With recent concern about aquatic contamination from potential teratogens including heavy metals, dioxins, pharmaceutical residues, polychlorinated biphenyls (PCBs), and synthetic estrogens,1–6 some public health agencies throughout the world have recommended limiting gestational fish consumption to minimise fetal harm associated with toxicant exposure.7 Conversely, a well-publicised research paper in Lancet concluded that adequate seafood consumption in pregnancy correlates with improved child development and that 'advice to limit seafood consumption could actually be detrimental'.8 This commentary will consider the issue of maternal fish consumption in the context of recent medical literature on nutrition and essential fatty acids (EFAs).
With discussion of intricate laparoscopic techniques, avant-garde reproductive interventions, complex microsurgery, and epigenetic therapies, many medical practitioners find the practice of dietary or nutritional therapy to be dull, alternative, and perhaps simplistic medicine. Exploration of the aetiological factors contributing to the global escalation in chronic disease,9 however, has revealed that some contemporary ill health results from nutritional compromise.10 Furthermore, recent research demonstrates that certain obstetric and paediatric afflictions might effectively be prevented by provision during pregnancy of the nutrients required for optimal development and sustained function.
The correlation between deficient folate and neural tube defects11 (NTDs) previously prompted widespread periconceptional supplementation and recent concerns about potential fetal sequelae (including cleft palate)12 of maternal biotin deficiency13 as well as considerable risk for NTDs with low maternal vitamin B12 status14 have fuelled intensified research on the link between gestational nutrient requirements and maternal and fetal outcome. Correlations including those of maternal selenium levels with pre-eclampsia15 and maternal vitamin D status with subsequent risk in the offspring for diabetes,16 asthma,17 adverse bone health,18 and multiple sclerosis19 illustrate the truism that the human being requires specific nutrients during gestation to carry out the molecular processes within cells and tissues, processes which on the macroscale influence both maternal physiology and fetal development. Considerable attention has recently focused on the role of EFAs in maternal and fetal metabolism.
Essential fatty acids.
EFAs refer to lipids that cannot be synthesised within the body and must be ingested to meet metabolic demands. The two families of essential lipids—omega-3 fatty acids (ω3FAs) and omega-6 fatty acids (ω6FAs)—are required for physiological functions including oxygen transport, energy storage, cell membrane function, and regulation of inflammation and cell proliferation.20,21 In pregnancy, EFAs are required for early development of the fetal-placental unit;22 docosahexaenoic acid (DHA), a type of ω3FA commonly derived from seafood, is vital for maternal homeostasis, as well as fetal brain and retinal development throughout the third trimester.23–25
Various studies have demonstrated that EFA deficiency as well as unbalanced consumption of ω3FAs and ω6FAs may be significant to health outcomes.26 With an overall 80% decline in ω3FA intake in the past century,6,27 combined with a noteworthy increase in ω6FA intake, epidemiological research suggests that EFA malnutrition may be a determinant of several chronic and degenerative disease states.20,26 Various recent papers and meta-analyses report an increased risk of a variety of condition such as heart disease,28 rheumatoid arthritis,29 breast cancer,30 hypertension,31 osteoporosis,32 and neurological33 and psychiatric disease34 in association with inadequate ω3FA consumption. Maternal and fetal research has also begun to evaluate the consequences of ω3FA insufficiency.23
Maternal and paediatric benefits of omega-3 fatty acid sufficiency
It has been hypothesised that sufficient gestational ω3FA intake may diminish the likelihood of preterm labour by the downregulation of prostaglandin formation.35 Juxtaposed with recent evidence that inadequate consumption of ω3FAs significantly increases the likelihood of early labour,36 an epidemiological study has reported a marked rise in preterm birth among white American women from 1981 to 1998.37 Furthermore, a prospective cohort study demonstrated that women who avoided seafood had a 7.1% incidence of preterm birth compared with a 1.9% risk for those eating fish once weekly.38 In addition, maternal consumption of cod liver oil and increased ω3FA:ω6FA intake ratio have been associated with longer gestations and higher birthweights,35,39,40 except in women with high pre-existing levels of ω3FAs.35 Whether the results from these interesting studies should change our clinical practice remains uncertain.
Hypertension complicates about 6% of pregnancies in the developed world. A cross-sectional case–control study found that pregnant women with low levels of ω3FAs were 7.6 times more likely to have pre-eclampsia than those with high levels of this EFA41 and that a 46% risk reduction for pre-eclampsia could be achieved by a moderate increase in the proportion of ω3FAs consumed.41 While meta-analytical reviews confirm a dose-dependent relationship between ω3FAs intake and blood pressure outside pregnancy,30,41 recent evidence suggests that blood pressure control later in life may also be affected negatively by inadequate maternal and neonatal intake of ω3FAs.42 While many studies show significant benefit in relation to hypertension, a controlled trial of supplementation in selected high-risk pregnancies found that additional ω3FA intake was associated with reduced recurrence risk for preterm delivery but had no impact on recurrence risks for intrauterine growth restriction or pregnancy-induced hypertension.43
The demonstrated correlation between lower gestational seafood consumption and higher rates of postpartum depression44 suggests that individual ω3FA indices may also be a determinant of postpartum mood status.45,46 Possibly associated with the limited seafood intake in North America, the incidence of postpartum depression is in the range of 12% compared with about 2% in Japan where fish consumption is high44 (although major cultural differences in the way the two societies are organised may also be important). Furthermore, low DHA in breast milk and maternal red cells, resulting from low gestational intake of EFAs,47 are commonly found in women with postpartum depression.44
Maternal ω3FA status also appears to correlate with fetal outcome. Ensuring a sufficiency of ω3FAs for women during pregnancy and lactation has been correlated with enhanced cognitive and behavioural functioning,48–50 improved sleep behaviour,51 and less risk of metabolic disorders such as type I diabetes in the developing offspring.52 In review, there is abundant evidence in the medical literature that links adequate gestational fatty acid status with maternal and fetal benefit.
Sources of gestational omega-3 fatty acids
To secure safe and appropriate ω3FA gestational intake, two principles merit consideration. Marine sources of ω3FAs contain required DHA, while plant foods generally do not; conversion from plant source ω3FAs to DHA is possible but requires energy and enzymatic availability. As ω3FAs and ω6FAs use the same enzymes, dietary intake ratio of these lipids can determine enzymatic availability. Accordingly, while some pregnant women produce sufficient DHA through biochemical conversion from plant source ω3FAs, direct DHA from fish intake secures provision of this required gestational nutrient.24
As a result of multiple potential teratogens contaminating seafood sources,1–6 however, some authors warn about risks associated with gestational seafood intake. With potentially long induction times from toxicant exposure to ultimate outcomes53 [as in the diethylstilbestrol (DES) tragedy], with insufficient research on the long-term impact of many contemporary aquatic contaminants (acting in isolation or synergistically with other pollutants), and with pronounced vulnerability of the fetus to seemingly minuscule levels of toxicants,53 debate continues as to whether risks from EFA insufficiency outweigh the risks of harm from seafood toxicants. In response, some have suggested that supplementation with fish oil rather than seafood consumption might be a preferred approach to providing required DHA. Recent research, however, has challenged this approach.
Most work examining fish oil use in pregnancy demonstrates benefit; a few recent studies, however, fail to confirm benefit and some outcomes appear to suggest harm.54,55 A major confounder in current work, however, is that oil prepared from fish liver (the major organ of detoxification) is often contaminated with the same toxicants including heavy metals23 found in source fish. Accordingly, consumers of regular fish oil consume toxicants that may affect physiological processes and influence metabolic outcomes. Methyl mercury, for example, is a common aquatic contaminant and can induce hypertension in animals56—this may account for adverse outcomes, such as gestational hypertension in consumers of regular fish oil.55 Furthermore, some toxicants can impair or modify absorption, utilisation, and metabolism of nutrients potentially resulting in adverse sequelae.
To address the toxicity concern, however, gestational EFA requirements can be secured with avoidance of toxic exposure by replacing gestational seafood intake with regular ingestion of plant source ω3FAs and supplementation with purified fish oil.57,58 Through distillation and subsequent toxicological testing, purified fish oil preparations are available. As well as obviating patient hazard, toxicant confounding in research can be precluded by use of purified preparations.
Concluding thoughts
The recent medical and scientific literature correlating micronutrient and dietary transitions with health sequelae makes it evident that nutritional status is a major determinant of health and wellbeing throughout life, including intrauterine development. Regardless of beneficial outcomes in research or epidemiological study, however, medical history has repeatedly demonstrated that translating new ideas and research findings into clinical practice is generally a slow process.59 It often takes a generation—corresponding to the time required for new trainees untainted by status quo ideas and biases to achieve positions of influence—before innovative clinical patterns emerge. While concerns about gestational folate deficiency were expressed in 1976,60 for example, it took two decades before routine supplementation was widespread.
Research confirms the need for essential ω3FAs in pregnancy, and population analyses suggest that deficiency is common. Accordingly, it is important that providers of maternity care be familiar with principles of clinical nutrition and possess the necessary tools to assess and manage concerns relating to nutritional health. In keeping with extensive epidemiological evidence linking obstetric and paediatric outcomes with gestational dietary habits, it is suggested that obstetricians and midwives should discuss the importance of a good diet with pregnant women and that in the absence of measured maternal ω3FAs levels, the plant source EFAs be regularly consumed during pregnancy and pure fish oil supplements be explored as a means to provide the necessary DHA required for optimal maternal and fetal outcome.
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