Contaminated fish are a danger to health

All of the world's oceans and rivers are polluted with toxic chemicals; mainly pollutants resulting from decades of increasing industrial activity. The health risks associated with the consumption of fish and fish oil supplements are due to the presence of these toxic pollutants, not to mention the saturated fat, animal protein and cholesterol they contain. Oily fish such as salmon, trout and mackerel may be contaminated with chemicals, while shark, marlin and swordfish contain high levels of mercury (Consumers' Association, 2002). Long-term exposure to these harmful substances poses a significant threat to health.  

POP goes the seawall:

A particularly nasty class of contaminants found in oily fish is known as persistent organic pollutants (POPs). These organic pollutants are made up of complex molecules which do not break down easily in the environment; hence the name 'persistent'. They build up in the food chain through bioaccumulation. This means that the level of chemicals found in an animal tissue rises the higher up the food chain the animal is. For example, larvae might eat some contaminated algae, a fish then eats the larvae, and a bigger fish eats smaller fish and so on. Fish at the top of the food chain such as tuna, shark, swordfish, tilefish and king mackerel, have the highest concentrations of POPs. Algae (which are right at the bottom of the food chain) are likely to contain much lower levels.

Farmed salmon are even more of a concern than wild salmon due to the concentration of POPs in the feed they eat and their fatty nature as POPs can accumulate in fatty tissue.

In 2001, officials from 90 countries signed a landmark UN treaty in Stockholm, designed to curb the use of some of the world's most dangerous and toxic chemicals (Stockholm Convention, 2008). They identified twelve particular substances for banning which later became known as the 'dirty dozen'. The list included eight organo-chlorine pesticides: aldrin, chlordane, DDT, dieldrin, endrin, heptachlor, mirex and toxaphene; two industrial chemicals: hexachlorobenzene (HCB) and polychlorinated biphenyls (PCBs); and two groups of industrial by-products: dioxins and furans.

The Stockholm Convention on POPs came into force on May 17, 2004, with agreement from an initial 128 parties and 151 signatories. Co-signatories agreed to ban nine of the 'dirty dozen,' limit the use of DDT to malaria control and curtail production of dioxins and furans. Several other substances are now being considered for inclusion in the Convention. These are: hexabromobiphenyl, octaBDE, pentaBDE, pentachlorobenzene, short-chained chlorinated paraffins, lindane, a- and ß-hexachlorocyclohexane, dicofol, endosulfan, chlordecone and PFOS. Not what you'd expect to see in a list of ingredients!

However, even though this is a step in the right direction, it will take many years for POPs (used in everyday items such as plastics, paints and pesticides) to break down. Furthermore, the environmental group, the World Wildlife Fund has expressed concerns that global warming could increase levels of POPs as higher temperatures may release chemicals which have been frozen in glaciers.


Polychlorinated biphenyls (PCBs) and dioxins are the main contaminants in oily fish (Consumers' Association, 2002). Although they are now banned, PCBs have not disappeared from the environment and won't for a very long time.

The Scientific Advisory Committee on Nutrition (SACN) is an advisory committee of independent experts that provides advice to the FSA, Department of Health and other government agencies and departments. SACN states that: "PCBs are very persistent chemicals which are ubiquitous in the environment and are generally present in low concentrations in foods, especially fat-containing foods including milk, meat and fish," (SACN, 2004).

As persistent lipophilic (fat-loving) compounds that accumulate in lipids, PCBs are particularly likely to be present in oily fish. They are implicated in heart disease, cancer, infertility and can harm developing foetuses by disrupting the development of male babies' reproductive organs.  


Dioxins were the primary ingredient in Agent Orange, the defoliant sprayed over Vietnam which has produced an appalling legacy of cancers and other diseases, miscarriages and birth defects. As stated, these insidious toxins build up in the body over time. The FSA points out that: "If a woman changes her diet when she becomes pregnant, or when she starts trying for a baby, this won't change the levels of dioxins and PCBs that are already in her body. So it's a good idea to limit the amount of oily fish eaten from a young age."

Furthermore, contrary to previous advice that these toxic pollutants are only found in oily fish, the FSA states that: "Recent surveys have shown that some other fish and crab may also have similar levels of dioxins and PCBs as oily fish. These fish are: sea bream, turbot, halibut, dog fish or huss, and sea bass." Now they tell us!

Mercury rising:

Methylmercury is the organic form of mercury found in the environment, usually as the result of industrial pollution. This toxin is commonly found in the marine environment and accumulates up the food chain particularly in older, larger fish. Mercury affects the kidneys, heart and central nervous system (CNS). Exposure is particularly hazardous for unborn children where the main organs and the CNS are still developing. Women and girls who may have children one day are advised to eat less oily fish than men and older women because mercury can harm the development of unborn children. The FSA states that adults should have no more than one portion of swordfish, shark or marlin a week because these fish contain high levels of mercury (FSA, 2009).

While some studies indicate that high intakes of oily fish, or fish oil supplements, are associated with a reduced risk of CVD, others have suggested that fish oils can increase the risk. In these studies, mercury is suspected as being responsible for the harmful effects seen as it can accelerate atherosclerosis and increase the risk of heart attack. Possible mechanisms by which mercury exerts these damaging effects include promoting the peroxidation of unsaturated fatty acids (Salonen et al., 1995). This is the process whereby damaging molecules called free radicals 'steal' electrons from the fats in cell membranes, resulting in cell damage.  

Mercury also forms an insoluble complex with the antioxidant mineral selenium. It binds it in an inactive form preventing selenium from acting as a cofactor for the protective enzyme glutathione peroxidase. This enzyme's main biological role is to protect the body from oxidative damage. Binding selenium in this way can lead to an increase in free radical activity. Mercury also has a very high affinity to sulfhydryl groups which play an important role in antioxidant reactions. Sulfhydryl groups are compounds that contain a functional group composed of a sulphur atom and a hydrogen atom (-SH). They feature as a functional group of the amino acid cysteine and so play an important role in many biological systems. By binding to sulfhydryl groups, mercury inactivates antioxidative compounds, again, increasing free radical activity. Taken together, these reactions indicate how mercury could inhibit important antioxidative mechanisms in humans. In other words, the high mercury content in fish can not only counteract the protective effects of the fatty acids found in oily fish, but may go on and do serious harm to health.

For non-oily fish, there seems even less reason to take the risk of exposing yourself to mercury. Numerous studies have reported an association between fish intake and coronary mortality. In 1995, one study investigated why men in Eastern Finland have an exceptionally high level of mortality from CVD (Salonen et al., 1995). This study measured hair mercury content in healthy individuals and then recorded CVD events (such as heart attacks) over the following years.

The mercury content in Finnish lakes is known to be relatively high; especially so in ground waters (water that supplies wells and springs) in Eastern Finland. This study found that an average daily fish intake of just 30 grams or more was associated with a two-fold increase in heart attack risk. For each additional 10 grams of fish per day there was an increment of five per cent in the five-year risk of heart attack. This indicates that mercury, even in subtoxic amounts, is a risk factor for coronary and fatal CVD. The authors concluded that because in many countries fish and seafood are contaminated with mercury and because fish is eaten all over the world, their findings could be of enormous public health importance.

A later study (from the same group) investigating the association between blood levels of the omega-3 fats EPA and DHA, and the risk of acute coronary events in middle-aged men from Eastern Finland, again looked at mercury levels in hair (Rissanen et al., 2000). This prospective population-based study of 1,871 men confirmed that high levels of DHA in the blood were associated with a decreased risk of acute coronary events. However, the men who fared best in this comparison were those with high DHA levels and low mercury content in the hair. It was concluded again that mercury could cancel out any protective effect offered by the fats in oily fish.  

A case-controlled study published in the New England Journal of Medicine in 2002 conducted in eight European countries and Israel provided further evidence linking mercury in fish to CVD. This study investigated the relationship between mercury levels in toenail clippings and DHA levels in adipose tissue with the risk of a first heart attack among men (Guallar et al., 2002). This study included 684 men in the coronary care units of participating hospitals who had experienced their first heart attack. Also, 724 men representative of the same population (but who had not had a heart attack) were selected to act as controls. Results showed that while higher DHA levels were associated with a lower heart attack risk, the toenail mercury level was directly associated with an increased risk of heart attack. The authors concluded that mercury may diminish the cardioprotective effect of fish intake.

They also warned that exposure to mercury is already a concern in specific high risk groups and suggested extending the US Food and Drug Administration's recommendation that pregnant women and women who may become pregnant do not eat swordfish, king mackerel, tilefish or tuna to include the entire population.  

A statement from the American Heart Association (AHA) acknowledges the discrepancy in the findings of research on fish oils and CVD (Kris-Etherton et al., 2002). They acknowledge that the contradictory findings of these studies could be explained by the adverse effects of mercury, which they say may diminish the health benefits of omega-3 fatty acids from fish. They conclude that while collectively, the research is supportive of the recommendation to include at least two weekly servings of fish (particularly fatty fish), this recommendation must be balanced with concerns about environmental pollutants, in particular PCB and methylmercury. This ambiguity can only add to the confusion!

They do however point out that the research supports the inclusion of vegetable oils (soya bean, rapeseed, walnut and flaxseed oils) and food sources (walnuts and flaxseeds) high in ALA in a healthy diet for the general population. This, at least, is a move in the right direction.  

Food Poisoning:

Food poisoning generally happens when someone eats food that is contaminated with bacteria or its toxins. According to the FSA, it is estimated that up to 5.5 million people in the UK are affected by food poisoning each year (NHS Direct, 2008d). However, it is difficult to know exactly how many people are affected as not all of those affected seek medical advice.

Most cases of food poisoning result from eating contaminated animal products (meat, poultry, eggs, dairy, fish and shellfish) as plants tend not to harbour the types of bacteria capable of causing food poisoning in humans. If plant foods do cause food poisoning it is generally because they have been contaminated with animal excreta, human sewerage or prepared with dirty hands.

While most food poisoning incidents are associated with meat and poultry, the links between fish, shellfish and food poisoning should not be discounted. Researchers from the Health Protection Agency Centre for infections in London looked at a number of cases of food poisoning in the UK between 1992 and 2003 (O'Brien et al., 2006). Of the 1,763 cases reported to national surveillance, only 55 were published in the peer-reviewed literature. As this type of publication tends to favour the unusual or novel event, coverage can lead to an over-emphasis of one food group over another. This has lead to a failure to pay enough attention to the role of contaminated fish and shellfish in food poisoning. The authors of this study suggest that relying solely on the literature has lead to an over-emphasis on controlling the milk supply and a failure to pay enough attention to the role of contaminated fish and shellfish in food poisoning.

When they are raw or haven't been cooked properly, fish and shellfish can contain a range of harmful viruses and bacteria. Shellfish (such as clams, mussels and oysters) are also filter feeders, constantly filtering water, which can lead to the accumulation of pollutants. The FSA recommends that elderly people and people who are unwell might want to avoid eating raw shellfish to reduce their risk of food poisoning (FSA, 2008d). Most shellfish is eaten cooked, but oysters are often served raw and can contain a virus called the Norovirus. Raw or partially cooked shellfish can also contain hepatitis A, which is a highly contagious virus of the liver.

For these reasons, the VVF argues that sea vegetables (seaweed) are the true 'fruits de mer' or fruits of the sea, not shellfish. Not only do they add flavour to food, each variety is a low-fat nutritional powerhouse, packed full of vitamins and minerals. Plus, you won't find elevated levels of dioxins, PCBs or mercury in sea vegetables, in contrast with fish and shellfish in whose body fats these pollutants accumulate over time.

The FSA's website provides an extensive list of precautionary measures people should take when buying, storing and preparing fish. It lists 18 different ways you can minimise the risk of being poisoned (FSA, 2009). Of course just avoiding fish and shellfish would be much easier. Indeed excluding all animal foods from the diet will dramatically decrease the risk of food poisoning.  

Fish and PhIPs:

Most people think the fool-proof way to avoid food poisoning from meat and fish is to cook it very well - some people practically cremate it to make sure. However, thoroughly cooking meat and fish can lead to even more serious problems.

Cancer-causing compounds called heterocyclic amines (HCAs) are formed when meats (such as beef, chicken and pork) and fish are cooked at high temperatures (Wilson et al., 2007). HCAs form when a chemical found in muscles called creatine reacts with amino acids (the building blocks of protein) and sugar at high temperatures. While meat and fish contain significant amounts of creatine, vegetables contain none. 

In 2005, the US federal government officially added HCAs to its list of known carcinogens (US Department of Health and Human Services, 2008). The most abundant HCA found in the human diet is called 2-amino-1-methyl-6-phenylimidazo[4,5-b] pyridine (PhIP). PhIP is associated with an increased risk of cancer of the colon, breast and prostate (Wilson et al., 2007).

Exactly how PhIP increases cancer risk is uncertain. It has been suggested that PhIP produces harmful molecules called reactive oxygen species which cause DNA strands to break thus increasing cancer risk (Wilson et al., 2007). Alternatively, researchers at Imperial College in London have reported that PhIP is a potent mimic of the hormone oestrogen (Lauber et al., 2007). This is very worrying, as many studies link higher oestrogen levels to breast cancer. This theory provides a plausible explanation for why diets rich in red meats are linked to an increased risk of breast cancer.

In 2005, the Physician's Committee for Responsible Medicine's Cancer Project revealed that grilled salmon was among the five worst foods to grill (PCRM, 2005). The Cancer Project nutritionists found that many commonly grilled foods contain alarmingly high levels of HCAs. The five foods containing the highest levels included: chicken breasts, steak, pork, salmon and hamburgers.

The amount of PhIP in some foods may typically lie in the region of 35 nanograms per gram. However, there are some reports of much higher levels of PhIP (Skog, 2002). Concentrations of PhIP in fish vary widely according to the type of fish and method of cooking. One study reported levels ranging from 1.7 to 73 nanograms per gram in salmon cooked at 200°C by various methods (Gross and Grüter, 1992).

The damaging action of HCAs may be reduced by foods containing antioxidants, such as vegetables and soya foods (Weisburger, 2002). However, the safest option is to avoid meat and fish altogether. Healthier options for grilling and frying include soya burgers, veggie sausages and Portabella mushrooms.