect (NTD) according to folate intake based on two retrospective studies. Intake values appear next to each point, in micrograms. Midpoint values for each reference intake category have been used for defining folate intake, and relative NTD risks have been linearly adjusted to a baseline absolute risk of 1.29 per 1,000 for a folate intake of 312 µg/d. Values that include folate supplements (indicated by a) are estimated in dietary folate equivalents (1 dietary folate equivalent = 1 µg food folate = 0.6 µg folate from fortified food or as a supplement taken with food = 0.5 µg supplemented folate when fasting). SOURCE: Data from Shaw et al. (1995c) and Werler et al. (1993).

plementation (800 µg/day of folic acid) on the risk of a first occurrence of NTD was prematurely terminated after 4,753 women had been enrolled (Czeizel and Dudas, 1992) (Table 8-8). No case of NTD was observed in the group taking multivitamins containing 800 µg of folate daily compared with six cases in the group receiving a trace element supplement (p = 0.029). The effect of supplemental folate alone was not assessed. Although no protective effect was observed in one case-control study (Mills et al., 1989), a significant reduction in risk associated with supplementation was seen in one cohort (Milunsky et al., 1989) and four other case-control studies

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