Fish: superfood or health risk?

As a lean, readily available protein, fish is widely considered a healthy food choice that billions of people around the world rely on for nutrition. Yet popular opinion diverges over the benefits and threats of a fish-heavy diet. Is there a downside to eating seafood or is it definitively part of a healthy eating pattern? 

Seafood, which includes fish and shellfish, offers health benefits for the general population—including pregnant women and those who are breastfeeding, according to the 2015-2020 Dietary Guidelines for Americans. In addition to protein, fish provides healthy omega-3 fatty acids, vitamins B12 and E, plus iron, selenium, zinc, and iodine. The guidelines note that eating about 8 ounces of seafood per week is linked to fewer cardiac deaths among people with and without pre-existing cardiovascular disease, based on scientific evidence.

But fish has also been the subject of public health advisories, due to the presence of contaminants that can be harmful to health. In this article, we’ll explore the benefits and harms of eating fish.

Fish and contaminants 

Most people have heard that fish can contain mercury, due to increasing pollution and manufacturing concerns. Seafood is the main source of human exposure to methylmercury, a powerful neurotoxin, according to a research article published in 2019 in Nature. In the United States, 82% of population-wide exposure to methylmercury comes from eating marine seafood and nearly 40% is from fresh and canned tuna, the authors wrote.  

When ingested at high levels, this heavy metal can cause nerve damage in otherwise healthy adults, young children, and during fetal development. This is enough of a concern that the FDA and EPA have offered guidelines for women who are or might become pregnant, breastfeeding mothers, and young children, including advice on which fish to eat based on mercury levels. 

However, the impact of mercury levels often found in the fish we eat is still the subject of sometimes contradictory clinical research. Much of the concern seems to revolve around infants. The CDC warns that breastfeeding mothers should limit exposure to mercury in their diets, however, even that advice is somewhat murky when it comes to fish. 

“Although mercury naturally occurs throughout the environment, the mother’s diet is the primary source of mercury exposure for most breastfed infants before they are introduced to complementary foods,” states the CDC. “When a mother eats fish, the mercury in the fish can be passed into her breast milk. However, the benefits of breastfeeding may be greater than the possible adverse effects of exposure to mercury through breast milk.” 

The aforementioned FDA guidelines offer a chart that categorizes fish type and recommended frequency in the diet, based on levels of mercury or other contaminants. The fish with the highest mercury levels include king mackerel, marlin, shark, orange roughy, swordfish, bigeye tuna, and tilefish. 

More recently, the discussion has focused more on microplastics. Data show that humans are ingesting microplastics and that some of them come from seafood. It’s a relatively new issue, but scientists are already raising questions about the risks of microplastic ingestion. For now, more research is needed to determine their impact.

Developmental and cognitive benefits of eating fish

When a pregnant woman eats mercury-heavy seafood, their unborn child may absorb it. While initial research showed that higher mercury levels could result in health problems for the child, more recent research has produced inconsistent results.  

One such study showed that fish consumption did not negatively affect size and birth weight in newborn babies. In fact, when compared with women who avoided fish entirely, fish consumption appeared to protect the birth weight of infants.

Another study evaluated more than 150,000 mother-child pairs to determine the impacts of maternal fish consumption on newborns and young children. Participants were split into two groups: one with children at 6 months of age and the other with children at 1 year. The results showed a positive correlation between fish consumption and motor development skills, with delays in motor skills progression reduced at 6 months and delays in problem-solving progression reduced after 1 year.

As a result, the 2015-2020 Dietary Guidelines recommend that pregnant women eat 8 to 12 ounces of fish (the kind with lower mercury levels) per week and that adults in general eat at least 8 ounces per week.

Fish, heart disease, and other illnesses

Substantial evidence suggests that eating 1-2 servings of fish per week (particularly oily fish like salmon, sardines, and herring) is beneficial for cardiovascular health due to high levels of omega-3 fatty acids and low levels of saturated fats. But the relationship between mercury in fish and heart health is less clear, according to a 2017 article in the journal BMC Cardiovascular Disorders. 

“We found no evidence that mercury exposure from regular fish consumption increases cardiovascular disease risk in a population of Spanish adults with high cardiovascular disease risk and high fish consumption,” the authors wrote. “This implies that the mercury content in fish does not detract from the already established cardiovascular benefits of fish consumption.”

In addition, a large study from 2011 contradicted previous evidence that mercury exposure might increase the risk of cardiovascular disease. In a large-scale analysis featuring two studies published in the New England Journal Of Medicine, higher mercury levels were not correlated with an increase in heart disease. In fact, the opposite was found: High mercury levels appeared to lower the risk of heart disease, an association they attributed to the increase in fish consumption. 

In terms of the impacts of eating fish on other diseases, one study looked at the relationship between eating fish and depression. The meta-analysis of a collection of published cohort studies reported that the risk of depression decreased as fish or omega-3 consumption increased. This inverse correlation appeared to affect women in particular, though the study did not pursue why.

Is eating seafood worth it?

Despite the risk of consuming contaminants, the USDA recommends moderate consumption of seafood for adults, children, and pregnant women.

Still, manufacturing and lifestyle choices have led to an increase in the contaminants found in our food chain. Exposing humans to potentially toxic contaminants is always a health risk, but much of the research surrounding mercury levels, PCBs, and microplastics remains inconclusive.  

Eating seafood, like all dietary decisions, comes down to striking a balance. For example, if you are concerned about mercury but still want the benefits of consuming seafood, check out the FDA chart mentioned above and choose species that tend to accumulate low or undetectable levels of this contaminant. 

As rates of lifestyle-based illnesses continue to increase, medical research is taking a closer look at diets as a leading cause. Fatty acids found in fish help to prevent certain illnesses and fish remains one of the best sources of highly nutritious food. So go ahead and enjoy some fish—just be sure to choose the species wisely.