– but what about the mercury? – but what about the mercury?
Oily fish is a great source of the essential omega-3 fatty acids, EPA and DHA, which are important for the development of the brain and nervous system of the unborn child. Pregnant women with low levels of these two omega-3 fatty acids have a statistically significant increased risk of preterm delivery compared with pregnant women who have high levels of EPA and DHA in their blood. This was shown in a study from SSI in Copenhagen, carried out in collaboration with Harvard T.H. Chan School of Public Health in Boston, the United States. The problem is that many women are afraid to eat oily fish during their pregnancy because of the risk of mercury poisoning. So which fish are the safest for consumption – and what about fish oil supplements?
If a baby is born before the 37th week of pregnancy, it is considered premature. The earlier the birth, the greater the risk of complications and late effects. On a global scale, premature delivery is the leading cause of death among children under the age of five, so there is every imaginable reason to make sure that the pregnancy is healthy, not just for the baby’s sake but also for the mother’s.
For decades, science has believed that a high intake of the two omega-3 fatty acids, EPA (eicosapentaenoic acid) and DHA (docosahexaenoic acid), which we primarily get from herring, salmon, anchovies, and other oily fish, could lower the risk of premature delivery. In their study, the scientists looked at data from a nationwide population study of 96,000 Danish babies that was based on questionnaires and registries. From the period 1996-2003, the scientists analyzed blood samples from 376 women, who had given very early birth (before the 34th week), and from 348 women, who had delivered at term. Blood samples were taken from all women during the first and second trimester of their pregnancies.
As much as ten times higher risk of premature delivery
By analyzing the blood samples, the researchers found that the women in the quintile with lowest blood levels of EPA and DHA were up to 10 times more likely to deliver prematurely compared with the quintile of women with the highest levels of the two omega-3 fatty acids. Also, the risk of giving birth too early decreased as levels of EPA and DHA went up. The study suggests that pregnant women can lower their risk of premature delivery by eating more oily fish or taking a fish oil supplement.
Nonetheless, the scientists warn against generalizing, as the study was conducted in Denmark, and other factors than diet may have weighed in. Still, studies rarely show such as positive an effect as the one seen among women with higher levels of omega-3. It should also be mentioned that oily fish contain a lot of selenium, a nutrient that has been shown to prevent premature delivery in other studies.
The Danish study was published in Ebiomedicine in August 2018.
Warning about mercury in oily fish
Many pregnant women are afraid to eat fish because of warnings about mercury, which can harm the brain and nervous system of the fetus. The problem with these warnings is that they deter pregnant women from eating fish, causing them to become deficient in omega-3, a type of essential fatty acids that is very important for normal pregnancy. It is therefore useful to learn a little about the food chain and to know in which ocean the fish has been swimming.
Fish from the higher part of the food chain contain more mercury than fish from below. Selenium, which is a trace element, is able to neutralize some of the mercury by forming a harmless compound called mercury selenide. However, once selenium has done that by forming a bond with mercury, the selenium is no longer available to the many different selenoproteins that are vital for numerous body functions and for ensuring a healthy pregnancy. In other words, predatory fish at the top of the food chain have an unfavorable mercury-selenium ratio and pose an increased risk that free (unbound) mercury can harm the fetus. The scientists behind the new study therefore recommend that the pregnant women follow the dietary guidelines for safe fish consumption that were issued by the American health authorities last year.
Eat fish from the lower part of the food chain
What is important (for pregnant women) is to avoid predatory fish such as tuna, perch, swordfish, shark, and halibut. Women trying to become pregnant, who are already pregnant, or who are breastfeeding should avoid eating fresh cuts of large predatory fish like tuna, and they should stay away from canned tuna (if it is white tuna or albacore tuna). This advice is based on studies showing that one in four children risks brain damage caused by accumulated mercury.
It is therefore safer to get the essential omega-3 fatty acids from oily fish such as herring, anchovies, salmon, and mackerel that are at the lower part of the food chain and normally have a favorable selenium-mercury ratio with more selenium than mercury. Salmon from the Baltic Sea tends to contain too many heavy metals and other types of pollution, so it is generally better to look for salmon from cleaner waters or organic salmon.
Omega-3 fatty acids from supplements
Many women, who are pregnant, fail to follow the official dietary recommendations of eating fish several times a week (at least 350 grams with 200 grams coming from oily fish). People who do not like fish or just find it difficult to eat enough should take a supplement. Fish oil based on free fatty acids have the best bioavailability. Fish oil should always be within the safe limits for peroxide value and content of environmental toxins. Fish oil supplements do not contain selenium, a micronutrient that is also important for a healthy pregnancy and full-term pregnancy.
Facts about premature delivery
Chris Sweeney, Harvard Gazette. Pregnant women encouraged to eat cold-water fish. MedicaleXpress August 3, 2018
Nicholas V.C. Ralston, Laura J. Raymond. Mercury´s neurotoxicity is characterized by its disruption of selenium biochemistry. 2018
Ralston,N., BBA- General Subjects (2018), https://doi.org/10.1016/j.bbagen.2018.05.009
DR-dokumentar: De ufødte børn 03-11-2014
Njord, V Svendsen: Næringsstoffer i fisk neutraliserer miljøgifte. Videnskab.dk 2012
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