The Waiting Room
Eating fish: a closer look at health effects
Tracy Morton MD
Fish form an integral part of life on Haida Gwaii. It is food, culture and economy for us and for all the people living on the coast. Because it is such a central part of our diets, the effects of frequent fish consumption deserve a close look. This article examines the nutritional aspects and health effects of eating fish, focusing on the fish available to the people of BC’s North Coast.
From a health perspective, fish is sexy food. Public and health professionals alike are inundated with articles about the benefits of eating fish. Relative to other meats, it is low in fat and calories. It contains lots of protein and is low in carbohydrates. The fats it does contain are especially healthy: low in saturated fats and cholesterol, and high in the superstar fat, the omega 3 fatty acids. More and more medical organizations, from the American Heart Association to Health Canada, recommend we eat at least two servings of fish per week.
Let’s look at the benefits in more detail. As discussed in previous articles, fats are a critically important part of the food we eat. Caring for one’s health includes paying attention to the types and amounts of fats we eat and learning something about what different fats do in the body. The healthiest of all fats is the omega 3 fats. Hundreds of studies show that diets high in omega 3s improve cholesterol and reduce rates of heart disease and stroke. They are nature’s “anti-inflammatories.” They reduce the risk of heart arrhythmias. Consuming foods with abundant omega 3s has been shown to reduce the severity of autoimmune diseases and allergies They have been shown to improve many mental health conditions, with the best evidence for the treatment of attention-deficit disorder.
The Western diet is typically poor in omega 3s and higher in the other types of fats, such as saturated fats—the type found in red meat and dairy products. There are both plant and fish sources of omega 3s. There are also many products that have omega 3s added, such as margarine. It turns out that the two omega 3s found in fish (abbreviated DHA and EPA) are more healthy than the one found in plants (ALA). In fact, for ALA to have beneficial effects, it must be converted to DHA, a step which is slow and inefficient for our metabolism, and which converts only 10 percent of ALA to DHA. Consuming fish provides biologically active omega 3s directly without the need for conversion.
There is no universal consensus on how much omega 3s we should consume daily. The Dieticians of Canada recommend 1000-1600mg of ALA or 100-160mg of DHA and/or EPA. To achieve a therapeutic effect, amounts ranging from 1000-4000mg of DHA+EPA are recommended, and it is next to impossible to consume the food containing 10 times that amount of ALA. Many people take omega 3 supplements to achieve this intake. A tablespoon of cod liver oil contains as much omega 3s as a three-ounce serving of salmon—about 2500mg. Other oily fish such as herring, anchovies and oolichan are concentrated sources of omega 3s.
Milligrams omega 3 (DHA+EPA) per 3 oz of some fish*
Salmon: 1100–1900 Mg
Halibut: 600-1120 Mg
Shrimp/prawns: 325 Mg
Cod: 150-240 Mg
Catfish: 220-300 Mg
Flounder: 960 Mg
Tuna: 440-1100 Mg
Red snapper: 290 Mg
Shark: 830 Mg
Swordfish: 970 Mg
Pollock: 450 Mg
King mackerel: 360 Mg
*equivalent to half the amount in one store-bought tin
Mercury in fish
Balanced against the omega 3 benefits that fish provides is the concern about the mercury content of fish. Mercury is a heavy metal found throughout our environment. Half is produced naturally through geological processes such as volcanic activity. The other half is produced through industrial activities including mining, coal-fired power plants and cement production.
When we think of mercury, we imagine the silvery substance found in old thermometers or in tooth fillings. This is the metal form of mercury, and in fact it is not that toxic (i.e. most experts do not feel that mercury amalgam fillings pose a health risk). Organic mercury is the much more toxic form of mercury, and it is the type found in fish. For those of us not working in one of the above industries, the majority of mercury in our bodies is there through the consumption of fish. All animals accumulate mercury from their diet and eliminate it at a certain rate. Fish are very slow at metabolising and excreting mercury, so mercury tends to build up in the fat stores as the fish ages. Thus, the longest living fish contain the highest levels of mercury. These include halibut, sturgeon, and large predatory fish like tuna, shark and swordfish. For freshwater fish there is a great deal of variability in the amount of mercury, as lakes and rivers may have markedly different amounts of dissolved mercury.
At high levels such as those following industrial accidents, mercury causes nerve and brain damage, skin rashes, kidney damage, vision loss and hallucinations. The expression “mad as a hatter” comes from mercury exposure in felt hat-makers in the 1800s, causing delusions and hallucinations. It is possible to experience some of these symptoms from heavy fish consumption of the species above. Though our bodies to have a mechanism for eliminating mercury, it is possible to accumulate mercury if one eats these fish on a frequent basis, say large servings several times per week.The biggest concern with mercury toxicity is with exposure during pregnancy. The developing baby is especially vulnerable to much lower levels, leading to the recommendations that the consumption of these fish be limited to just two or three meals per month.
How about salmon? Shellfish? Fortunately, these fish are low enough in mercury that they could be consumed daily with no long term concerns about toxicity.
Overall, the net benefits of consuming fish outweigh the concerns of mercury exposure, unless one has a particular liking for halibut or is frequently consuming fish caught near a mine or coal-fired power plant. If it is accessible, consider replacing red meat with more fish in your diet.
Regular columnist Julie Chaplin, has taken a break this issue to welcome a new baby to her family. Congratulations Julie!
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