Environmental factors, including those related to diet, are believed to contribute significantly to the cause of many forms of cancer. Dietary fat intake is among the most widely studied dietary risk factors for breast and prostate cancers.
In recent years, increasing attention has been paid to the intake of specific fats rather than total fat intake, and notable among these have been fish oils. Long-chain EPA and DHA, which are polyunsaturated omega-3 fats contained primarily in fatty fish, have been shown consistently to inhibit the proliferation of breast and prostate cancer cell lines in the test tube and to reduce the risk and progression of these tumors in animal experiments.
Regarding fish consumption, the concentrations of EPA and DHA in fish oil vary between fish species, with relatively high concentrations found in fatty species native to cold waters, such as salmon, mackerel, sardines, and herring, and relatively low concentrations in lean fish, such as sole, halibut, and cod. The interpretation of "total fish consumption" in epidemiologic studies can therefore be problematic, because the absolute and relative amounts of fatty acids reflected in this measure vary greatly among populations.
Many studies examined fish consumption in relation to breast and prostate cancer risk, although only a few accounted for the type of fish consumed or examined the intake of specific marine fatty acids. The studies also varied greatly with respect to important methodologic factors, such as sample size, adjustment for potentially confounding variables, the detail and quality of the dietary assessment, and the duration of follow-up.
In addition, epidemiologic studies to date have not examined intakes of specific fat in relation to endometrial and ovarian cancers. Clinical and experimental studies of these cancers also have been scarce.
How Fish Oil Prevents Cancer
Several mechanisms have been proposed by which the intake of marine fats may lower the risk of cancer. Among the most important of these is the inhibition of eicosanoid biosynthesis from arachidonic acid (AA; 20:4n-6), an omega-6 fat metabolized in the body from linoleic acid. Eicosanoids are a class of compounds derived from polyunsaturated acids and include prostaglandins, hydroxyeicosatetraenoic acids, and leukotrienes.
Prostaglandins are oxygenated, unsaturated cyclic fats that perform a variety of hormone-like actions. Those converted from AA by the cyclooxygenase-2 enzyme, notably prostaglandin E2 (PGE2), have been linked to carcinogenesis in several types of studies:
Tumor cells typically produce large amounts of AA-derived PGE2, which may impede immune system function, possibly through its role in the generation of suppressor T cells. Fish oils inhibit cyclooxygenase-2 and the oxidative metabolism of AA to PGE2.
EPA and DHA also have been shown to inhibit lipoxygenase which metabolizes AA to hydroxyeicosatetraenoic acids and leukotrienes. Hydroxyeicosatetraenoic acid has been linked to the suppression of apoptosis, the stimulation of angiogenesis, stimulation of tumor cell adhesion, and expression of the invasive phenotype.
Lipoxygenase inhibitors have also been discussed recently as a potentially important class of chemopreventive agents.
Eiocosanoids derived from AA also may be involved in other processes related to cancer progression, as well as cancer initiation. These include:
Estrogen can be metabolized along 2 major pathways, to 16-{alpha}-hydroxyestrone or to 2-hydroxyestrone. 16-{alpha}-Hydroxyestrone is considered to be the more biologically active of the 2 estrogen metabolites and has been observed to increase mammary epithelial cell proliferation rates in experimental studies.
In contrast, 2-hydroxyestrone may decrease proliferation and has been associated in some, but not all, studies with reduced breast cancer risk. Thus, "Western" diets that are rich in linoleic acid may decrease the production ratio of 2-hydroxyestrone to 16-{alpha}-hydroxyestrone and thereby increase cancer risk.
Several studies focused specifically on DHA and its role in the development of breast and prostate cancers. For example, DHA may activate peroxisome-proliferator activated receptor-{gamma}, ligands of which have shown antiproliferative effects in vitro on prostate cancer cell lines. DHA also has been shown to improve the response of breast tumors to cytotoxic agents.
Differences in Fish Oil Consumption
Studies of fish oil consumption trends have shown inverse associations between per capita consumption of fish oil and the incidence of and mortality rates from prostate and breast cancer. Moreover, the shift toward a Western diet usually involves a concurrent decrease in omega-3 fat intake and increase in omega-6 fat intake, such as that observed in Japan over the past several decades (with a concurrent rise in breast cancer incidence).
Whereas the intakes of these two classes of fats were, for most of human history, similar in quantity (i.e., an intake ratio near unity), modern diets now heavily favor the intake of omega-6 fats. Indeed, the results of several human and animal studies suggest that reductions in breast cancer and PGE2 biosynthesis can best be achieved with a relatively high intake ratio of omega-3 to omega-6 fats.
Hence, the processes that ultimately modulate the concentration of tumor growth -- enhancing eicosanoids may depend more on the relative concentrations of specific fatty acids in the diet than on their absolute concentrations.
The concentrations of EPA and DHA relative to those of other fats contained in fish vary between species, and relatively high concentrations are found in fatty fish, such as salmon, mackerel, sardines, and herring, species that are generally native to cold waters. Lean fish, which typically are native to warmer waters, tend to have lower concentrations of EPA and DHA and may sometimes have higher concentrations of AA.
For example, a 100-g serving of Pacific herring contains 1.0 g EPA and 0.7 g DHA (19). In contrast, a 100-g serving of haddock contains 0.1 g each of EPA and DHA. Thus, different types of fish may have different effects on processes related to cancer development.
Factors Complicating Proper Interpretation of Fish Oil Clinical Studies
For studies that examined only total fish consumption in relation to cancer risk, assumptions regarding the type of fish consumed (and, therefore, EPA and DHA intake) can be made from the per capita intake of fish oil. For example, total fish consumption in a Scandinavian population might reflect a greater intake of fatty fish than would the same total fish consumption in a population in the United States, because the per capita intake of omega-3 fats and the per capita intake ratio of omega-3 to omega-6 fats in Scandinavia are up to 5- and 10-fold, respectively, those in the United States.
Data from a few experimental studies suggest that the strength of the association with fish oils may be reduced in the presence of high antioxidant intake, because both the former and the latter inhibit the formation of AA-derived peroxidation products. This has been put forth as a potential reason for the largely negative results of studies in the United States, where supplementation with antioxidants is widespread.
However, this explanation is not entirely convincing because the formation of cytotoxic peroxidation products is only one of several mechanisms that may underlie the association between fish oils and cancer risk. Nevertheless, adjustment for dietary antioxidants in ecologic and analytic studies of omega-3 fatty acids to date has been infrequent.
Intake of fish oils also has been observed to inhibit the metastasis of human breast cancer cell lines growing as solid tumors in animal models. Hence, the association between fatty acids and cancer risk may be clarified further through the analysis of epidemiologic data that take into account various follow-up (or induction) periods, that are from studies with repeated assessment of diet during the follow-up period, and that provide information on cancer at various stages of growth and progression.
In conclusion, the development and progression of breast and prostate cancers appear to be affected by processes in which EPA and DHA play important roles.
Given the lack of studies that examined the intake or tissue concentrations of specific fish oils and the fact that most studies of fish consumption did not account for the type of fish consumed, there are still too few data from epidemiologic studies to evaluate the strength, consistency and dose response of the relation between fish oil intake and human cancer.
Although there is ample evidence from test tube and animal studies that these essential fats can inhibit the progression of tumors in various organs, particularly the breast and prostate, the evidence from epidemiologic studies is less clear. Although most of the studies did not shown an association between fish consumption or fish oil intake and the risk of hormone-related cancers, the results of the few studies from populations with a generally high intake of fish oils are encouraging.
American Journal of Clinical Nutrition, Vol. 77, No. 3, 532-543, March 2003
This is one of the best reviews I have seen to date that expands on the specific science of why fish oils are effective in the management and prevention of cancer.
The biochemistry and science in the article is detailed and in-depth, however, considering fish oils are such a major factor in optimizing health, I thought it was important to make an exception and include the article despite its largely scientific nature; the article intelligently and succinctly reviews the hard science behind the reason for fish oil’s benefits.
Always remember omega-3 fats are essential to your, and your children's, health. The best source for omega-3 is fish oil and cod liver oil, as, unlike other sources such as walnuts or flax seeds, it is high in two fatty acids crucial to human health, DHA and EPA. These two fatty acids are pivotal in preventing heart disease, cancer, and many other diseases. The human brain is also highly dependent on DHA - low DHA levels have been linked to depression, schizophrenia, memory loss, and a higher risk of developing Alzheimer's.
It important to realize that spring is around the corner and for many of you that will mean that you will be getting regular sun exposure. If that is true for you it is important to switch from cod liver oil to fish oil at this time of year to prevent vitamin D toxicity.
The best way to know whether your vitamin D levels are within safe limits is to check your blood level of vitamin D, but I know it's likely that many of you will not be doing that. So be on the safe side and switch over to fish oil now unless you know for sure that your vitamin D levels still require some further boosting.
When choosing your fish oil or cod liver oil, it is important to remember that not all brands are the same. In my research--and in my clinical experience with my patients--I have found that 1) the liquid form is superior to capsules; 2) that certain brands definitely seem inferior to others; 3) that the Carlson's brand of fish oil/cod liver oil is of an exceptional quality and purity. It is likely not the only brand in the world of such high quality, but up to this point it is the only one of such quality that I can recommend, so I now offer the Carlson's fish oil and cod liver oil on Mercola.com; you can also check your local health food store to see if they carry it.
As for fish itself, patients consuming nearly every type of fish these days are showing high levels of mercury in their systems. In short--and sadly, as it would otherwise be one of the healthiest meats on the planet--I now advise against consuming any fish from any source, including fresh water, farm-raised or ocean, because most are contaminated with mercury. If you do consume fish, you should be certain the provider can demonstrate that the fish are free of detectable levels of mercury and other toxins (Carlson's fish oil/cod liver oil has been entirely purified so you don't have to worry about these toxins.)
So what is the difference between fish oil and cod liver oil? In short, fish oil should be consumed in warm weather months and warm climates, while cod liver oil should be consumed in cool weather months and cool climates. Cod liver oil is high in vitamin D (and vitamin A); those in cool weather climates do not generally get enough vitamin D. However, in warm weather, their vitamin D is usually sufficient, as sunshine is one of the primary methods of obtaining this vitamin; if you consume too much vitamin D in these warm weather months, you do risk overdosing.
A reasonable dose for both fish oil and cod liver oil is one teaspoon for every 50 pounds of body weight daily.
Generally, our diets contain not only far too little omega-3, but far too many omega-6 fats. Experts looking at the dietary ratio of omega-6 to omega-3 fatty acids suggest that in early human history the ratio was about 1:1. Currently most Americans eat a dietary ratio that falls between 20:1 and 50:1. The optimal ratio is most likely closer to the original ratio of 1:1. For most of us this means not only increasing our omega-3 intake through fish oil, but also greatly reducing the omega-6 fatty acids we consume.
Please recognize that we get ALL the omega-6 (and omega-9) fat we need from food. We do NOT need to take any supplements for these fats. Many of the omega fat supplements you see in health food stores will only serve to worsen your health, not improve it, as they contain omega-6 fats that will only worsen your omega-6 to omega-3 ratio.
I strongly recommend avoiding sunflower, corn, soy, safflower, and canola oil, and products that contain these oils. This also means no hydrogenated or partially hydrogenated fats, no margarine, no vegetable oil and no shortening. These oils are full of omega-6 fats and will only worsen your omega-6 to omega-3 ratio.
Acceptable oils include high-quality extra virgin olive oil, coconut oil, avocados and organic butter... or better yet, grass-fed organic butter.
Another way to improve your omega 6:3 ratio is to change the type of meat you are eating. Since I can't recommend fish due to their high levels of mercury and other toxins these days, the best meats are those raised primarily or entirely on grass diets. Nearly all cattle found in grocery stores and restaurants are grain-fed before slaughter, so if you eat this type of beef, it will typically worsen you omega 6:omega 3 ratio.
Though it may sound unusual to some, two of the best meats on the planet--in terms of nutrition value and their incredible taste--are bison and ostrich. And on Mercola.com, I offer bison meat and our exclusive omega-3-rich ostrich meat, both of which are free-ranged, fed primarily grass/alfalfa diets, and contain no antibiotics or steroids. Whether from my site or elsewhere, I urge you to try bison and ostrich.
You can also consume a more "gamey" meat like venison, or try grass-fed beef, which I also highly recommend on Mercola.com. The grass-fed beef I recommend is even higher in omega-3 than fish, with a 6:3 ratio of 0.16 to 1 (this information is from a study done at Iowa State University in August 2001). It is still hard to find in grocery stores, however, and unlike bison and ostrich--which both taste like prime beef--some find the taste of grass-fed beef too lean.
On a side-note, you should be cautious of stores advertising grass-fed beef that really isn't true grass-fed. ALL cattle are grass-fed to some small extent, but the key is what they are fed the months prior to being processed. Most all cattle are shipped to giant feed lots and fed corn to fatten them up. Your best bet is to call the person who actually grew the beef, NOT the store manager, to find out the truth.
Another effective way to determine if the meat is really from a grass-fed animal is to purchase the ground beef and slowly cook it until it's done, then drain and collect all the fat. Grass-fed beef will contain a relatively small amount of such fat compared to traditionally prepared ground beef (which is why some find the taste of grass-fed too lean!)
In summary:
1) Most Americans, young and old, are highly deficient in omega-3, and one of the best things you can do for yourself and your children is routinely consume fish oil in warm months and cod liver oil in cool months, as they are high in the best kind of omega-3. 2) You should cut out or reduce the oils and foods high in omega-6 fats, as Americans get far too much of them. This includes corn, sunflower, soy, canola and safflower oil, margarine, vegetable oil and shortening. 3) You should eat healthy meats, as store-bought beef and other common meats can worsen your omega-6 to omega-3 ratio. Bison and ostrich are two great-tasting and very healthy choices, and game animals or beef that is truly grass-fed are other good options.
1) Most Americans, young and old, are highly deficient in omega-3, and one of the best things you can do for yourself and your children is routinely consume fish oil in warm months and cod liver oil in cool months, as they are high in the best kind of omega-3.
2) You should cut out or reduce the oils and foods high in omega-6 fats, as Americans get far too much of them. This includes corn, sunflower, soy, canola and safflower oil, margarine, vegetable oil and shortening.
3) You should eat healthy meats, as store-bought beef and other common meats can worsen your omega-6 to omega-3 ratio. Bison and ostrich are two great-tasting and very healthy choices, and game animals or beef that is truly grass-fed are other good options.
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