Good news for fish lovers: a British study of several thousand pregnant women has found that the amount of fish they ate had very little effect on the amount of mercury in their blood. In fact, their diet appeared to be responsible for less than 20% of the mercury detected in their blood.

The study suggests that far less mercury comes from what people choose to eat than had previously been thought. In addition, such toxic chemicals pose a greater threat to developing children than they do to adults.

Herbal tea was an unexpected dietary source of blood mercury, the highest after oily fish and white fish.

Concerns of possible toxicity from mercury in fish are what prompted the FDA to recommend in 2004 that pregnant women and young children limit their fish consumption to 12 ounces (2 meals) or fewer of fish or shellfish that are low in mercury per week, recommendations that still stand today. But the current study suggests that these guidelines may need to be reviewed.

The Avon Longitudinal Study of Parents and Children (ALSPAC) — also known as Children of the 90s — is a long-term health research project studying residents of the City of Bristol and the surrounding area, about 120 miles west of London. More than 14,000 women enrolled while pregnant in 1991 and 1992, and the health and development of their children has been followed in great detail ever since.

The mercury portion of the study looked at nearly 4,500 women who had filled out detailed dietary questionnaires and also gave a blood sample while pregnant that was tested for its mercury content. The researchers sought to determine how much eating seafood (fish and shellfish) contributes to blood mercury levels during pregnancy and also how much other foods and drinks contribute. Information was collected on 103 separate food and drink items including white fish, oily fish and shellfish.

By comparing an individual's intake of specific food items to the amount of mercury found in their blood samples, the researchers were able to determine how much each food was contributing to the amount of mercury in the women's blood. The three fish items together (white fish, oily fish and shellfish) accounted for 7% of total blood mercury. The entire 103 food items surveyed accounted for 16.6% percent. Herbal tea was an unexpected dietary source of blood mercury, the highest after oily fish and white fish.

A slightly different statistical analysis changed the findings only a little: fish responsible for 8.75% of blood mercury, diet as a whole responsible for 19.8%.

Where is the rest of the mercury in the blood coming from? That's not clear. People are exposed to mercury from many other sources including dental fillings and as an air and water pollutant.

Mercury waste enters the environment from incineration, the burning of fossil fuels and from some fungicides and pesticides. Some also may come from smoking cigarettes. But little seems to enter our bodies from what we eat and drink.

The authors conclude that advice to pregnant women to limit seafood intake is unlikely to reduce their blood mercury level substantially.

Many studies have suggested that eating fish is good for the health, helping, for example, the heart and the brain. While much interest has focused on the omega-3 fatty acids found in fish, it's still not clear what exactly in fish is responsible, just that eating fish and good health seem to go hand in hand.

But the threat of mercury contamination has made some people reluctant to eat any fish at all, particularly pregnant women. Which is a shame, because many varieties of fish and seafood are very low in mercury. It's mostly the larger fish at the top end of the food chain (shark, swordfish, tilefish, king mackerel) that are high in mercury.

Sardines, salmon, tilapia and catfish are four common types of fish that are extremely low in mercury, as are scallops, shrimp, clams and oysters. Canned light tuna (not albacore) is also low but not as low as any of the fish or shellfish just mentioned.

The study appears in Environmental Health Perspectives and is freely available.