Wetenschap - 28 november 2002

Mercury levels in fish undo protective effect of oily fish in the human diet

Mercury levels in fish undo protective effect of oily fish in the human diet

According to scientists a diet containing a lot of fish should lower the risk of a heart attack. Not so, according to research coordinated by Professor Frans Kok of the Human Nutrition and Epidemiology Group at Wageningen University. In a recent article published in the New England Journal of Medicine the researchers suggest that the effect of mercury in fish almost completely negates the protective effect.

A couple of hundred of the seven hundred heart patients in the latest research followed carried out by Professor Kok had done exactly what they should have done according to advice from nutritionists. They ate lots of fish, thus providing their bodies with the protective fatty acids that fish contain. According to researchers the fats in fish are good for the heart and blood vessels. "Omega-3 fatty acids present in fish diminish the chance of blood clots and reduce the amount of fats in the blood," explains Kok. "In addition they seem to have a positive effect on cholesterol levels and the heart rhythm." However, when an international research group, of which Kok was team leader, examined seven hundred men in eight European countries and Israel, they found little evidence of the protective effects of fish.

The researchers also had data from healthy males, which meant that they were able to observe that eating more fish did not automatically lead to a lowered risk of heart attack. "We took samples of subcutaneous fat from the patients and measured the fatty acid composition of these," describes Kok. "This enabled us to calculate how much fish the subjects had eaten over a number of years." Contrary to expectations, the chance of a heart attack did not automatically decrease when there was a higher amount of fish fatty acids in the human fat sample.

The researchers found the cause of this in the toenails of their subjects. They measured the concentration of mercury in the toenails to get an idea of the quantities of mercury that the men had consumed. They discovered that the concentration of mercury in the toenails was much higher when there was more fatty acid from fish in the fat samples. "Most of the mercury humans consume comes from fish," says Kok, "So it wasn't really that much of a surprise."

About two thirds of the mercury found in the sea comes from environmental pollution. Most of this is inorganic mercury which is converted by fish and algae into the far more dangerous methyl mercury. This poisonous compound deactivates enzymes in the human body that are needed to protect cells, and it also contaminates cholesterol and encourages the formation of blood clots.

The researchers were able to separate the effects of mercury and fish fatty acids by statistical analysis. The group with a higher intake of mercury had a fifty percent higher chance of a heart attack than the group with lower mercury intake. But because the higher risk group had absorbed the mercury through fish they still received some protection from the fatty acids. Without this, the chance of a heart attack would have been 116 percent higher. "Nevertheless we don't think that people should stop eating fish," continues Kok. "Not all fish contain the same amount of mercury."

Dr Michiel Kotterman of the fish research institute Rivo examined which types of fish were worse sources of mercury. Without exception these were fish at the top of the food chain. Highest scorers were the swordfish and the shark, which contain about one milligram of mercury per kilogram. Tuna, trout, pike and perch have a middle score. One kilogram of these types of fish contains a tenth to a half of the amount found in swordfish and shark. Shellfish have a low mercury score.

The amounts of mercury in cultured fish vary. In the years when El Ni?o does not disturb sea currents, the raw materials for fish feed come from the cleaner waters around South America, and the concentrations of mercury in salmon and other farmed fish are low. In the years that El Ni?o is more active the fish-feed manufacturers have less clean raw materials to work with.

Another factor is also where the fish is caught. "The highest levels of mercury were found in a group of Spanish subjects, who had probably eaten large amounts of locally caught fish, and we believe this may be a significant factor." Regional differences may also explain why so many other researchers have found evidence of the protective effects of fish. "I'm certainly not saying on the basis of this research that it is not worth eating fish," says Kok. "What is clear though is that the levels of mercury and other heavy metals in fish must be drastically reduced."

Willem Koert

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