By Carol Ann Rinzler

The next time your teen brother, nephew, son, or friend tells you he’s a grown-up, smile pleasantly and hand him a tuna sandwich. In fact, the brain of an older teen — think 15, 16, 17, and 18 — is particularly “plastic,” meaning particularly able to build the new connections required to learn new things like high-school math, science, history, and literature.

In 2009, scientists at Sweden’s Gothenburg University reported that 15-year-old boys who eat fish more than once a week score higher on intelligence tests than their non-fish-eating friends.

Not being totally chauvinistic, the Gothenburg researchers soon filed another report, this one on teenage females. Sure enough, those who ate fish did better in school. So when you’re serving tuna on rye, make sure your teen brother, nephew, son, or friend shares that tuna sandwich with his sister, niece, or girlfriend. And vice versa.

Naturally, there is a catch in the day’s catch. Some fish contain an unhealthful amount of methylmercury, a compound produced by bacteria that chemically alter the naturally occurring mercury in rock and soil or the mercury released into water through industrial pollution.

Little fish eat mercury-contaminated algae, bigger fish eat the smaller fish, people eat the bigger fish, and the mercury ends up there. The larger the fish and the longer it lives, the higher its mercury content is likely to be. To reduce the risk to fetal development, the 2015–2020 Dietary Guidelines for Americans advisory committee advises pregnant woman to avoid the Mercury Big Four (shark, swordfish, king mackerel, and tilefish) and limit their weekly consumption of seafood to two or less 4-ounce servings of seafood high in omega-3s and low in mercury.

Luckily, you can also get DHA from foods other than fish. For example, many infant formulas now contain DHA derived from fermented algae, and health-food stores sell algae-based DHA supplements for adults. One plain egg yolk contains 20 milligrams/0.02 grams of DHA; an egg yolk from an egg laid by a hen fed a DHA-enriched diet may contain up to 200 milligrams/0.2 grams of DHA. Some nuts and seeds (think walnuts and flaxseed) contain alpha linolenic acid, a precursor converted by our bodies to DHA.

This table shows the DHA and mercury content of some popular fish and shellfish.

DHA & Mercury Content of Popular Fish & Seafood
Fish Grams of DHA per 100 g/3.5 oz Mercury Content*
Anchovy 1.3 Low
Catfish 0.13 Low
Clams 0.15 Not detectible
King mackerel 0.23 High
Salmon, Atlantic, wild Salmon, Atlantic, farmed 1.43 0.62 Low Low
Sardine, canned, drained 0.51 Low
Scallop 0.10 Low
Shark, raw 0.53 High
Swordfish 0.68 High
Tilefish 0.73 High
Trout, mixed species 0.68 Low

* Low = less than 0.12 ppm; high = more than 0.730 ppm.

Source: USDA, Appendix G2: Original Food Guide Pyramid Patterns and Descriptions of USDA Analyses, Addition A: EPA and DHA Content of Fish Species, 2005; FDA, Mercury Levels in Commercial Fish and Shellfish, 2006