Omega-3 oils and prostate cancer - should men take fish oil supplements?

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Omega-3 intake reduces the risk of many cancers but not prostate - Professor Robert Thomas explains why he avoids fish oil supplements

A massive UK study, published last week, provided some new intriguing data on the anti-cancer benefits of omega-3 and 6 fatty acids but highlighted further concerns with an increased risk of prostate cancer - especially if taken in supplement form.

Omega-3 and omega-6, otherwise known as polyunsaturated fatty acids (PUFAs) are found in oily fish, meat from grass-fed or wild animals and vegetables such as seaweed, walnuts, avocados and chai seeds.

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Cod liver oil omega-3 capsules may bring an increased risk of prostate cancer to men Cod liver oil omega-3 capsules may bring an increased risk of prostate cancer to men
Cod liver oil omega-3 capsules may bring an increased risk of prostate cancer to men | Shutterstock

PUFAs have enormous health benefits for the body

As well as being used for energy production, PUFAs are essential building blocks for many hormones, enzymes and cell walls. They help reduce cholesterol levels and are the fats which protect our brains and vasculature. This is why men and women with low intake of these healthy diets are more prone to dementia, strokes and heart disease. In terms of cancer, they also help our immunity recognise early cancer cells as foreign in order to kill them before they take a foothold.

Likewise, they help help dampen down excess inflammation which can result in cells dividing too fast, hence forming too many toxic free radicals, which damage the DNA creating mutations - the first step in the cancer pathway.

This latest study was a further evaluation of The UK Biobank cohort which has followed the dietary habits and conducted nutritional-related blood tests on over a quarter of a million participants, for 13 years.

Although there have been previous studies looking at associations between PUFA intake and cancer incidence, they have been inconsistent. Because this study was so statistically robust and respected amongst scientists, the findings have firmly established that adequate intake of these healthy fats, reduces the risk of many cancer especially for the biggest killers which arise in the breast, lung and bowel.

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In absolute terms, there was about a five per cent difference between people with low levels and adequate levels of PUFAs.

Before you rush out stock up on fish oil supplements there was one important caveat, which seems to have been missed by many articles in the media. The exception to this trend was with prostate cancer, the most common cancer in men. The UK biobank study actually showed a slight increased risk in men with the highest omega-3 levels. This association supports two previous large studies have shown the same thing, so is likely to be true.

The first called the Prostate Prevention Study which was set up to evaluate whether a drug called finasteride could reduce risk. In a sub-analysis of this data, men with the highest omega-3 intake had a greater subsequent risk of prostate cancer. The same was found in another large prospective study which was primarily designed to find out if vitamin E and selenium reduced prostate cancer risk (The SELECT study). Researchers found that they made the risk worse, as did omega-3.

Why could fish oil supplement increase prostate cancer risk

There are several hypotheses why this trend occurs in men but the one with most validity stems from the use of fish oil supplements rather than foods containing omega-3 and 6. Some fish oils have been found by consumer watchdogs to contain dioxins and PCB, as they use fish from polluted seas or fish that are higher up the food chain, such as king mackerel, swordfish and large tuna.

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Furthermore, if exposed to heat, oxygen and light, fish oils are oxidised into toxic substances such as aldehydes, which can cause mutations in our cells. Many men take cod liver oil, which, although rich in omega-3, has very high levels of vitamin A which, in supplement form, has also been associated with a higher cancer risk, especially in smokers.

The main issue with fish oil supplements is that the vast majority are preserved with Vitamin E which in the SELECT study, mentioned above, was associated with an increased prostate cancer risk. Other studies have also highlighted concerns for other conditions. Sportsmen randomised to take a vitamin E and A supplements, versus placebo, actually had increased muscle damage and inflammation, hindering the recovery of muscle damage.

The Queensland skin cancer study also reported a higher rate of recurrent skin cancers in those taking vitamin E capsules and the CARET Study showed an increased risk of lung and prostate cancer.

It is not clear why the concerns with fish oil supplements only seem to apply to men and mainly prostate cancer. Clearly more research data would be welcomed as more than 55,000 men develop prostate cancer each year, so any lifestyle factors to reduce the risk would potentially save a lot of lives as well as discomfort and costs of medical treatments. As a rule, in view of this data, I try to consume my PUFAs from the natural sources listed below and advise men to do the same.

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That said, if people do not routinely eat many PUFA-containing foods, in view of the importance of maintaining omega-3 and 6 levels for other cancers and conditions, supplementation, with cautions, may have a role. It is very important to buy fish oils from reputable sources, store them in a cool dark place and consume them before the use-by date. It would make sense to buy brands which have lower levels of vitamin E (tocopherols) having some preservative actions with alternatives such as rosemary. It would also be sensible to alternate getting omega-3 from fish with algae.

In summary, natural sources of PUFA omega fats include fatty fish such as salmon, mackerel, sardines, herring, trout, and anchovies. Shellfish such as oysters, clams, and crab contain smaller amounts of omega-3s. Algae and seaweed are a good source of both types of long chain omega-3 fatty acids (DHA and EPA) so useful for vegetarians and vegans.

Some plants contain short chain omega-3s (ALA) which then have to be converted to the active units (EPA and DHA). These include flaxseeds, chai seeds, avocado, walnuts, and leafy green vegetable spinach, kale, and Brussels sprouts.

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