Foods and Toxins To Avoid /

Mercury and Fish

01 In Brief

Fish is nutritious and the Australian Dietary Guidelines recommend that we consume 1-2 serves per week. However some fish are high in mercury, which is toxic to the developing neurological system. The Food Standards of Australia and New Zealand have published guidelines that limit the amount and types of fish that should be consumed by women planning to become pregnant, are pregnant, breast feeding and children under 6 years. 

02 What Do I Need To Know?

SOURCES OF MERCURY

• Fish is a nutritious food that provides many health benefits. It is a good source of protein, Vitamin D and iodine and is an excellent source of omega 3 fatty acids. The Australian Dietary Guidelines recommend that people consume at least 1-2 serves of fish/seafood a week. 

• Mercury occurs naturally in the environment, in air, water and food. It is found in nature in various forms, which include organic, inorganic, and metallic - the organic form being the most hazardous. In the aquatic environment inorganic mercury is converted into a toxic organic form called methyl-mercury by the microorganisms found in the sediment of seabeds. This accumulates in the aquatic food chain in fish and shellfish.   

• All fish will contain some mercury and the amount depends on where the fish lives, its age, size and feeding pattern. Larger predatory fish, at the top of the food chain and who live longer tend to have higher levels of mercury.  

THE EFFECT OF MERCURY

• Once ingested, mercury is readily absorbed into the blood stream and can cross the blood brain barrier and the placenta resulting in higher mercury levels in the fetus compared to the mother. 

• The effect of mercury on the human nervous system is well recognised although the severity is dose-dependent. The adult’s nervous system can tolerate much higher levels than the growing brain of the fetus and young children. 
  • In adults high levels of exposure results in loss of cells in specific areas of the brain. The first observed effects are tingling of the hands, feet and lips that may occur many months after exposure. Then follows loss of coordination, visual problems and loss of hearing and speech problems. Mad Hatters disease is a well known example. 
  • In the fetal brain methyl-mercury causes more extensive damage and in severe cases the effects are very similar to cerebral palsy. Minamata disease in Japan is a well-documented example of exposure to very high levels of mercury from a contaminated water supply.
• Two major studies have been conducted on the effects of mercury on Island populations whose diet mainly consists of fish: 
  • Faroe Islands - children's attention span, learning and memory were studied. Here people eat Pilot whale meat daily (average concentration of mercury of 1.6 micrograms/kg). Results showed that all the children were adversely affected and more subtly, their visual, spatial and fine motor skills were also abnormal.
  • Seychelles - resuls showed that there were no adverse affects on children's learning or behaviour. despite fish being eaten on a daily basis. Here, fish has a far lower concentration of mercury, (0.3 mcg/kg). In some tests, beneficial effects were observed and could be explained by the known benefits of  fish intake. 

RECOMMENDATIONS

• The mercury we ingest is stored in fat tissue, brain and bones. Our bodies can remove mercury but it is a slow process, which sometimes takes several months. The daily excretion is only 1 per cent of the body stores and the half life of mercury is estimated to be 70-80 days.

• FSANZ recommends decreasing intake of, or simply avoiding, fish that is high in mercury for: 
  •  Women planning to become pregnant;
    
  •   Pregnant women;  
  •  Young children up to and including six years of age.  

• For the general population who consume moderate amounts of fish the levels of mercury do not pose a health risk. Fish oil supplements are considered safe at the recommended dosage.  

• Maximum levels of mercury content of commercial fish are regulated by the ANZ Food Standards code: 
  • Fish known to have high levels of mercury restricted to upper limit of 1.0 mcg/kg: 
    • Billfish – swordfish, marlin, southern blue fin tuna, barramundi, ling, orange roughy, rays and sharks; 
  • All other fish and crustaceans 0.5 mcg/kg. 

• A table of guidelines has been developed for the Australian environment and is based on currently available scientific data.  

The FSANZ ‘Advice on Fish Consumption’ has been specifically developed for the Australian population and reflects local knowledge of our diets, the fish in Australian waters that we eat, and their mercury content.

Number of serves of different types of fish that can safely be consumed:

Table from FSANZ

03 What Others Say

• Mercury in Fish - Food standards of Australia and New Zealand

FSANZ Consumer Information Mercury and Fish

•  NSW Food authority

Fish and Mercury FAQS NSW FA 

Mercury in Fish Wallet card

04 I Want To Know More

• A review article and advice with more detailed information from FSANZ Food Standards Australia and New Zealand 

FSANZ further information mercury and fish

• FDA US fact sheet: advice as it relates to US conditions 

FDA Fact sheet methyl-mercury in fish and shellfish

• Canadian advice regarding canned albacore tuna 

Health Canada mercury and fish

• Faroe Islands study

Impact of prenatal methyl-mercury Faroe Islands study

05 Clinicians Tools and Resources

• NHMRC Australian dietary guidelines

Australian Dietary Guidelines

Last updated: 13/07/2011 by Dr Elizabeth Hallam*/Claire Galea

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