Seafood, Omega-3s and Mercury

A peninsula with long coastlines, Korea has always eaten from the sea. Small dried anchovies are stir-fried as myeolchi-bokkeum and simmered for stock, mackerel is grilled or braised, pollock turns up fresh, frozen and dried, and squid, clams, oysters, crab and shrimp fill stews, pancakes and jeotgal. Seafood brings protein and valuable fats to the table. It also raises a common question about mercury, which matters most during pregnancy and early childhood. This page sets out what the main health agencies say.

What omega-3 fats are

Omega-3s are a family of polyunsaturated fats. Harvard's Nutrition Source describes three main forms: ALA, found in plant foods such as walnuts, flaxseed and soybean oil, and EPA and DHA, found mainly in fish and other seafood. The body can convert a little ALA into EPA and DHA, but not efficiently, which is one reason fish is valued as a direct source.

Oily, darker-fleshed fish are the richest. The Korean Food Composition Table lists raw mackerel from Korea's southern waters at about 2.96 g of omega-3 fatty acids per 100 g, including roughly 2,060 mg of DHA and 650 mg of EPA.

What research says

Harvard notes that the strongest evidence for omega-3 fats concerns heart health, while also pointing out that many large trials of omega-3 supplements have not shown the same benefit as eating fish. The NIH's National Center for Complementary and Integrative Health likewise describes the evidence for supplements as modest and suggests that the advantages of seafood may come partly from its other nutrients and from the wider eating patterns of people who eat fish. Research is ongoing. Harvard's general suggestion is to eat fish or other seafood once or twice a week, favoring oily fish.

Mercury and the US fish advice

Fish can contain methylmercury, and levels differ widely by species. The US Food and Drug Administration (FDA) and Environmental Protection Agency (EPA) publish joint advice for people who are or may become pregnant, those who are breastfeeding, and children aged 1 to 11. It sorts fish into three groups. Many species common in Korean cooking fall into the lowest-mercury "Best Choices" group:

• Best Choices: anchovy, Pacific chub mackerel (the species sold as godeungeo), pollock, squid, clam, oyster, crab, shrimp, flounder and sole, cod, herring, sardine, salmon and canned light tuna.
• Good Choices: albacore and yellowfin tuna, halibut, monkfish, rockfish and white or Pacific croaker, among others.
• Choices to Avoid: king mackerel, marlin, orange roughy, shark, swordfish, bigeye tuna and tilefish from the Gulf of Mexico.

Two familiar Korean fish, hairtail and yellow croaker, are not named on the chart. Note too that "mackerel" covers several species: chub mackerel is a Best Choice, while king mackerel is on the avoid list.

For adults in the advice's target group, the FDA suggests 8 to 12 ounces (about 225 to 340 g) of fish a week, which works out to two or three servings from the Best Choices list or one from Good Choices. Children are advised to eat fish twice a week from Best Choices, in age-appropriate portions of about 1 ounce (28 g) at ages 1 to 3, rising to about 4 ounces by age 11. The FDA highlights fish as a source of omega-3s, iron, iodine and choline that support a child's brain development.

Korean guidance

Korea's Ministry of Food and Drug Safety (MFDS) has given similar advice. In a 2014 statement it recommended that pregnant women, women who may become pregnant and nursing mothers limit tuna, swordfish and shark to no more than 100 g a week, while stressing that fish should not be cut out altogether. Its monitoring between 2004 and 2008 found average methylmercury levels below the national limit, with canned tuna much lower than large fresh tuna. In 2015 guidance reported by the Kyunghyang Shinmun, the ministry put the weekly limit for people eating only ordinary fish and canned tuna at about 400 g.

Putting it on the table

• Rotate familiar fish: Grilled or braised mackerel, pollock in saengseon-jeon, anchovy banchan and shellfish stews all draw on lower-mercury species.
• Watch large tuna: In MFDS testing, fresh tuna averaged about seven times the methylmercury of canned tuna (0.21 versus 0.03 ppm), so sashimi-grade tuna is the one to limit. See hoe culture.
• Mind the salt: Salted mackerel, dried anchovies and jeotgal are high in sodium. See sodium in Korean food.
• Raw seafood in pregnancy: Mercury is not the only concern. Raw fish and raw crab dishes such as ganjang-gejang carry food-safety risks; see food safety at home.
• Children: For portion and texture advice, see Korean food for children. Seaweed, another sea food, is covered in seaweed and iodine.

Anyone who is pregnant, planning a pregnancy or feeding a young child should treat these lists as a starting point and discuss their own diet with a healthcare provider.