Fish is a fantastic healthy source of proteins and fats. On the other hand, mercury in fish is toxic and can cause serious health problems. This article explores how mercury enters the food chain, why it is such a problem once it is there, and the considerations you should make when choosing whether to eat fish.
To get the scoop on our top tips, you can jump forward to see how to keep your mercury from fish consumption at safe levels.
What is mercury and how does it get into the food chain?
Mercury in fish is the result of natural processes. Mercury is a heavy metal element which is naturally occurring in soil, water and air. It can be released into the environment by industrial processes such as mining and burning coal, or natural events like eruptions. It exists in three different forms: elemental, organic and inorganic. Elemental mercury is a silvery metal which is liquid at room temperature and used in old thermometers. But, it is the organic form of mercury called Methylmercury (MeHg) which is a cause for concern in fish.
Seawater contains methylmercury in low concentrations. Tiny algae organisms called phytoplankton absorb methylmercury and are eaten by small fish. Small fish are eaten by larger fish. At each stage the mercury in fish is retained because small amounts of the toxic mercury are stored in body tissues and not excreted.
Why is mercury contamination such a problem?
Over time, the small repeated exposures to mercury can increase the concentration in the smaller fish. A process called biomagnification occurs where this increase in mercury continues up the food chain. Further up the food chain, larger predatory fish can contain mercury concentrations 10 times higher than the fish that they consume. Larger, longer lived fish like shark, marlin or tuna contain more mercury than small fish and seafood.
Humans are particularly susceptible to methylmercury because it can cross both the blood-brain barrier and the placenta. Mercury toxicity is associated with kidney problems, neurological issues such as memory loss and lack of concentration, and there is increasing evidence that it is connected to high blood pressure and heart attacks. Because it can pass through the placenta and in breastmilk, it can be particularly harmful to pregnant or breastfeeding women and their children.
What are the personal considerations when deciding whether to eat fish?
Mercury in fish accumulates in the meat of the fish and so it cannot be trimmed off or discarded with the skin or other waste parts. It is unseen and has no smell, so you cannot tell how much methylmercury is contained in a fish just by looking at it. However, for most people, the benefits of eating fish outweigh the negatives provided that they make informed choices.
Fish are an important source of omega-3 fatty acids and other healthy proteins and fats. In the UK, the Scientific Advisory Committee on Nutrition actually advises that most people do not eat enough fish, particularly oily fish, and should be encouraged to increase their weekly intake. Their recommendation is that people should eat at least two portions of fish a week, one of which one should be oily.
Because the mercury concentrations in larger fish are much higher it is recommended that adults should eat no more than one 140g portion of shark, marlin or swordfish a week to keep within the protective guideline of 3.3 micrograms per kilogram of body weight per week (µg/kg bw/week). The mercury concentration in tuna, while lower than that of shark, marlin or swordfish is higher than that in other fish like cod or salmon so the recommendation is that you eat no more than two 140g portions of fresh tuna, or four 140g portions of canned tuna, per week.
For women that are pregnant or planning to get pregnant within the next year the recommended maximum mercury consumption reduces to 1.6 µg/kg bw/week. This is because mercury can harm the development of a foetus.
How to keep your mercury from fish consumption at safe levels:
Eat 2–3 servings (227–340 grams) of a variety of fish every week.
Choose lower-mercury fish and seafood, such as salmon, shrimp, cod, and sardines.
Avoid higher-mercury fish, such as shark, marlin and swordfish. Eat them no more than once a week and avoid completely if you are pregnant or planning to become so.
Following these tips will help you to reap the benefits of eating fish, while keeping your mercury exposure below the recommended levels.
What does this mean for hospitality and catering?
Mercury is a form of chemical contamination in food, but unfortunately one that cannot be removed in the kitchen. You may want to consider sourcing fish with lower mercury concentrations such as freshwater salmon, or ensure that you vary the species of fish available so that it is not always shark, marlin or swordfish on the menu. You could include some information about mercury toxicity in your menu to help people make informed choices about what they eat.
If you found this article about mercury in fish useful, you might be interested in finding out about other forms of contamination in food and how they can be prevented. iQualifi’s online level 2 in Food Safety and Hygiene will help you understand how food can become contaminated with allergens, bacteria or chemical contamination and help you understand how to minimise the risks to health.