Are fish oil supplements putting you at risk of prostate cancer?
By Peter Howe, Director of the Clinical Nutrition Research Centre, University of Newcastle
A report published in the Journal of the National Cancer Institute late last week shows a potential link between omega-3 fatty acids and the risk of developing prostate cancer.
But it may be premature for people to reduce their omega-3 intake until we have a better understanding of what lies behind this apparent association.
Omega-3 fatty acids are a class of polyunsaturated fats found in marine and plant oils that are considered essential for health. Plants contain alpha-linolenic acid (ALA), the short-chain omega-3 precursor that is elongated to form the more beneficial long-chain acids (EPA, DPA and DHA) found in marine sources.
This conversion is limited in humans; less than 1% of ALA is converted through to DHA. Hence the Nutrient Reference Values for Australia and New Zealand and the National Heart Foundation recommend consumption of pre-formed long chain omega-3 obtainable from fish, fish oil and krill oil, in addition to consumption of ALA from plant sources such as canola, chia, flaxseed, soy and various nuts.
News of potential adverse association with prostate cancer comes at a time when the public have been heeding the message that the omega-3s derived from marine sources are beneficial for cardiovascular health, counteracting inflammatory conditions, such as rheumatoid arthritis, and possibly improving mood and cognitive performance.
A potential link between prostate cancer and consumption of ALA was reported more than a decade ago but was subsequently dismissed due to lack of supportive evidence. And with the realisation that ALA contributes little to the health benefits ascribed to omega-3, attention has focused more recently on the effects of the long-chain omega-3s.
We can now assess the "omega-3 status" of an individual from the levels of these fatty acids accumulating in certain fractions of the blood. And a strength of the current report is that it examined associations between blood levels of long-chain omega-3 rather than estimating intakes from dietary records, which is notoriously imprecise.
The report indicated that DHA, the omega-3 considered most important for conferring cardiovascular benefits, has the strongest association with high-grade prostate cancer – the authors included their latest findings in a meta-analysis in the same report, which indicates a 48% increase in relative risk of high-grade prostate cancer.
Most cancer risk factors are associated with unhealthy diets and lifestyles, so, on one level, this conflicts with current nutrition advice promoting increased omega-3 consumption to reduce the risk of chronic disease. It also challenges conventional thinking on the benefits versus risks of omega-3 supplementation.
The right dose
Up until now, long-chain omega-3s have been considered very safe. There was a minor concern that fish oil supplementation may prolong bleeding, although the risk is no greater than that posed by a low dose of aspirin. Both can help reduce the risk of potentially lethal blood clots.
Australia has adopted the generally-recognised-as-safe limit of three grams of omega-3 fatty acids per day (equivalent to around ten standard fish oil capsules) as recommended intake.
The level recommended for reducing the risk of chronic disease, particularly for maintaining heart health, is only 0.5 grams per day (one-sixth of the upper limit) but, on average, Australians consume only 0.2 grams of omega-3 per day.
That means, ideally, they should be consuming an extra 0.3 grams per day, equivalent to the omega-3 content of a typical fish oil capsule or a weekly serve of oily fish.
Too much of a good thing?
There are cases where, at excessive intakes, beneficial nutrient supplements, such as antioxidants, may have detrimental effects.
But that doesn't explain the present finding. The levels of omega-3 that the prostate cancer study authors refer to are not extreme – they can be achieved through consumption of fish alone, without popping capsules.
The range of omega-3 levels from low risk to high risk as described in the study are within the range attainable by variations in the amount of omega-3 consumed in typical Australian diets.
While this observation raises concern, there's a lot more we need to understand before we can properly balance the risk-versus-benefit equation for omega-3 consumption.
In the absence of trials that can demonstrate a causal relationship, we need to have a clearer understanding of possible underlying mechanisms. It's known, for example, that omega-3 oils can influence certain male reproductive mechanisms, such as sperm production and motility, but there is currently no hypothesis to explain how they might influence prostate cancer risk.
At the same time, evidence is mounting for protective effects of omega-3 consumption in breast and other cancers
Until now, the risk factors for fish oil supplementation have been considered to be minimal. Even if a causative relationship between omega-3 consumption and prostate cancer is confirmed, consumers will need to weigh up their cancer risk against the known cardiovascular benefits before giving up on omega-3 fatty acids.
Peter Howe has received funding from NHMRC and the ARC. He has also had support from various companies including company partnerships with ARC Linkage Grants.
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