Evaluation of Food Fortification with Folic Acid for the Primary Prevention of Neural Tube Defects

Study Results and Interpretation

Knowledge and use of folic acid supplements

There was a significant increase from Phase I to Phase II in the proportion of women aged 19-44 who knew the importance of folic acid in Newfoundland (two sites combined) (from 33% to 46%, p<0.001) and in Kingston, Ontario (from 36% to 51%, p<0.001). The proportion of women taking a vitamin supplement containing folic acid increased significantly between the two time periods in Newfoundland (from 17% to 28%, p<0.003), but increased non-significantly in Kingston, Ontario (from 33% to 39%, p> 0.05) (Table 2). In Ontario, the proportion of women who reported taking supplements with at least 400mg folic acid per dose increased between Phase I and Phase II (from 17% to 26%, p<0.005) (data available in full report). Information about folic acid dosage was not collected in Newfoundland.

In the Phase II Newfoundland sample, women who were trying to conceive or who were sexually active and not using birth control were more likely to be taking supplements containing folic acid than women who were using birth control or who had taken permanent measures to prevent pregnancy (see full report). There was a 37 percentage points increase in use of supplements containing folic acid among women with a chance of pregnancy in Phase II in Newfoundland (p=0.02). Although there was also an increase in the Ontario sample, the results were not statistically significant (Table 3).

This study suggests that the message to take folic acid is not getting through to enough women of childbearing age, despite a range of national and local educational initiatives, including development and dissemination of Canadian clinical practice guidelines (Canadian Task Force on the Periodic Health Examination 1994), well-publicized national conferences and local public health campaigns. Although we have shown some increase over three years in folic acid supplement use, considerable room for improvement remains.

Dietary assessment

There was no statistically significant change in the average daily intake of naturally occurring folate among either women aged 19-44 or seniors between Phase I and Phase II (p=0.19 and p= 0.18, respectively). Seniors generally had dietary folate intake slightly higher than women of childbearing age. In Phase II, the average daily intake of naturally occurring folate was 290 mg/day for seniors and 260 mg/day for women aged 19-44 (data available in full report).

The implementation of mandatory fortification resulted in an average dietary intake of 70 mg/day of folic acid in women aged 19-44, and 74 mg/day of folic acid among seniors. It is noteworthy that for the women the average daily folic acid intake due to food fortification was less than the approximately 100 mg that was previously predicted for women of child-bearing age (Food and Drug Administration 1996, Turner and McCourt 1998). The maximum dietary intake of folic acid due to fortification for an individual woman was 235 mg/day, and for an individual senior was 219 mg/day (data available in full report).

Income was a significant predictor of folate intake. Both women of childbearing age and seniors living in households with income below the Low Income Cutoff (LICO) had lower folate intake. A detailed examination of the dietary sources of folate for the women of childbearing age showed that the difference in consumption for those below the LICO was due solely to lower consumption of naturally occurring folate and not due to less consumption of cereal grain products that were the target for fortification with folic acid.

The dietary folic acid intake due to fortification did not exceed the Tolerable Upper Intake Level (UL) of 1,000 mg folic acid/day (Institute of Medicine 1998) for any of the participants. It is important to note that this part of the study excluded persons taking vitamin supplements containing folic acid. While it was not possible to estimate the proportion of people in the general population who may be consuming more than 1,000 mg/day of folic acid from fortification and supplementation combined, it is likely that this proportion is small. The average dietary intake and maximum intake of folic acid due to fortification were 70 mg/day and 235 mg/day, respectively, for women aged 19-44 years, and 74 mg/day and 219 mg/day, respectively, for seniors. The average folic acid dose in folic acid containing over-the-counter supplements marketed in Canada is about 350 mg/day (Health Canada unpublished information).

Blood analysis - folate

For all sites combined, mean serum folate and RBC folate increased significantly from Phase I to Phase II in both women of childbearing age and seniors (p<0.001). For both age groups, there was a corresponding decrease in mean plasma HCY levels post fortification (Tables 4 and 5).

The results of this study provide strong evidence of improved blood folate status in women aged 19-44 following mandatory fortification with folic acid. Among seniors, the improvements in folate indices and the moderate decrease in mean plasma HCY are a positive result, especially with regard to risk of cardiovascular disease.

Blood analysis - vitamin B₁₂

There was a significant increase in mean vitamin B₁₂ levels in women of childbearing age and seniors between Phases I and II (p= 0.020 and p< 0.001, respectively) (Tables 4 and 5). Prior to mandatory fortification the proportion of seniors in this study with low vitamin B₁₂ (<133 pmol/L) was 18.8%. Following fortification the proportion of elderly with low vitamin B12 levels declined to 11.8% (p=0.032) (data available in full report). In vitamin B₁₂ deficiency, plasma MMA is usually elevated. Plasma MMA is believed to be a better indicator of vitamin B₁₂ status at the tissue level than serum vitamin B12 levels are. In this study, no significant changes were seen in mean plasma MMA levels for women of childbearing age from all sites or for seniors between Phases I and II (Tables 4 and 5). However, there was an increase in the proportion of women of childbearing age with MMA values above the upper reference value of 0.37 mmol/L. Also, for the Newfoundland sample of women, there was a statistically significant increase in mean plasma MMA. There was no significant change in the proportion of abnormal MMA values in seniors (data available in full report).

Among seniors, the results showed no significant difference in mean haemoglobin concentrations, mean corpuscular volume (MCV), or proportion with abnormally high MCV (>99 fL) or low haemoglobin (<120 g/L) concentrations (data available in full report).

These results show no evidence of a deterioration in vitamin B₁₂ status among seniors. Furthermore, there is no evidence of improved folate status resulting in masking of the haematological manifestations of vitamin B₁₂ deficiency among seniors as a group. There was no evidence of deteriorating vitamin B₁₂ status among young women participants based on vitamin B₁₂ measurements. The upward trend in plasma MMA levels and higher proportion of abnormal values among young women is being further evaluated. It is unlikely that this is a direct effect of folic acid fortification and this observation is not consistent with any known effects of folic acid on vitamin B₁₂ status.

Incidence of NTDs

Prior to 1998, the annual incidence rate of NTDs in Newfoundland varied greatly over time, with the lowest rate of 2.72 per 1,000 births in 1978, and the highest rate of 5.02 per 1,000 births in 1995. The average incidence rate of NTDs between 1976 and 1997 was 3.40 per 1,000 births. A drop is seen in 1998, in which the rate of NTDs was 1.60 per 1,000 births, from 3.06 per 1,000 births in the previous year. The decreasing trend continued after 1998 (Figure 1). The incidence of NTDs for the years 1991-2001 is presented in three periods in Table 6. The mean annual incidence was 4.35 per 1,000 births during 1991-1993 and 5.02 per 1,000 births during 1994-1996 (1994-96 vs 1991-93, relative risk [RR] 1.15, 95% CI 0.86-1.54, p = 0.95), and 4.37 per 1,000 births during 1994-1997 (1994-97 vs 1991-93, RR 1.01, 95% CI 0.76-1.34, p = 0.54). The total annual incidence of NTDs fell by 78% after the implementation of folic acid fortification, from an average of 4.36 per 1,000 births during 1991-1997 to 0.96 per 1,000 births during 1998-2001 (RR 0.22, 95% CI 0.14-0.35, p <0.0001). It is worthwhile to note that there has been no significant increase in the proportion of NTDs from terminated pregnancies since 1994.

The 65% increase in the proportion of women in the Newfoundland sample who were taking vitamin supplements containing folic acid, from 17% in Phase I to 28% in Phase II, suggests that an increasing trend in folic acid supplementation may have played a role in the declining NTD rate in Newfoundland. In this study it was not possible to determine the individual contribution of supplementation and fortification to the trend in NTDs.

Limitations

There are several limitations in this study. We have documented the incidence of NTDs among live births, stillbirths and terminated pregnancies known to have an NTD. It was not possible to include NTDs that may have occurred in pregnancies that resulted in a spontaneous abortion or a termination that occurred for reasons other than a congenital anomaly.

This study, as with other studies of fortification in Canada, was challenged by the fact that there was no precise date when exposure to food fortification with folic acid began. The addition of folic acid to white flour and enriched pasta and cornmeal was permitted as of December 1996. Although this requirement did not come into force in Canada until late 1998, the Phase I (November 1997 to March 1998) subjects of our study may have consumed at least some food fortified with folic acid. This would result in an underestimate of improvements in blood folate status due to fortification, and might lead us to miss adverse effects on vitamin B₁₂ status. On the other hand, the fact that we observed such marked improvements in blood folate status leads us to conclude that there was a real increase in exposure to folic acid through fortification over the study period.

Another limitation of this study is the possible underestimation of folic acid intake due to fortification. Our calculations were based on the assumption that manufacturers are fortifying flour at the required level.

It has been suggested that allowance for "overages" is resulting in higher amounts in the affected products (Choumenkovitch et al. 2002). Also, for enriched pasta, the required level of fortification is from a minimum of 0.20 mg/100 g pasta to a maximum of 0.27 mg/100 g. In our calculations we assumed the minimum level of fortification.

The sampling procedure showed a Phase I 60% and Phase II 65 % response rate for the dietary questionnaire and blood sampling among women aged 19-44 years in Newfoundland, with no difference between urban and rural response rates. In contrast, the response rates for Ontario were 28% for Phase I and 37% for Phase II (data available in full report). Thus the results in Newfoundland may better reflect the province's women of childbearing age compared to Ontario's results. These findings may not be representative of the rest of Canada because of population differences in factors such as genetic background and dietary behaviour. These differences may also affect the generalizability of the NTD incidence results.

The sample response rate for the dietary questionnaire and blood sampling in seniors was 45% both in Phase I and in Phase II (see full report). Many of the refusals to participate were due to illness of the eligible person. Furthermore, seniors residing in long term care settings were not included. Thus our sample population of seniors may be healthier than the general population age 65 and over in Newfoundland.