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Fish and shellfish are tasty additions to a healthy diet. They provide high quality protein, are low in saturated fats and provide the essential omega-3 fatty acids.
Fish contain DHA, an omega-3 fatty acid known to be important to neurological development. Fish is also one of the only natural food sources of vitamin D, and is a good source of choline. Both vitamin D and choline play a role in neurological development. Women with diets higher in fish have higher blood concentrations of vitamin D, choline and DHA. They also have infants with higher blood concentrations of these important nutrients.
In the last trimester of pregnancy, the transport of DHA to the baby increases. Currently, we do not know if the mother’s diet is the only source of DHA or if the placenta is able to produce some. However, studies comparing the mother’s diet and blood concentrations demonstrate that both mother and baby have higher levels of DHA when DHA is increased in the diet.
Fish consumption in pregnancy is also related to an increase in birth weight and head circumference, two measures of intrauterine development and a predictor of health status later in life. This benefit comes when women eat at least one serving of fish per week, and was present even though women obtained most of their protein from meats.
Fish has a downside as well. The down side is mercury, which enters the water from the atmosphere and can be present in fish. Mercury is known to cause neurological damage. This risk is increased in unborn babies and young children whose brains are developing very quickly. Nearly all fish and shellfish contain traces of mercury, but some contain higher levels than others.
If you want the benefits of fish for your unborn child, without the drawbacks of mercury, you need to know how much fish is “safe” to eat. In 2005 without benefit of studies on the overall effects, leading health authorities recommended pregnant women or women who may become pregnant limit their fish intake to two servings per week of low mercury fish.
In response to the recommendations to limit fish intake, women have reduced their fish intake more than is considered necessary to reduce mercury exposure. For example, in one study women only consumed fish once per week even though recommendations were for twice per week. Some women find the recommendations too confusing to remember, so avoid eating fish completely. Although the recommendations are for pregnant women, fish consumption has decreased for all women.
Researchers are investigating the effects of eating fish during pregnancy to provide better information for women to make decisions. However, this is a more difficult question to answer than you may initially think. Intentionally testing different intakes of fish for pregnant women to see when it becomes a problem for the baby would be unethical. Women who eat a lot of fish may have other differences in their diet, lifestyle and health than women who do not eat fish – which means the differences in the health of the infants are just as likely due to other factors. To figure out if the theoretical risk of mercury damage from eating fish happens, we need to use other research tools.
One tool is simulation – where information about women is put into a computer program that helps figure out what would happen if the women ate more fish. In one such study where eating 100 grams of fish per day was tested (assumed to be more than the largest amount any person may reasonably eat), most fish species had very small effects on the IQ of the newborns.
In another simulation using the hair concentrations of mercury in the mother, researchers found a small positive effect of IQ when eating fatty fish, and a minor adverse effect on IQ eating lean fish. Both these simulations find that the DHA and other positive compounds in fish seem to neutralize the neurological effects of the mercury in the fish.
The most comprehensive long-term study of the effects of mercury exposure through consuming fish during pregnancy has been tracking children for over 17 years. In this study, researchers have measured 90 different developmental endpoints for children who are now 17 years old and found children whose mothers eat more fish actually do better, despite the greater exposure to mercury.
Mercury exposure, or how much mercury enters your body, is dependent on many things. This includes where you live, what type of fish you eat, how much fish you eat, and the where those fish lived.
Mercury enters the environment as industrial pollution from several types of facilities, and is released naturally from volcanic eruptions and other weathering. In the United States, coal-burning power plants are responsible for about 50% of the mercury air pollution.
Once in the air, mercury travels through the environment and is able to accumulate in fish muscle. In 2008, the U.S. Geological Survey examined a sample of the waterways and found 27% of the sites sampled had fish with mercury concentrations higher than the recommended upper limit for human consumption. The highest concentrations of mercury are found in lean predatory fish, such as pike or perch.
Commercially bought fish are caught from a variety of regional sources. Depending on where you live, they may provide a lower-mercury option than the fish you are able to obtain from local waterways.
Certain fish are known to have higher levels of mercury and should be avoided. According to the United States Food and Drug Administration these fish include shark, swordfish, king mackerel and tilefish. The UK Food Standards Agency has the same recommendation as the USFDA and adds the recommendation against eating marlin. Food Standards Australia and New Zealand recommend against eating billfish (swordfish / broadbill and marlin), shark/flake, orange roughy and catfish. Health Canada recommends pregnant women limit their consumption of shark, swordfish and fresh or frozen tuna (not canned) to once a month. Fish with higher mercury concentrations typically cost more than other types of fish.
The five most commonly consumed low mercury fish in the U.S. are shrimp, canned light tuna, salmon, Pollock and catfish. DHA concentrations are highest in cold water fish such as salmon, tuna, mackerel and sardines.
If you catch your own fish, you can check on the mercury levels by contacting your state/provincial or local health department. In the US, you can also check with the Environmental Protection Agency to get updates.
Some women do not like fish, or would like the benefits of eating fish without any potential risks of mercury. For these women, supplements may provide a beneficial option. As with any supplement, questions arise about taking large quantities of one nutrient without the other naturally occurring compounds an individual would receive in the foods.
There are three theoretical concerns from DHA supplementation. The first relates to the anti-inflammatory effects of DHA. Pregnant women already have a decreased immune response due to the hormones of pregnancy. Adding a DHA supplement may further decrease her immune function.
A second concern relates to DHAs effects on the blood system, specifically lowering blood pressure and reducing clotting ability. Adding a DHA supplement may increase bleeding risk for women with problem pregnancies or who have a bleeding disorder.
A final concern has to do with the metabolism of DHA in the body. In laboratory studies, DHA oxidation produces mutagenic and carcinogenic by-products. These have not been identified in studies of DHA use by humans, but some researchers are concerned DHA supplements will introduce these potentially harmful compounds without the naturally occurring anti-oxidants in foods that contain DHA.
Researchers are examining the effects of DHA supplementation in pregnancy. In one such study, women were given 3 capsules of marine algae-oil derived DHA per day for a total of 600 mg per day. These women were compared to women eating their normal diet and taking a placebo capsule. The women were told to take as many as they could tolerate per day, up to 3 capsules. The study group did have a higher DHA concentration in the cord blood at birth, had a gestation that was just over two days longer on average and higher birth weight and length than control groups.
In a study on fish consumption and birth weight, researchers found the use of fish oil supplements did have an effect, but this effect was much weaker than simply eating one serving of fish per week. A review of studies on the effects of supplementation during pregnancy on neurological development found most trials were of poor quality. The one well-conducted study identified found no effect on overall development with supplements.
It is also important to remember that it is nearly impossible to remove all contaminants from a natural fish oil supplement.
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