Seventy five years ago, long chain polyunsaturated oils (PUFA also known as polyunsaturated oils) were added to the list of essential nutrients. Since then there have been many publications reporting low rates of death from coronary heart disease (CHD) among Greenland Eskimos due to fish consumption. Today, fish is considered a health food. Now, we all know the importance of eating fish to obtain the daily and much needed omega-3 oils into our modern day diets not only in the prevention of CHD but also in preventing sudden death from this disease.
The omega-3 polyunsaturated oils: eicosapentaenoic acid (EPA also known as 20:5 omega-3) and docosahexaenoic acid (DHA also known as 22:6 omega-3) found in fish are linked with optimal brain function, cognition, and improved eye and skin health. Some studies, furthermore, show that although some omega-3 oils may be protective against certain cancers and may have a therapeutic effect on depression and specific autoimmune diseases including lupus erythematosis, psoriasis, and arthritis yet other omega-3 oils derived from certain other sources can actually increase the risk of certain types of cancers. This is because the omega 3 oils obtained from vegetable origin (α-linolenic acid) is a precursor of long chain polyunsaturated omega-3 as found in fish and the endogenous conversion of α-linolenic acid in the human body to EPA and DHA is limited and inefficient. Therefore the source of omega-3 fatty acid can influence a disease outcome.
When comparing different types of fish, lower risk appears more strongly related to intake of oily or fatty fish (salmon, herring, sardines, mackerel), rather than lean fish (cod, catfish, halibut). Some common fish that contains high levels of omega-3 per serving of 100g (about a 4 oz.) are:
1. Atlantic Mackerel - 2.45g
2. Chinook Salmon - 2.09g
3. Pacific Sardines - 1.87g
4. Atlantic Herring - 1.67g
5. Tuna - 1.57g
6. European Anchovies - 1.44g
White fish also contains omega-3 fish oils although at much lower levels than the ones mentioned above.
There are many other benefits that can be obtained by eating fish. Fatty fish also contains Vitamin D and selenium. One scientific study suggest that the beneficial effects of omega-3 oils particularly on stroke and CHD, may be due to vitamin D or selenium, or additional components of fish other than omega-3 oils. This study does however, pool the omega 3 oils obtained from vegetable origin (α-linolenic acid) and fish omega-3 oils when doing the comparison between omega-3 with vitamin D or selenium and this may have distorted the final analysis.
Vitamin D has gained increasing recognition for reducing the risk of many chronic and infectious diseases and mortality rates especially from epidemiological studies. Vitamin D inadequacy is common worldwide and classically causes osteomalacia and rickets. More recently, the contribution of low vitamin D status to increased falls and fracture risk has become appreciated. Now, Vitamin D is the current wonder compound. One recent study indicates that high vitamin D status appears to correlate to reduced risk of several cancers, including colorectal cancer. Recently, interest has also extended to studying vitamin D's benefits for the brain. Low vitamin D status is being potentially associated with a multitude of conditions (including Alzheimer disease, osteoarthritis, multiple sclerosis, and hypertension) and higher overall mortality. Consuming fish is inversely correlated with Alzheimer's disease (AD) prevalence and a 2010 review also describes how Vitamin D deficiency may be a cause for autism.
The two major naturally relevant forms of Vitamin D are vitamin D2 (ergocalciferol) and vitamin D3 (cholecalciferol). Synthetic Vitamin D is D2 yet these supplements do not necessarily work in reducing deficiency. It is the naturally occurring vitamin D that is by far the most widely used form for correcting vitamin D deficiency. Research also indicates that omega-3 fats may also be better absorbed from food than supplements. Selenium is also one of those minerals that cannot be added to our food once it has been processed. Taking these into consideration, it would seem that the best way of obtaining omega 3, vitamin D and selenium might be from the natural source especially from fish. This statement is further strengthened by a recent study where low intake of fish, polyunsaturated oils (PUFA) and vitamin D deficiency has been suggested to play a role in the development of schizophrenia. This was a huge study with more than 33,000 subjects (although it was self-assessment questionnaire), so the outcomes are more relevant. Fish is necessary as part of our healthy diet, there is no doubt about that.
Government policy documents and health professional organizations recommend we eat more fish. We should be aiming to eat at least two portions and up to four portions of fish per week, one of which should be oily. Conversely, there are many of us who worry about what else we may be eating with our fish. It is true that most fish will contain some mercury or PCBs or both or other environmental contaminants, just like any of the other food we consume. These contaminants build up in the fatty tissues of fish and other animals and by frequent consumption of this contaminated food they accumulate in your body over time and may result in health problems, ranging from small, hard-to-detect changes to birth defects and cancer. Environmental contaminants such mercury, lead, arsenic, PCBs and dioxins, brominated flame retardants, arsenic, lead, cadmium, and other organic and inorganic contaminants are inadvertently present in a wide variety of our food. These environmental contaminants are discussed more in detail in our book "Is Your Food Killing You?"
Mercury is one of the main concerns of a contaminant found in fish. Mercury is a reactive heavy metal emitted from natural sources (volcanoes) and human sources (coal-fired electric power plants, gold mining, institutional boilers, chlorine production, and waste incineration). From the atmosphere, mercury cycles from rainwater into lakes and oceans, where it is converted by bacteria into methylmercury. Inorganic mercury is poorly absorbed following ingestion, and elemental mercury does not readily cross tissue barriers. In contrast, methylmercury is readily absorbed and actively transported into tissues. Fish therefore absorb methylmercury mostly from their food, but also from the water as it passes over their gills. Concentrations of methylmercury in aquatic species depend on levels of environmental contamination and on the type of fish. Generally, larger, older predators (e.g., swordfish, shark) have higher tissue concentrations, while smaller or younger, shorter lived species (e.g., shellfish, salmon) have very low concentrations. In addition, the large predatory fish near the top of marine food chains are more likely to have high levels of mercury than fish lower in marine food chains due to the process of biomagnifications. Since methylmercury binds to proteins, it is found throughout fish tissue, including muscle tissue that makes up fish steaks and fillets. Fish can also absorb organic chemicals (such as PCBs, dioxins and DDT) from the water, suspended sediments, and their food.
In humans, health effects of very high mercury exposure following occupational or industrial accidents include paresthesias, ataxia, and sensory abnormalities in adults, and delayed cognitive and neuromuscular development following exposure to the unborn child. Health effects of chronic low level mercury exposure seen with fish consumption are less well established. The public is aware of the potential harm from mercury in fish but lacks clear understanding of who is at risk or which seafood species contain mercury. Adult men, and women who are not planning to become pregnant, are less at risk from mercury exposure than children. Methylmercury crosses the placenta, and foetal exposure correlates with maternal exposure. Methylmercury is a neurotoxicant and high exposures over long periods of time may adversely affect the developing foetal brain and central nervous system. It can take 5 years or more for women in their childbearing years to rid their bodies of PCBs, and 12-18 months to significantly reduce their mercury levels.
Recent work shows public confusion about the benefits of fish, challenged by the evidence of mercury and other contaminants in fish, resulting in controversy and confusion over the role of fish consumption in a healthy diet. The low levels of pollutants in oily fish that can build up in our bodies, pregnant and breastfeeding women, girls and women who one day may have a baby are advised not to eat more than two portions of oily fish a week. Mothers who consume contaminated fish before becoming pregnant are said to have children who are slower to develop and learn yet the fact is, that high maternal seafood consumption of the type known to be contaminated with mercury, has been associated with fewer - not more - autistic markers in the offspring. With the bulk of messages about fish consumption focused on risk, the benefits may be lost to consumers. Yet no such warning is given to these vulnerable groups when contaminants such as mercury are found in high fructose corn syrup (HFCS), which is used as a sugar additive in a wide variety of the processed foods we consume.
One study done in 2006, gives a good overview of the levels of omega-3 oils and contaminants in commonly consumed fish, shellfish, and other foods (see Table 2). From this it is obvious that some fishes have more contaminants than others and this will also depend not just on the type of fish but also where it has been raised or farmed. Some fish and vegetable oils have more omega-6 (Linoleic acid) polyunsaturated fatty acid. For instance, tilapia has a much higher content of omega-6 than omega-3 oils yet most of us will think we are getting a good source of omega-3 by eating tilapia! Although omega-6 oils are required by the body, current evidences suggest that it is the ratio of omega-3 to omega-6 is that is crucial in preventing disease and cancer.
Our modern day diet consists of too much of the omega-6 oils even in foods that we may think are natural. Take something as simple as eggs: feeding hen with one type of feed may produce eggs with a particular ratio of omega-6: omega-3. Supermarket eggs have a 20:1 ratio whereas range-fed eggs have a 1.5:1 ratio of omega-6: omega-3. When you see eggs or any other products on supermarket shelves that state these have been enriched with omega-3, all it means is that there is slightly more omega-3 in these products but there is still a high amount of omega-6! Meat from grass-fed animals has two to four times more omega-3 oils than meat from grain- fed animals. One recent publication, showed that a 35% fat diet with an omega-6: omega-3 ratio of 28:1 caused obesity that progressively increased over four generations of mice. Although most studies like this have their own flaws, they nevertheless together suggest that an imbalance of these PUFA is capable of causing obesity in mice that worsens over several generations.
The half-life of omega-6 in fat tissue is about two years, so reducing it is a long-term prospect. Our advice has been to eat fish to obtain the essential omega-3 oils. Also, consider choosing a wider variety as this will not only help maintain a balanced omega-6: omega-3 ratio but will help reduce the environmental impact. Young children are advised to avoid some types of fish, such as shark, swordfish, king mackerel, or tilefish, and to eat up to 12 ounces per week of fish and shellfish that are lower in mercury, such as shrimp, canned light tuna, salmon, pollock, and catfish.
The list below shows the amount of various types of fish that a woman who is pregnant or planning to become pregnant can safely eat. Small children should reduce portion sizes.
Fish that contains least mercury (less than 0.09 parts per million) are:
Anchovies
Herring
Mackerel (North Atlantic, Chub)
Salmon (Fresh wild)
Sardines
Depending on where you live, you can enjoy at least one of these omega-3 rich fish at least 2 times a week. If you live in the Atlantic, the Atlantic Mackerel would be the best choice as it provides the highest omega-3 fish oil and the lowest mercury levels and hardly any PCBs. Wild Alaskan Salmon has hardly any mercury but has higher levels of PCBs.
Fish with high amount of mercury (0.3 to 0.49 parts per million) are:
Mackerel (Spanish, Gulf)
Tuna (Canned Albacore)
Tuna (Yellowfin)
Of these fish, eat three servings or less per month
If trying to avoid contaminants from fish, then avoid these fish: (mercury levels of more than 0.5 parts per million)
Mackerel (King)
Tuna (Bigeye, Ahi)*
Farmed salmon
A more in-depth analysis of mercury and other contaminants in fish is available here. Also remember that cooking can alter the omega-3 content in your fish depending on how which method you choose to cook your fish. More about the cooking process is covered in our book.
In conclusion, if you are worried about mercury and PCB or other contaminants, it is not just from the fish source that you should be worried about. Mercury (along with aluminium and residual formaldehyde, all of which are nasty compounds) can be found in vaccines such as in tetanus and diphtheria toxoids. In our article entitled "Environmental exposures and Cancer risk", we show that environmental pollutions can be found in the foods we consume daily! Reduce the intake of these foods instead if you are worried about eating contaminated foods! Gain the benefits of omega 3 from fish by eating the right kinds of fish and cooking it correctly.
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