Fish is regularly promoted as an important part of a healthy diet. With benefits ranging from reduced cardiovascular risk to the prevention of dementia, it’s little wonder health authorities are talking up the value of eating fish. We’re now also learning the importance of fish and omega-3 fats from fish for children’s development, with reports showing these poly-unsaturated fats are important in preventing learning difficulties and minimising ADHD, dyslexia and more.

More recently, though, concerns have been expressed over the levels of mercury in fish and mothers-to-be have been advised to limit their fish intake. Worried about the health of their unborn babies and not wanting to expose them to this toxic metal, many pregnant women have elected to remove fish completely from their diet.

However, given the health benefits to be had from eating omega-3s, is totally removing fish from your diet the best strategy? Let’s examine the links between fish, mercury and health in general, as well as the situation with mothers-to-be. By not eating fish, are you missing out?

The benefits of eating fish

Fish is a great source of low-saturated fat protein. It’s also high in the minerals zinc and iodine, along with vitamin B12. But, more significantly, fish is one of our primary sources of two types of omega-3 polyunsaturated fats: eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). It’s these omega-3s that have excited nutritionists and health bodies, as there is evidence they will reduce your risk of cardiovascular disease and of having a heart attack.

By not eating fish, are you missing out?

Omega-3s are also good for your brain. They are important in the brain’s structure, with estimates that they make up 8 per cent of your brain. These fats are used in the myelin sheath, a fatty layer surrounding neurons that enables the speedy transmission of messages to and from your brain. Low levels of omega-3s are thought to interfere with neurotransmitter production, which may be why a 2002 New Zealand study of more than 4500 adults found people who ate fish had better mental health. Omega-3s are now often used in the treatment of depression and fish consumption has also been shown to slow cognitive decline and reduce the risk of both dementia and Alzheimers.

Ongoing research into the health benefits of omega-3s is suggesting they can benefit and prevent a wide range of conditions including arthritis, inflammatory bowel disease and autoimmune conditions. Omega-3s are also a current subject of research in the area of children’s learning and development. In 2005, 100 children with learning and motor coordination problems were given fish oil supplements for six months. They showed improvements in behaviour, reading, spelling and short-term memory. An Australian study, also of more than 100 children, found omega-3s improved ADHD-related problems, including inattention, hyperactivity and impulsivity. This kind of result is why an adequate intake of omega-3s is recommended for children’s learning and mental development.

Beyond omega-3

Not only does fish contain omega-3 polyunsaturated fats but it’s also one of the best dietary sources of the mineral iodine. Iodine deficiency is common in Australia and New Zealand. Soils in both countries are low in iodine, meaning it’s not part of the collection of minerals taken up by plants and very little iodine enters our food chain. However, iodine is necessary for the proper functioning of the thyroid gland, which produces hormones necessary for physical growth and development. A low iodine intake will also inhibit a child’s mental development and ability to learn. If your kids are not eating fish, they are possibly not getting enough omega-3 fats or iodine.

Omega-3s are also a current subject of research in the area of children’s learning and development.

Fish is also important in pregnancy. As well as providing important omega-3 for development of the unborn baby’s central nervous system, fish is a good source of protein. Recent research is suggesting that eating fish while pregnant may protect your baby from developing asthma and other allergic conditions such as eczema and hayfever.

In recent years, though, concern about mercury levels has tarnished fish’s healthy reputation. Many people, particularly mothers and pregnant women, have stopped eating fish in an attempt to reduce their mercury exposure and avoid toxicity. Despite cutting out fish, few people have supplemented their diets with alternate sources of either omega-3s or iodine. Given the health benefits from both, this is problematic.

What is mercury?

The substance at the heart of this dilemma is mercury, a heavy metal that occurs naturally in the environment. However it’s also present in our world as a result of pollution. Mercury is used in the manufacture of a number of common products, including:

• Car parts
• Batteries
• Fluorescent lightbulbs
• Medical products
• Thermometers
• Dental amalgams

When these products are thrown away and not disposed of carefully, they end up in landfill. As the products degrade, mercury is exposed and free to pollute both our land and waterways. Mercury is highly toxic and most industrialised countries have made efforts to both limit its use and ensure its safe disposal. There have been campaigns in some countries to ban its use entirely.

Mercury does nasty things to your body. It damages the central nervous system, endocrine system, kidneys and other organs. Given the harm it can do to your health, it’s wise to limit your exposure to this heavy metal. Fortunately, most of us don’t come into contact with enough mercury to cause serious problems. While toxic exposure can occur, in Australia it’s as a result of industrial exposure.

How does it get into fish?

Mercury is found in our waterways as a result of pollution and landfill run-off. Anaerobic bacteria found in lakes, rivers, soils, wetlands and the oceans convert mercury into an organic form called methylmercury. As well as being highly toxic, methylmercury accumulates in organisms, working its way up the marine food chain by a process called biomagnification. The anaerobic bacteria that work on mercury are consumed by plankton, which are then eaten by small fish, which are in turn consumed by larger and larger fish.

Each larger fish absorbs the body burden of mercury of the smaller fish it consumes. So the danger to your health from mercury exposure comes from eating the larger fish, those at the top of the marine food chain. These fish contain concentrated levels of mercury that may be 10 times that found in smaller fish.

Should you be eating fish?

Despite the genuine concerns about mercury in fish, the proven health benefits that come from eating it far outweigh the potential for harm. Simply put, without fish you are missing out on important omega-3s that will reduce your risk of cardiovascular disease, depression, dementia and a whole series of other health problems.

It’s wise, though, to be careful about which fish you eat and to limit your intake of the larger fish, those higher up the food chain. The following fish contain the highest levels of mercury. While it’s not necessary for most people to completely avoid these fish, you would be best to limit your intake to once a week only.

• Swordfish
• Shark (also known as flake)
• Broadbill
• Marlin
• Orange roughy (also known as deep sea perch)
• Catfish

Those types aside, a number of fish species are low in mercury and can be eaten on a more regular basis. See the table on page ???. These fish can be enjoyed canned, frozen or fresh, as processing and cooking fish does not alter mercury levels.

The truth about tinned fish

Tinned fish is one of the most popular ways to consume fish. It’s an easy, versatile, portable food that can be added to sandwiches, salads and cooked dishes. A full-grown tuna is a large fish, it’s higher up the food chain and therefore likely to contain higher amounts of mercury. However, the fish used for tinned salmon and tuna are much smaller fish, generally less than one year old, and they have not yet accumulated problematic levels of mercury. They are therefore safe to eat on a regular basis. In contrast, tuna steaks and fresh tuna in sushi should be limited to once a week only.

Pregnancy and children

More than other adults, pregnant women need to be particularly careful about their fish intake. The toxic dose of mercury for unborn babies is much lower than for adults. Their smaller bodies mean they are at risk from much smaller doses of mercury. In pregnancy, mercury exposure can affect both the development and functioning of the central nervous system of the foetus.

This concern over the effects of mercury on unborn babies has received widespread media exposure. An oversimplification of the message, however, has led many mothers-to-be to cut fish out of their diet completely. If you are pregnant or breastfeeding, you’re right to be concerned about mercury, but if you exclude fish altogether, you are missing out on two nutrients important to your baby: iodine and omega-3 essential fatty acids. Instead of cutting out fish, it’s better to include fish in your diet more regularly but be careful of the type of fish you choose. Avoid the larger fish and those in the above list.

Eat two to three servings per week of the low-mercury fish listed in the table. As mentioned, fish is also important in the diet of young children. As with mothers-to-be, it’s wise to limit children’s intake of the larger fish. Include 2-3 servings of the low-mercury fish in children’s weekly diet.

Other sources of omega-3s and iodine

While fish is the main source of DHA and EPA omega-3s, there is a third group called alpha-linolenic acid (ALA). These omega-3s can be found in:

• Nuts, especially walnuts, hazelnuts and brazil nuts
• Flaxseeds
• Omega-3 eggs
• Soybeans
• Some margarines
• Vegetables oils such as canola, soybean and flaxseed
• Some green vegetables such as Brussels sprouts, silverbeet, spinach and salad greens

If you don’t eat fish, it’s a good idea to include at least five servings of these foods in your diet each week. If you do eat fish, it’s still advisable to eat these foods regularly to boost your levels of ALA omega-3 polyunsaturated fats.

Low mercury fish

• Mackerel
• Silver warehou
• Atlantic salmon
• Tinned salmon and tuna
• Herrings
• Sardines
• Squid and octopus
• Snapper
• Trout
• Trevally
• Whiting
• Anchovies
• Bream
• Mullet
• Garfish