Research reportSeafood consumption, the DHA content of mothers’ milk and prevalence rates of postpartum depression: a cross-national, ecological analysis
Introduction
Mothers are the sole source of nutrients for the fetus during development. Without adequate nutritional replenishment, mothers can become depleted of critical nutrients during pregnancy with adverse consequences for both mother and infant. Hibbeln and Salem Jr. (1995) previously proposed that mothers may be at a higher risk of suffering postpartum depression when they become depleted of omega-3 essential fatty acids, in particular docosahexaenoic acid (DHA). Maternal DHA status can be reduced by half during pregnancy and not fully restored at 26 weeks postpartum (Holman et al., 1991, Al et al., 1995, Otto et al., 1997). DHA is selectively transferred from the maternal circulation to the fetus by a placental protein (Cambell et al., 1998). An adequate supply of maternal DHA is necessary to support optimal neurological development of both fetus and infant (Salem Jr., 1989, Willatts et al., 1998, Birch et al., 2000). DHA is highly concentrated in synaptic neuronal membranes and has unique membrane biophysical properties critical to synaptic function (Salem Jr., 1989). Synaptic growth cones, the progenitors of synapses and dendrites, preferentially accumulate DHA during neuronal development (Auestad and Innis, 2000, Martin, 1998). Synaptic growth cones become depleted of DHA when rat mothers consume diets restricted in omega-3 fat content (Auestad and Innis, 2000). Shortening of dendrites and less dendritic branching occurred in rat pups whose mothers were fed diets deficient in essential fatty acids (Wainwright et al., 1998). When supplied in infant formula at adequate doses, DHA and arachidonic acid (AA) improve infant cognitive development and visual acuity (Willatts et al., 1998, Birch et al., 2000, Clandinin et al., 1997). DHA and AA cannot be synthesized de novo by either the mother or the fetus. Production of DHA and AA from essential precursors is inadequate to support the developmental needs of infants; thus preformed sources are required (Salem Jr. et al., 1996). Fish and seafood are prominent dietary sources of preformed omega-3 fatty acids DHA and eicosapentaenoic acid (EPA).
DHA and or EPA depletion due to pregnancy may have adverse neuropsychiatric consequences for mothers. A growing body of data supports the proposition (Hibbeln and Salem Jr., 1995) that an inadequate intake of omega-3 fats is associated with major depression and other affective disorders. Several investigators have reported lower plasma and erythrocyte concentrations of omega-3 fats among depressed subjects (Adams et al., 1996, Peet et al., 1998, Edwards et al., 1998, Maes et al., 1996). In a recent placebo-controlled clinical trial, the treatment response to EPA plus DHA among subjects with bipolar affective disorder was robust enough that the trial could be stopped after 4 months (Stoll et al., 1999). The treatment group had significantly fewer depressive episodes. A significant antidepressant effect of 1 g/day of EPA alone among subjects with treatment resistant depression has also recently been described (Peet et al., personal communication). However, since EPA is also converted in to DHA in vivo (Salem Jr. et al., 1996), it is difficult to definitively determine the relative effects of each fatty acid. One recent study reported that seafood consumption greater than twice/week is associated with a lower risk of both depression (odds ratio=0.63) and suicidal ideation (odds ratio=0.57) (Tanskanen et al., 2001). Consistent with these findings, a prior cross-national analysis reported that greater seafood consumption was strikingly correlated with a lower lifetime prevalence rates of major depression (r=−0.84, p<0.005) (Hibbeln, 1998). Since greater seafood consumption protects women from depletion of DHA and EPA during pregnancy (Al et al., 1995, Makrides et al., 1996), the central hypothesis of this study was that the prevalence rates of postpartum depression should be lower in countries with greater rates of seafood consumption. The DHA content of mothers’ breast milk is a biological marker for maternal omega-3 fatty acid status in the postpartum (Al et al., 1995, Makrides et al., 1996). Thus, a secondary hypothesis was that higher concentrations of DHA in mothers’ milk would predict lower rates of postpartum depression across countries.
In this cross-national ecological study, several strategies were employed to evaluate and reduce the influence of confounding factors. Only published studies using the Edinburgh Postpartum Depression Scale (EPDS) were included to ensure greater uniformity and comparability of the category of major postpartum depressive symptoms. The EPSD was specifically designed to evaluate the prevalence and severity of postpartum depression (Cox, 1994). In contrast to other depression rating scales, the EPDS excludes biological symptoms that are common to both depression and normal postpartum states, e.g., sleep difficulties (Cox, 1994). The EPDS has been translated into more than 17 languages and has demonstrated excellent selectivity and in multiple validation studies (O’Hara, 1994, Boyce et al., 1993, Jadresic et al., 1995; Abou-Saleh and Ghubash, 1997, Carpiniello et al., 1997, Fossey et al., 1997, Da-Silva et al., 1998, Lawrie et al., 1998, Lee et al., 1998). Since validation studies were not conducted in all countries included in this study, differences in diagnostic yield may have existed. However, a conservative interpretation is that a score above a high cut-off (e.g., 12/13) indicated severe depressive symptoms, rather than a major depression. Adverse personal, social and economic conditions clearly increase the risk of suffering severe depression symptoms in the postpartum period (O’Hara and Swain, 1996). Thus, the impact of the following factors were evaluated using the available published data: study time postpartum, low socioeconomic class, the percentage of young mothers, the percentage of mothers without partners, the percentage of mothers with secondary education and the influence of Asian cultures.
Section snippets
Methods
Literature searches were conducted using Internet Grateful Med and Pubnet Med (National Library of Medicine, Bethesda, MD, USA) using applicable search terms including: postpartum depression, postnatal depression, Edinburgh Postnatal Depression Scale, cross-cultural depression, breast milk, mothers’ milk, fatty acids, omega-3 fats and essential fats. The fatty acid compositions of mothers’ milk were reported as area percent or calculated as weight percent. DHA, EPA and AA values were averaged
Statistical methods
In the primary analyses, a simple Pearson’s product moment correlation and a Spearman’s rank coefficient test compared the content of DHA, EPA and AA in mothers’ milk to the prevalence rates of postpartum depression across countries. Residuals were examined and curve fitting was performed. In the secondary analyses, potential confounding risk factors that are known to predict postpartum depression were evaluated. Multivariate analyses that simultaneously included all potential confounding
Results
The total sample size of this analysis included 14 532 subjects over 23 countries described in 41 studies. Only three studies contained less than 100 subjects and inclusion or exclusion of the studies did not significantly alter the results. Twenty-two different countries were identified that met study criteria and reported of point prevalence rates of major postpartum depressive symptoms (Table 1, Table 2). Data on the prevalence of postpartum depression in Iceland were reported as a secondary
Discussion
The findings in these cross-national analyses were clearly consistent with the hypothesis (Hibbeln and Salem Jr., 1995) that inadequate dietary intake of omega-3 fats and the subsequent maternal depletion of omega-3 fats during pregnancy are associated with an increased risk of major postpartum depressive symptoms. Both lower concentrations of DHA in mothers’ milk and lower national rates of seafood consumption were robustly correlated with higher rates of major postpartum depressive symptoms
Acknowledgments
Robert Rawlings, M.A. provided valuable statistical assistance in the preparation of this manuscript. This study was supported in part by a Young Investigators Grant from the National Association for Research on Schizophrenia and Depression (NARSAD).
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