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Will too much fish kill you?

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Aaron Deere is a sports nutritionist, functional medicine consultant and advanced personal trainer. He is based in London.

Omega-3 is an essential fatty acid, which means that it can’t be synthesised in our bodies so we need to consume them from our diet. They are important for normal metabolic function, and research suggests adequate consumption provides a whole host of other health benefits, including reducing the risk of cardiovascular disease, certain cancers, mental health disorders and inflammation.

There are six types of omega-3, of which three are important to humans. Eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), which are found in fish, are the two compounds purported to have the greatest health benefits. The other, alpha-linolenic acid (ALA) which is found in plant oils, don’t deliver the same beneficial effects on many health problems, according to epidemiological evidence.

The consumption of fish is a key component of a balanced diet, and the current UK recommendation is two portions per week, one white and one oily.  This target would provide us with approximately 0.45g/day of omega-3 fatty acids[1].

Surprisingly, despite this modest recommendation – and the fact that the UK an island – the current average for fish consumption sits well under this target, at approximately a third of a portion per week[2].

Heavy metal
We know that eating fish is the best way of providing our bodies with the omega-3 it needs to achieve optimal health. So why is the recommended weekly target of fish consumption so low considering the multitude of reported health benefits associated with this essential fatty acid?

The tissues of many species of fish contain elevated metal concentrations exceeding the nationally- and internationally-agreed quality standards for fish and meat, so eating high amounts of fish can potentially lead to increased consumption of these heavy metals that negatively impact health. The concentration of metal in fish occurs from contaminated suspended matter in the water and sediments, and from organisms that fish eat.

The uptake of metals also plays a role in the reduction of species diversity, because metal-tolerant species become dominant thanks to their ability to tolerate excessive accumulated amounts of heavy metals in their bodies[3].

Mercury rising
Mercury and arsenic are two toxic compounds that have been shown to bio-accumulate within certain species of fish. These elements have no beneficial effects in humans, and there is no known homeostatic mechanisms related to them. With the exception of occupational exposure, fish are acknowledged to be the single largest contributor of mercury in humans[4].

Mercury has been shown to accumulate with fish age and in relation to the position within the food chain. This leads to higher mercury concentrations in older, fatty predatory species such as tuna, halibut, redfish, shark and swordfish.

Tuna in particular have the ability to concentrate large amounts of mercury, with a majority of it stored in internal organs and muscle[5].

The negative impact of elevated mercury levels are many and numerous. It has been found to play a key role in various disorders including neurological, immunological, motor, reproductive and even genetic. Recently it has also been linked to diseases such as Alzheimer’s, Parkinson’s and autism[6].

Plenty of fish in the sea
The levels of heavy metals present in some fish makes it difficult to determine fish consumption as part of a healthy, balanced diet. The potential for harm from these heavy metals suggests eating less fish known to accumulate mercury, and eating a wide range of safer species.

Without being a complete substitute for fish in the diet, fish oil supplementation may offer a safe method of ensuring adequate omega-3 fatty acid intake. As methyl mercury is water-soluble, mercury and other heavy metals stay behind in the liver, flesh, intestines and other parts that are left behind, when oil is extracted from the fish[7]. An independent analysis of 16 different fish oil products validated the process, with no significant contamination by heavy metals found in any of the products tested[8].

Within the body DHA is found in the highest quantities compared to other omega-3 fats, with most organs containing up to 30 times that of EPA. Plasma DHA saturation has been shown to occur at approximately 2g per day, with a large number of studies suggesting this figure as a safe amount as a daily supplement.

When choosing a fish oil supplement it may be difficult to identify the better brands, and which have higher levels of EPA and DHA, or are free from potential contaminants. The International Fish Oil Standard (IFOS) is a third-party independent laboratory that tests products to establish a rating and posts all results on its website.

Don’t be lured by the term ’pharmaceutical grade’ on any fish oil supplement. Manufactures often advertise their product with this phrase but no formal FDA definition exists. This is because there must first be an established pharmaceutical standard against which the substance can be measured, and this does not exist for fish oil. So this phrase is used to describe the strength of the product, not the quality.

In for the krill
An alternative omega-3 supplement to fish oil is krill oil. It contains a lower concentration of EPA and DHA than fish oil, but research suggests comparable findings on health markers such as inflammation and oxidative stress. One issue with krill is that it contains enzymes within their digestive tracts that cause them to deteriorate rapidly once caught, so processing must occur at sea and which comes with potential logistical and financial implications. There have also been potential ethical concerns related to the harvesting of krill because of their position as a primary food source in the aquatic food chain.

1,2 Scientific Advisory Committee on Nutrition (2004) Advice on Fish Consumption: Benefits and Risks. London, FSA
3  R. Dallinger, F. Prosi, H. Segner, H. Back (1987) Contaminated food and uptake of heavy metals by fish: a review and a proposal for further research.Oecologia. Volume 73, Issue 1, pp 91-98
4 D. Bahnick, C. Sauer (1994) A National Study of Mercury Contamination of Fish, Chemosphere, 29, pp. 537–546.
5 Oehlenschläger, J., (2002): Identifying heavy metals in fish. In: H.A.Bremner (ed.): Safety and quality issues in fish processing. 95-113
6 F. Zahira (2005) Low dose mercury toxicity and human health. Environmental Toxicology and Pharmacology. Volume 20, Issue 2, pp351–360.
7  Harris, W (2004) Fish oil supplementation: Evidence for health benefits. Cleveland Clinic Journal of Medicine vol. 71 3 208-210
8 Consumer Reports, 2003

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