Historically, fish is one of the most important sources of food known to man. Large proportions of the World’s population rely on fish as their daily source of animal protein, and fish is also used as animal feed in large quantities. Fish contributes greatly to the nutritional needs of some of the poorest communities. However, all fish contain methyl mercury (MeHg), a known neurotoxin and environmental poison. The quantity present in the muscle tissue of fish varies widely depending on a number of factors such as age, location and species. Methyl mercury accumulates up the food chain and as a result the large predatory fish tend to have much higher in their tissues than smaller fish. Shark, tuna, merlin and swordfish consumption on a regular basis can cause hair mercury levels as high as 10 µg / g1.
Generally, mercury is known to be toxic in all forms. Exposure to methyl mercury causes irreversible damage to the brain and spinal cord, which result in neurological deterioration and eventually death. Because methyl mercury is toxic to the nervous system, it is particularly harmful at times when the nervous system is developing, such as in unborn children and during the early years of life. A number of environmental accidents, for example in Japan and Iraq, resulted in the spilling of methyl mercury from industry into the water supply. This is known to have caused severe damage to the unborn children of pregnant mothers exposed to the toxic fish in a dose response manner. The effects of long term and lower levels would tend to be more subtle and may not be obviously apparent.
As well as containing pollutants, fish are also important sources of certain nutrients. Fish contains the long chain polyunsaturated fatty acids docosahexanoic acid and eicosapentanoic acid which are incorporated into cell membranes and are essential for the proper functioning of the nervous system. These long chain fatty acids are particularly important to young children to allow the development of a properly functioning brain. Fish would therefore appear to be both beneficial and detrimental to the nervous system because of its polyunsaturated fatty acid content and its methyl mercury content respectively. Recommendations are to avoid the most polluted fish such as farmed salmon and the larger predator fish, but maintain adequate intakes of the long chain polyunsaturated fatty acids from fish sources. Some fish oil supplements have the methyl mercury and other pollutants removed and are also recommended.
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