The results of this research support previous studies on the nutritional and social factors associated with depression
[5, 42, 43]. In this study, women with an EPDS score ≥10 were more likely to be born outside of Canada; to report having more chronic health conditions, more life stress and less social support during the current pregnancy; and to have consumed fewer micronutrients from supplements, most notably selenium and omega-3. The logistic regression showed that prenatal EPDS ≥10 (at second and third trimesters) increased the odds of postpartum depressive symptoms, while prenatal selenium intake from supplements and postnatal social support were protective (decreased the odds) of postpartum depressive symptoms. Of the nutrients evaluated, all intakes were likely to be higher in women with EPDS <10 than those with EPDS ≥10.
The role of selenium in relation to mood is not as well studied as some other nutrients (e.g. omega-3s, folate, and zinc) that have been associated with depression. A recent nested case–control study by Pasco and colleagues found low intake of selenium (<8.9 μg/MJ/day) was associated with almost a three-fold increase in the likelihood of major depressive disorder (OR 2.95, 95% CI 1.00-8.72) after adjusting for age and socio-economic status
. A randomized trial by Mokhber and colleagues reported significantly lower mean EPDS scores in the group of primigravid pregnant Iranian women, aged 16 to 35, taking selenium supplements compared to a comparison group after controlling for sociodemographic and health history variables
. Hawkes and Hornbostel in a study of eleven healthy men found lower selenium status associated with worse mood scores
This study reinforced the importance of social support for pregnant women in the prenatal and postnatal periods, the benefits of which have been well-documented for the reduction of the risk and symptoms of mood disorders and depression
[43, 46, 47]. While there is little research into the mechanism by which environmental factors such as social support affect brain biochemistry in humans, animal studies indicate that improving external stimuli through an enriched environment may reverse the effects of stress-related events, at the behavioral, endocrine, and biochemical levels
The findings from this study also revealed the need for evaluation for prenatal depressive symptoms. While there are programs in many jurisdictions to evaluate new mothers for the risk of this condition, prenatal mental health still receives little attention as part of prenatal care
[49, 50]. Since antenatal depression may be more common than postnatal depression, and women with antenatal depression are more likely to be depressed postnatally
, prenatal depression screening should be as widely practiced as postpartum screening.
One limitation of this study was the effect of multicollinearity, which is a common problem in nutrition studies, particularly those that examine the intake of nutrients from supplements as supplements include multiple nutrients and many are in similar amounts and combinations due the nutrient recommendations by health agencies. Another limitation is that the present study did not have access to serum levels of selenium or any of the other nutrients discussed in this paper, thus there is no information as to whether supplementation affected biological levels. A third limitation was that dietary (food) intake was not included in the analysis. Although dietary intake was estimated in these women at two or three trimesters during pregnancy (data which will be available at some future date), selenium intake is difficult to estimate due to variations in the food supply and the inaccuracy of nutrient databases. The high supplement intake may mitigate dietary intake in this case, and would not be likely to change our findings. We did not have lab values at present to assess anemia in our sample. Although anemia is known to be a risk factor for depression, we also know that women are routinely supplemented with iron when anemia is diagnosed during pregnancy. In this study, for women below RDA, 82.9% scored <10 EPDS, 17.1% scored ≥10 EPDS; for women above RDA, 88.5% <10 EPDS, and 11.5% were ≥10, p = 0.84 (see Table
2), indicating that there was no difference of scoring ≥10 EPDS between being above or below the RDA for iron. While we recognize this is not the ideal measure for iron associated form of anemia, it served as a proxy for iron status. Another limitation was possible reporting error associated with recall of supplement intake (e.g. dosage and frequency). Recording of brand names and labels of supplements (e.g. types and amounts of nutrients) by research assistants, as well as the use of repeated measures, helped to minimized potential reporting bias. However, we acknowledge a major limitation with the Supplement Intake Questionnaire is self-reporting; thus we did not know whether the women actually consumed their supplements as they said they did. Furthermore, if a woman missed her trimester visit then this was treated as missing data and no estimation was made. Thus, there was some degree of projection as the women were asked for information for the entire trimester at the trimester midpoint and not the end, making this a limitation to our estimates. A limitation in the analysis was that we did not assess for interaction among the nutrients due to the absence of any a priori rationale from the literature. In addition, as data on prenatal nutrition status was not available, we were unable to test whether the effect of prenatal micronutrient supplementation on postpartum depression varied by prenatal nutritional status. For future research, we would like to assess whether specific nutrients interact to impact postpartum depression, and also whether prenatal nutritional status modified the association of supplementation and postpartum depression.
One of the strengths of this study was the temporal relationship of supplement intake and postpartum depressive symptoms could be established. Another strength was that supplement intake was collected repeatedly over time to obtain an average usual intake of the nutrients in the prenatal period, rather than using a single measure as in most studies. A third strength was the ability to control for a large number of factors associated with postpartum depressive symptoms in our analyses because of the variables (demographic, social, lifestyle) collected prospectively.
Implications for future research include examining objective measures of pre- and postnatal micronutrient status for: 1) relations between pre/postnatal blood nutrient levels (or changes in pre/postnatal blood levels) and postpartum depression, 2) moderating effect of prenatal blood nutrient levels on the relations between prenatal micronutrient supplementation and postpartum depression, and 3) interactions amongst nutrients and their effects on postpartum depression.