A few prior studies [39, 40] in Bangladesh have documented the daily phytate intake of women in their typical consumed diet. However, there is no study that reports phytate intake and its effect on the bioavailability of major micronutrients from the typical diet of pregnant women in Bangladesh. Even the two randomized trials, involving pregnant women in Bangladesh, (JiVitA-3  and MINIMat ), which investigated effects of maternal multiple micronutrients [41, 42] and early food supplementation  on maternal hemoglobin , birth weight [41, 42], infant mortality [41, 42], and adverse birth outcomes [41, 42], have not reported the phytate intake of the studied pregnant women and its effect on the bioavailability of minerals from maternal diet.
Dietary phytate intake
Previous studies found a wide variation in phytate intake among the diets of rural women of Bangladesh. For example, Bhargava et al.  reported a very high mean daily phytate intake (around 2300 mg), whereas Arsenault et al. , in another study, reported phytate intake of approximately 650 mg. The wide difference in daily phytate intake between these two studies was probably due to the variations in energy intake among the studied rural women. In our study, we found that the average phytate intake of the rural pregnant women was 695 mg; this finding is in agreement with Arsenault et al.’s reported phytate intake of rural women in Bangladesh .
The average daily phytate intake among rural pregnant women in our study was lower than that in Ethiopia  and Guatemala , while it was reported that the women of childbearing age in Sweden , Italy  and Finland  consumed about 180 mg, 293 mg, and 370 mg of phytate per day, respectively. A few studies on women in other Asian countries indicate that average phytate intake is higher compared to Western countries. Taiwan has reported an average phytate intake of 690 mg/day , whereas Indian women consume as much as 1560–2500 mg of phytate per day. Nititham et al.  reported that the daily phytate intake of non-urban women in Thailand was 1139 mg, while women in South Korea  showed phytate intakes of 752 mg/day.
Phytate to minerals molar ratios for bioavailability
This study focuses on diet-related factors, especially the inhibitory effect of phytate on calcium, iron and zinc bioavailability from the consumed diet of rural pregnant women in Bangladesh. The influence of phytate on the bioavailability of essential micronutrients depends not only on the phytate contents of the diet, but also the phytate and micronutrients’ interaction. We have used phytate to minerals molar ratios to forecast the inhibitory effect of phytate on the bioavailability of minerals from the consumed diet of the pregnant women [18, 19, 23, 25]. Moreover, we have used cut-off values for each of the molar ratios to identify the proportion of pregnant women affected by the inhibitory effects of phytate intake on calcium, iron and zinc bioavailability. We have reported the mean phytate to minerals molar ratios and further segregated the molar ratios by quintiles of phytate intake from the diet. All the molar ratios in our study were found to increase with the concomitant increment of phytate intake across the quintiles.
The mean phytate to calcium molar ratio was found to be 0.27 in our study, which is more than the critical value (>0.24) to impair calcium absorption. The effect of phytate intake on calcium bioavailability among the women of Bangladesh has not been studied or reported yet. In our study, we found that more than half of the pregnant women have phytate to calcium molar ratio above the critical value, which indicates that the calcium bioavailability of this proportion of women was affected by phytate intake. Low bioavailability of calcium from the diet of the pregnant women might have resulted in low total serum calcium level. Though in our study we had not measured the serum calcium levels of our studied pregnant women, the available literature has shown low to marginal levels of serum calcium in the pregnant women of Bangladesh [11–13].
We have not found any study which reports the phytate to iron molar ratios of pregnant women in Bangladesh. Mean phytate to iron molar ratio of the rural pregnant women of Bangladesh was reported to be 12.8 in our study. In addition, the mean phytate to iron molar ratio was found to be significantly (p < 0.05) higher in the highest phytate intake group compared to other groups. Across all the groups, the mean phytate to iron molar ratios was higher than the cut-off value. The cut-off value of phytate: iron > 1 will significantly reduce iron absorption from the diet . Considering the cut-off value, dietary iron among all the pregnant women in our study was inhibited by phytate intake. Due to the lack of biochemical data for serum iron status, we have failed to correlate and translate these findings to the extent of iron deficiency.
The mean phytate to zinc molar ratio of the rural pregnant women in our study was 11.2, which is soundly supported by findings from Arsenault et al. who studied women in rural Bangladesh . Besides this, the 2011-12 Bangladesh National Micronutrient Status Survey  has reported a mean phytate to zinc molar ratio of 12.0 among rural Non Pregnant Non Lactating (NPNL) women in Bangladesh. The reported ratio in our study was lower than that of pregnant women in Guatemala  and China , but higher than young Korean women .
Unlike phytate to iron and calcium molar ratios, the mean phytate to zinc molar ratio in our studied pregnant women was lower than the cut-off value. Our data propose that phytate affects zinc bioavailability on 12 % of the rural pregnant women sampled. Though the phytate has little effect on zinc absorption in our studied pregnant women, the available literature on women from Bangladesh has demonstrated a very high prevalence of zinc deficiency. The prevalence of zinc deficiency among rural NPNL women of Bangladesh was reported to be 57.5 %, while mean phytate to zinc molar ratios were found to be 12.0 among these women . This high prevalence of zinc deficiency was probably due to the inadequate intake (54.2 % of RDA) of zinc among the women, and phytate to some extent affects the bioavailability of zinc. However, the mean phytate to zinc molar ratio of the pregnant women’s diet in our study was comparatively low. Inadequate total zinc intake, and to some extent, phytate, might have resulted in too little absorbable zinc from the diet . Perhaps it is necessary to redefine the cut-off value of phytate to zinc molar ratio among women in Bangladesh.
The molar ratio of (phytate × calcium) to zinc was used to predict the synergistic effect of both phytate and calcium on zinc absorption [37, 50]. When considering the low calcium intake of the rural pregnant women, (phytate × calcium) to zinc molar ratio might not be a good indicator for predicting the interaction of phytate and calcium to zinc absorption . In our study, we found that none of the women had (phytate × calcium) to zinc molar ratio > 200.
Predictors of calcium, iron, and zinc bioavailability
As we have mentioned earlier, we measured the inhibitory effects of phytate intake on the bioavailability of calcium, iron and zinc from the diet of the pregnant women on the basis of phytate to minerals molar ratios. Moreover, we have tried to find out the determinants that significantly predict the variance in molar ratios, and hence the bioavailability of calcium, iron, and zinc from the diet. From the multivariate models we have used in predicting the inhibitory effects of phytate intake on the bioavailability of calcium, iron and zinc, we would like to infer that four predictors – namely, phytate intake, inadequate micronutrient intake, gestational age, and energy intake - significantly predicted the variance in phytate to minerals molar ratios. Our models have predicted about 92 %, 88 %, and 89 % of variance in phytate to calcium, iron, and zinc molar ratios, respectively.
Among these predictors, phytate was the strongest inhibitory predictor of calcium, iron and zinc bioavailability from the diets of pregnant women. The inhibitory effect of phytate on bioavailability increases with the increment of phytate intake. In our study, we found that phytate to calcium, iron, and zinc molar ratios would be expected to be respectively 0.05, 2.48, and 1.96 points higher for every 100 mg increment in daily phytate intake. The explanation for these findings could be that plant based foods consumed by the pregnant women in Bangladesh [14, 15, 51] contain high level of phytate, which is known as a potent inhibitor of calcium, iron and zinc absorption . Phytate chelates calcium, iron and zinc, forming insoluble chemical complexes in the gastrointestinal tract that cannot be digested or absorbed , and thus makes these minerals not available for absorption; rather, they are probably excreted through faeces with minerals-phytate complexes .
Another strong inhibitory predictor of the bioavailability of the micronutrients was inadequate intake of calcium, iron and zinc in the diets of these pregnant women. Inadequate intake of calcium, iron and zinc reduce the efficiency of absorption of these minerals from the diet by reducing minerals-receptors complexes and/or available amount of minerals for absorption [52–54]. Arsenault et al. had reported that respectively 0, 16 and 22 percent of rural Bangladeshi women displayed adequate calcium, iron, and zinc intake . Inadequate intake of calcium, iron and zinc among these women and the pregnant women in our study was due to the consumption of a major portion of their daily energy intake from white rice , which is relatively very low in calcium, iron, and zinc . In addition, the dietary patterns of pregnant women in low-resource settings are characterised by a lack of diversity, with infrequent consumption of nutrient-rich foods and milk and milk products [5, 55, 56].
Our study has also shown that gestational age has a significant effect on the bioavailability of zinc, iron, and calcium. Women who were close to the end of their pregnancies had higher phytate to zinc, iron and calcium molar ratios. This effect might be due to the inadequate intake of zinc, iron, and calcium of the pregnant women compared to the increasing needs for these micronutrients as pregnancy progresses . As pregnancy proceeds, an alarmingly high number of pregnant women of Bangladesh claim to eat less than pre-pregnancy levels .
Limitations to the study
A major limitation of this study was that the dietary assessment of the pregnant women was collected using a single 24-h dietary recall. Although one 24-h recall is sufficient to provide estimates of the average nutrient intake of a large sample, the day to day variation in intake might provide random errors in the average nutrient intake. Hence, it was not possible to conclude with assurance that the phytate and micronutrients intake of the pregnant women represented their habitual intake.
However, the available literature regarding the micronutrient intake of pregnant and NPNL women of Bangladesh are well aligned with our findings. Besides, other studies from Bangladesh have reported the seasonal variations in micronutrient intake of women. Therefore, the collection of data concerning a single point in time has boundaries to depict the accurate picture of phytate and micronutrient intake of the pregnant women over the whole gestational period.
A further limitation of our study is that we have not collected the biochemical data of the pregnant women. Due to the lack of biochemical data regarding the serum concentrations of calcium, iron and zinc, we are unable to correlate the molar ratios with biochemical deficiencies of these minerals.