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Other common
name(s): fish oil, fish oil supplements, marine oil, cod
liver oil
Scientific/medical
name(s): alpha-linolenic acid, eicosapentaenoic acid, and
docosahexaenoic acid; also caalled n-3 fatty acids or n-3
polyunsaturated fatty acids
Description
Omega-3 fatty acids are important nutrients that are involved
in many bodily processes. The body cannot make these fatty acids and
must obtain them from food sources or from supplements. Three fatty
acids compose the omega-3 family: alpha-linolenic acid,
eicosapentaenoic acid, and docosahexaenoic acid. Alpha-linolenic acid
(ALA) is found in English walnuts, in some types of beans, and in
canola, soybean, flaxseed/linseed, and olive oils. The other 2,
eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), are found
in fish, including fish oil and supplements.
Overview
Some studies in animals have found that fish oils rich in
omega-3 fatty acids suppress the formation and growth of some types of
cancer. Studies in humans have produced conflicting results. A recent
re-analysis of 40 years of research suggests that omega-3 fatty acid
supplements do not reduce cancer risk. Evidence is mixed as to whether
fish oil supplements improve cancer-related weight loss.
Studies have shown that the fatty acids in fish oil help
protect against heart disease and reduce risk factors such as
triglycerides. In one study, fish oils appeared to help improve heart
rhythm problems that can cause sudden death. However, they may also
increase cholesterol and reduce blood clotting. The omega-3 fatty acid
from vegetable oils (ALA) has not shown as strong an effect in studies
to date, although it may reduce risk of fatal heart disease.
How is it promoted for use?
Some people believe that omega-3 fatty acids protect against
the spread of solid-tumor cancers (those that form solid masses) that
are related to hormone production, particularly breast cancer. Some
also believe that omega-3 fatty acids inhibit the growth of colon,
pancreatic, and prostate cancer. Some people and groups advocate use of
omega-3 fatty acids to protect against cardiovascular disease and fatal
heart attacks. Others believe that omega-3 fatty acids help rheumatoid
arthritis, Crohn's disease, eczema, asthma, kidney failure, depression,
and more.
What does it involve?
Oils from some cold-water fish such as sardines, salmon,
herring, mackerel, halibut, striped bass, tuna, shark, and cod have
high concentrations of the omega-3s fatty acids DHA and EPA. Oil from
flaxseed contains more alpha-linolenic acid than any other known plant
source (see Flaxseed Oil).
Other plant sources of ALA include great northern beans, kidney beans,
navy beans, and soybeans (see Soybeans).
Omega-3 supplements, such as fish oil, fish oil capsules, and
cod liver oil (also called marine oils), are available at pharmacies
and natural food stores.
Some nutritionists recommend eating a diet rich in fish
containing omega-3 fatty acids, eating 1 to 2 teaspoons of flaxseed or
flaxseed oil daily, or taking daily supplements containing 1 to 2 grams
of omega-3s. Omega-3 fatty acids are very unstable and spoil easily, so
food manufacturers often remove them from foods to increase shelf life.
What is the history behind it?
Cod liver oil became popular in 19th century England as a
vitamin D supplement for sun-deprived children. In the 1950s, a German
scientist named Johanna Budwig, PhD, discovered essential fatty acids
and developed a diet that she said would fight cancer. Dr. Budwig
claimed that many of her patients experienced tumor reduction within 3
months, and she stated that some experienced even more dramatic
results. Dr. Budwig has reportedly used omega-3 fatty acids in
combination with other nutrients to treat thousands of people with
cancer and other diseases.
In 1996, the American Heart Association released a report
stating that eating foods containing omega-3 fatty acids is reasonable
and possibly helpful in reducing risk of heart disease. More recent
recommendations have suggested eating 2 or more servings of fatty fish
per week and eating foods that are high in alpha-linolenic acid such as
flaxseed, canola oil, soybeans, and walnuts to reduce cardiovascular
disease risk. People who already have cardiovascular disease are
recommended to eat more of these products or take supplements.
What is the evidence?
Although some research supports the anti-cancer claims made
for omega-3 fatty acids, some does not. Some studies even show an
increase in disease when omega-3 supplements are used. The strongest
evidence for the health benefits of fatty acids from fish is in the
area of heart disease and its risk factors. The relationship between
omega-3 fatty acids, cancer, and other diseases is not as well known.
In 2006, researchers reviewed 38 studies conducted over the
past 40 years on the effects of omega-3 fatty acids. Researchers looked
at studies that showed positive effects, no effects, and negative
effects of omega-3 fatty acids on the development of cancer. In the
final analysis, it appeared that there was no effect overall.
Researchers concluded that omega-3 supplements are unlikely to prevent
cancer.
A clinical study published in the journal Cancer concluded
that omega-3 fatty acids seemed to prolong the survival of cancer
patients who were also severely malnourished. An earlier small study
looked at patients with advanced pancreatic cancer and severe weight
loss. It compared use of EPA mixed with a high-protein, high-calorie
supplement to use of the supplement without EPA. After 8 weeks, the
study found that EPA did not help the patients gain weight. A more
recent review of all research on the use of EPA for cancer-related
weight loss found only 5 studies. The reviewers concluded that there
was not enough information to determine whether EPA helped people with
cancer-related weight loss.
Studies have shown that the fatty acids in fish oil help
protect against heart disease and reduce risk factors such as high
blood levels of triglycerides. In one study, fish oils appeared to help
improve heart rhythm problems that can cause sudden death. However,
they may also increase cholesterol and reduce blood clotting.
Alpha-linolenic acid (ALA), the omega-3 fatty acid from vegetable oils,
has not shown as strong an effect in studies to date, although it may
reduce risk of fatal heart disease.
Research is also focusing on the role of omega-3 fatty acids
in relation to omega-6 fatty acids. Omega-6 is another type of
essential fatty acid that is found in many vegetable oils (such as
corn, safflower, and sunflower oils), cereals, snack foods, and baked
goods. Unlike omega-3s, omega-6s are plentiful in the typical American
diet. Some researchers believe one cause of Americans' high rates of
cardiovascular disease may be an imbalance in the ratio of omega-3 to
omega-6 fatty acids. Ideally, the ratio of omega-3 to omega-6 fatty
acids in the human body is 1-to-1. However, the typical American diet
is low in omega-3s and high in omega-6s. Many people have ten to twenty
times more omega-6 fatty acids than omega-3 fatty acids in their
systems.
A large study followed more than 34,000 women from 1980 to
1998, observing their fish intake and the ratio of fish fatty acids to
omega-6 fatty acids in their diets to determine how the ratio affected
their colorectal cancer risk. Women who took in more omega-3 fatty
acids did not have a lower colorectal cancer risk, but they had fewer
large benign colorectal tumors, or adenomas. This study suggests that
omega-3 fatty acids may not reduce colorectal cancer risk but may slow
its growth. More research is needed to find out if this holds true.
The American Cancer Society's most recent nutrition guidelines
recommend eating a balanced diet that includes 5 or more servings a day
of vegetables and fruit, and choosing whole grains over processed and
refined foods. Limiting intake of red meats and animal fats (including
dairy fats) is also recommended in order to help reduce cancer risk. A
good way to do this is to choose fish, poultry, or beans for some meals
rather than beef, pork, or lamb. The guidelines note that although a
diet high in fish can help lower heart disease risk, the clinical
evidence regarding cancer is uncertain.
Are there any possible problems or
complications?
This product is sold as a
dietary supplement in the United States. Unlike drugs (which must be
tested before being allowed to be sold), the companies that make
supplements are not required to prove to the Food and Drug
Administration that their supplements are safe or effective, as long as
they don't claim the supplements can prevent, treat, or cure any
specific disease.
Some such products may not
contain the amount of the herb or substance that is written on the
label, and some may include other substances (contaminants). Actual
amounts per dose may vary between brands or even between different
batches of the same brand.
Most such
supplements have not been tested to find out if they interact with
medicines, foods, or other herbs and supplements. Even though some
reports of interactions and harmful effects may be published, full
studies of interactions and effects are not often available. Because of
these limitations, any information on ill effects and interactions
below should be considered incomplete.
Not enough is known about omega-3 fatty acids to determine
whether they are safe in large quantities or when taken with other
drugs. Omega-3s may increase total blood cholesterol and inhibit blood
clotting. People who take blood-thinning medications (anticoagulants)
or aspirin should not take extra omega-3 because of the risk of
excessive bleeding.
The source of some omega-3 fatty acids may also be a health
concern. Many larger predatory fish contain toxic chemicals absorbed
from pollution. Swordfish, shark, and tilefish (golden bass or golden
snapper), for instance, are high in omega-3 fatty acids but may also
contain high levels of mercury. King mackerel, a lesser source of
omega-3s, may also have high mercury levels. Grouper, red snapper, and
fresh or frozen tuna may have more moderate amounts of mercury. Other
large fish, such as tuna and salmon, may contain other chemicals such
as dioxin and polychlorinated biphenyls or PCBs, although fresh or
frozen salmon usually has low mercury levels and large amounts of
omega-3 fatty acids. Some studies have shown that farm-raised fish may
carry more toxins than fish caught in the wild. Unfortunately, there is
no way for a consumer to know what might be present in any particular
fish, although some fish are inclined to have higher levels of
contamination than others.
The precise risks and benefits of eating these fish are not
known at this time. Experts recommend that adults vary the type of fish
they eat as part of a healthy, balanced diet to reduce the chances of
getting too many contaminants. Mercury poses the greatest risk to young
children and unborn babies. Young children and women who are pregnant,
trying to get pregnant, or nursing should not eat fish likely to be
highly contaminated. They should also limit their intake of moderately
contaminated fish.
For men and middle-aged or older women (after menopause), the
benefits of eating fish may outweigh the risks of mercury or other
contaminants. Even so, experts suggest limiting intake of the
most-contaminated fish to one serving per week. Most refined fish oil
supplements have little or none of these contaminants.
Prolonged use of fish oil supplements can cause vitamin E
deficiency, which is why vitamin E is added to many supplements. Fish
liver oils (such as cod liver oil) can cause toxic levels of vitamins A
and D if overused. Supplements may also cause fishy breath odor,
belching, or abdominal bloating. They may also increase a tendency
toward anemia in menstruating women. Women who are pregnant or
breast-feeding should talk to their doctor before adding extra omega-3
to their diets.
People who are allergic to fish may have serious reactions to
fish oil or supplements made from fish and should avoid them. People
who are allergic to nuts should avoid supplements that are made of the
type of nuts to which they react.
Relying on this type of treatment alone and avoiding or
delaying conventional medical care for cancer may have serious health
consequences.
Additional Resources
More information from your American Cancer
Society
The following information on complementary and alternative
therapies may also be helpful to you. These materials may be found on
our Web site (www.cancer.org)
or ordered from our toll-free number (1-800-ACS-2345).
References
American Heart Association. Fish and Omega-3 Fatty Acids.
Accessed at:
http://www.americanheart.org/presenter.jhtml?identifier=4632 on June
10, 2008.
American Heart Association. Fish, Levels of Mercury and
Omega-3 Fatty Acids. Accessed at:
http://www.americanheart.org/presenter.jhtml?identifier=3013797 on June
10, 2008.
Bagga D, Capone S, Wang HJ, et al. Dietary modulation of
omega-3/omega-6 polyunsaturated fatty acid ratios in patients with
breast cancer. J Natl
Cancer Inst. 1997;89:1123-31.
Burns CP, Halabi S, Clamon G, et al. Phase II study of
high-dose fish oil capsules for patients with cancer-related cachexia. Cancer.
2005;103:651-652.
Covington MB. Omega-3 Fatty Acids. American Family Physician.
2004; 70:133-140.
de Lorgeril M, Salen P, Martin JL, Monjaud I, Boucher P,
Mamelle N. Mediterranean dietary pattern in a randomized trial:
Prolonged survival and possible reduced cancer rate. Arch Intern Med.
1998;158:1181-1187.
Dewey A, Baughan C, Dean T, Higgins B, Johnson I.
Eicosapentaenoic acid (EPA, an omega-3 fatty acid from fish oils) for
the treatment of cancer cachexia. Cochrane
Database Syst Rev. 2007:CD004597.
Doyle C, Kushi LH, Byers T, et al. Nutrition and physical
activity during and after cancer treatment: an American Cancer Society
guide for informed choices. CA:
a Cancer Journal for Clinicians. 2006;56:323-353.
Godley PA, Campbell MK, Gallagher P, Martinson FE, Mohler JL,
Sandler RS. Biomarkers of essential fatty acid consumption and risk of
prostatic carcinoma. Cancer
Epidemiol Biomarkers Prev. 1996;5:889-895.
Gogos CA, Ginopoulos P, Salsa B, Apostolidou E, Zoumbos NC,
Kalfarentzos F. Dietary omega-3 polyunsaturated fatty acids plus
vitamin E restore immunodeficiency and prolong survival for severely
ill patients with generalized malignancy. Cancer.
1998;82:395-402.
Hites RA, Foran JA, Carpenter DO, Hamilton MC, Knuth BA,
Schwager SJ. Global assessment of organic contaminants in farmed
salmon. Science.
2004;303:226-229.
Huang YC, Jessup JM, Forse RA, et al. n-3 fatty acids decrease
colonic epithelial cell proliferation in high-risk bowel mucosa. Lipids. 1996;
31:S313-5317.
Kushi LH. Byers T. Doyle C. et al. American Cancer Society
Guidelines on Nutrition and Physical Activity for cancer prevention:
reducing the risk of cancer with healthy food choices and physical
activity. CA: a Cancer
Journal for Clinicians. 2006;56:254-281.
Maclean CH, Newberry SJ, Mojica WA, Khanna P., et al. Effects
of Omega-3 Fatty Acids on Cancer Risk: A Systematic Review. JAMA.
2006;295:403-415.
Memorial Sloan Kettering Cancer Center. Omega-3. Accessed at:
http://www.mskcc.org/mskcc/html/69316.cfm on June 10, 2008.
Moses AW, Slater C, Preston T, Barber MD, Fearon KC. Reduced
total energy expenditure and physical activity in cachectic patients
with pancreatic cancer can be modulated by an energy and protein dense
oral supplement enriched with n-3 fatty acids. Br J Cancer.
2004;90: 996-1002.
Norman PE, Powell JT. Vitamin D, Shedding Light on the
Development of Disease in Peripheral Arteries. Arteriosclerosis, Thrombosis,
and Vascular Biology. 2005;25:39.
Oh K, Willett WC, Fuchs CS, Giovannucci E. Dietary marine n-3
fatty acids in relation to risk of distal colorectal adenoma in women. Cancer Epidemiol Biomarkers Prev.
2005; 14: 835-41.
Rajakumar K. Vitamin D, cod-liver oil, sunlight, and rickets:
a historical perspective. Pediatrics.
2003 Aug;112:e132-e135.
Yetiv JZ. Clinical applications of fish oils. JAMA.
1988;260:665-670.
Note: This information may not cover
all possible claims, uses, actions, precautions, side effects or
interactions. It is not intended as medical advice, and should not be
relied upon as a substitute for consultation with your doctor, who is
familiar with your medical situation.
Last Medical Review: 11/01/2008
Last Revised: 11/01/2008
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