Information sheet: Fish, Omega-3 fatty acids and cancer

Obesity Prevention Policy

Key messages and recommendations

Oily fish including swordfish, Atlantic salmon, gemfish, Spanish mackerel and canned fish such as sardines, salmon and tuna are rich sources of omega-3 fatty acids.
There are also a range of plant sources of omega-3 fatty acids, including canola oil and margarine, soybeans, soybean oil, linseeds (flaxseeds) and linseed oil, walnuts and walnut oil and leafy vegetables.
There is some evidence that eating fish is linked to a reduced risk of bowel and liver cancers.
There is strong evidence that consuming Cantonese-style salted fish increases the risk of nasopharyngeal cancer. There is strong evidence that consuming foods preserved by salting (including pickled vegetables and salted or dried fish, as traditionally prepared in East Asia) increases the risk of stomach cancer.
Consumption of omega-3 fatty acids are associated with a reduced risk of heart disease and relieving inflammatory conditions such as rheumatoid arthritis and inflammatory bowel disease.
Cancer Council recommends:
- Eating fish (preferably oily types) at least twice per week.
- Including some plant foods and oils that are rich in omega-3 fatty acids in your diet.

Background

Oily fish including swordfish, Atlantic salmon, gemfish, Spanish mackerel and canned fish such as sardines, salmon and tuna are rich sources of omega-3 fatty acids. Meats such as beef, chicken and lamb contain smaller amounts of omega-3 fatty acids. There are also a range of plant sources of omega-3 fatty acids, including canola oil and margarine, soybeans, soybean oil, linseeds (flaxseeds) and linseed oil, walnuts and walnut oil and leafy vegetables.

Omega-3 fatty acids are associated with a range of health benefits, such as lowering the risk of heart disease and relieving inflammatory conditions such as rheumatoid arthritis and inflammatory bowel disease. Research suggests that there is limited suggestive evidence that eating more fish may protect against liver and bowel cancers.^[1] One recent study suggests stronger evidence of the link with reduced bowel cancer risk,^[2] however the overall body of evidence supporting the protective effect of fish remains limited. Limited suggestive evidence is evidence that is suggestive of a direction of effect but not adequate to permit a judgment of a protective relationship. Cancer prevention recommendations that are based on strong evidence do not include eating fish because the evidence is not yet strong.

There are other benefits to consuming fish, including reduced risk of developing dementia in older adults, a reduced risk of cardiovascular disease, stroke, and age-related macular degeneration in the eyes. The main omega-3 fatty acids are α-linolenic acid (ALA), and the long chain fatty acids eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). ALA is an essential fatty acid for humans, as it is required for normal bodily function. The body is unable to make ALA and must obtain it from the diet. A small amount of EPA and DHA can be formed in the body, however most are obtained directly from dietary sources.

The Australian Dietary Guidelines includes recommendations for fish, tofu and nuts, grouped with meat, poultry, eggs, seeds and legumes.^[3] The guidelines recommend that adults consume 2 serves of fish, preferably oily fish, a week.^[3] Additional omega-3 fatty acids can be consumed by including tofu, nuts, oils and leafy greens in the diet. Fish oil supplements that contain large quantities of EPA and DHA are also readily available.

Including more fish and plant-based foods in the diet can displace red and processed meat. Eating too much red and processed meat are associated with increased risk of bowel cancer.^[1]

Biological mechanism

Further research is required to improve our understanding of the potential benefit of fish and omega-3 fatty acids in reducing cancer risk, and the underlying mechanisms involved. Experimental studies (including animal and in vitro studies) have shown that omega-3 fatty acids have multiple mechanisms in preventing cancer:^[4][5][6]

Omega-3 fatty acids have been shown to reduce cell proliferation, probably by modifying the inflammatory response and reduce oxidative stress.^[5]
Fish can also contain high levels of vitamin D and selenium, which independently protect against liver cancer.^[7][8]

Epidemiological evidence

Epidemiological evidence on the association between fish and cancer is summarised in the World Cancer Research Fund Continuous Update Project Expert Report 2018.1 The evidence for fish consumption and the prevention of bowel and liver cancers was limited but generally consistent:

Eleven of 18 studies in a meta-analysis found an 11% decreased risk of bowel cancer per 100 grams of fish consumed per day.
A systematic literature review of six studies found that there is a 6% decrease in risk of liver cancer for every 20g/day increase in fish intake (RR 0.94, 95% CI 0.89-0.99).

There is insufficient evidence that the consumption of fish and omega-3 fatty acids help prevent other types of cancer.

Preservation and processing

There is strong evidence that consuming Cantonese-style salted fish increases the risk of nasopharyngeal cancer (WCRF CUP). There is strong evidence that consuming foods preserved by salting (including pickled vegetables and salted or dried fish, as traditionally prepared in East Asia) increases the risk of stomach cancer.^[1]

Current consumption levels

The 2011–2012 National Nutrition and Physical Activity Survey (NNPAS) reported that seven out of ten Australians consumed meat, poultry and game products and dishes the day before the survey.^[9] On average, Australians consumed 1.7 serves of meat and alternatives per day, and 14% of people met the recommended daily number of serves on a usual basis.^[9] Fish and seafood made up just under 10% of the energy from this food group consumed.^[9]

Dietary recommendations

Consistent with the Australian Dietary Guidelines, Cancer Council recommends:

Eating fish (preferably oily types) at least twice per week; and
Including some plant foods and oils rich in omega-3 fatty acids in your diet.