Since the results showed that the negative impact of fine particulate matter on birth weight was only significant in infants born to mothers who reported low fish consumption in pregnancy, this may suggest that a higher maternal fish intake confers protection against the harmful effect of prenatal exposure to ambient toxicants. We estimated that in infants whose mothers reported low fish consumption (<91 g/week), the average birth weight deficit attributable to higher prenatal exposure to fine particulate matter (>46.3 μg/m3) amounted to 133 g. However, the interaction term between low fish consumption and prenatal exposure to fine particulate matter was insignificant.
The biological mechanisms whereby PM2.5
might cause adverse pregnancy outcomes are still unclear. PM2.5
is a proxy measure of a wide range of toxic agents present in the environment, which mainly occur through combustion processes that generate other toxic agents as well. Usually, the ambient fine particle fraction contains constituents of soots including PAHs, tobacco and wood smoke, organic compounds, sulfates and metals [35
]. All these substances may potentially affect intrauterine fetal growth.
Our observation suggesting a protective effect of higher fish consumption against harmful ambient hazards in fetal development is consistent with the actual body of knowledge on the importance of n-fatty acids in health and disease [36
]. The intake of fish or fish oils has long been hypothesized to prevent certain chronic conditions such as cardiovascular events [37
] or colorectal cancer [39
]; however, the mechanisms by which it might affect health status are still under debate. Recent evidence suggests that fish and n-3 fatty acid consumption may attenuate inflammation and oxidative stress in humans [41
]. Since fish micronutrients are assumed to possess antioxidant activity, they may protect tissues and cells by reacting with oxygen free radicals generated by metabolites of PAHs or other ambient toxicants present in fine particulate matter and counteract oxidative DNA damage [44
]. Animal studies have already shown that reactive oxygen species and oxidative stress are associated with poor fetal growth [48
]. Evidence from studies of pregnant women also suggests that oxidative stress plays a role in low birth weight [50
]. However, when considering the potential effects of fish micronutrients on birth outcomes, we have to be aware that fish intake may only be a marker for other underlying factors affecting birth outcomes.
Over the past decade there has been a considerable increase in fish consumption, which has mainly been attributed to general knowledge about the nutritional value of fish. However, the effect of maternal fish intake during pregnancy is actually a very sensitive issue since many types of fish are a major source of exposure to methylmercury [52
]. As the toxic effect of methylmercury in humans may depend on the balance between n-3 fatty acids and methylmercury in the fish consumed, the US Food and Drug Administration has advised pregnant women and women who may be pregnant not to eat swordfish, king mackerel, tilefish, shark or fish from locally contaminated areas [56
Our study has some limitations and among them we have to mention the fact that the women in our study sample differ from the broader population of women in Poland since we excluded women with conditions that strongly affect fetal growth, such as cigarette smoking, multiple pregnancy or pre-existing diabetes or hypertension. Our data on fish consumption were based on the FFQ method, which is useful for ranking individuals but does not help to assess the absolute intake of various nutrients. The FFQ method is also sensitive to systematic errors in reporting, and therefore our results must be interpreted cautiously. Moreover, in our study we did not collect information on the exact amount and type of fish consumed by the study participants.
On the other hand, we must mention the strength of our study, which comes from the fact that the most important confounders for neonatal birth weight, such as tobacco smoking by mothers during pregnancy or the presence of chronic diseases, were removed through the entry criteria. Other factors that are thought to affect the probability of delivery of newborns with lower growth, such as maternal height or prepregnancy weight, gestational age, the gender of the child and the season of birth were also accounted for. To avoid other important potential methodological limitations of previous papers regarding the characterization of exposure to environmental toxicants, our study included the assessment of total personal individual exposure to fine particulate pollutants from all potential sources, both indoors and outdoors.
Personal monitoring of exposure to fine particles among pregnant women in our study was performed over a short period of 48 h in the second trimester of pregnancy. To evaluatethe correlation betweenthe levels of PM2.5 measured over 48 h in the second trimester of pregnancy with those in the last trimester, a series of repeated measurements in both trimesters was carried out in a subsample of 85 pregnant women. The correlation coefficient of log transformed PM2.5 measurements in the second and the third trimesters of pregnancy appeared to be significant (r = 0.360, p = 0.0007). This gives us a certain degree of confidence that the measurements of the total personal level of exposure to fine particles taken in the second trimester may be representative for the period of pregnancy as a whole.
Since birth outcomes such as weight at birth alone are inadequate summary measures of the health effects of fetal exposure, future studies should use more refined assessment methods for development not only in terms of anthropometric features but also in terms of the neurodevelopmental performance of infants. The planned longer-term follow-up of our cohort should enable us to determine the sequelae of prenatal exposure to environmental toxicants together with eventual protective effects of maternal nutrition.
In conclusion, our results suggest that a higher consumption of fish by women during pregnancy may reduce the risk of adverse health effects of prenatal exposure to toxicants and highlight the fact that a full assessment of adverse birth outcomes resulting from prenatal exposure to ambient hazards should consider maternal nutrition during pregnancy as well. The results should be a strong incentive for other studies aimed at showing the important role of maternal nutrition during gestation in preventing detrimental health effects of prenatal exposure to various toxicants.