Monday July 1 2013
Oily fish are rich in polyunsaturated fatty acids
"Eating two portions of oily fish [a week] could protect women against breast cancer," reports the Mail Online website. The story comes from an analysis of the best available evidence on the link between oily fish and breast cancer risk.
Researchers were particularly interested in assessing the effects of a type of fatty acid called omega-3 polyunsaturated fatty acid (n-3 PUFAs). These fatty acids are found in oily fish such as salmon and tuna, and some plant sources.
The analysis included more than 800,000 women. Just over 20,000 of these women developed breast cancer during follow-up. Women with the highest intake of n-3 PUFAs from fish (marine) sources were found to have a 14% reduction in risk of breast cancer compared with women with the lowest intake.
However, as with all observational studies and reviews, the pooled results may be affected by factors (confounders) other than marine n-3 PUFA intake. For example, women who eat a lot of fish may be more likely to lead healthier lifestyles, such as not smoking.
But a link between n-3 PUFAs and a reduced cancer risk is plausible – n-3 PUFAs are known to reduce the production of the hormone oestrogen, which can stimulate abnormal cell growth.
Overall, this review is a good summary of the current state of knowledge about the link between n-3 PUFA intake and risk of breast cancer.
Where did the story come from?
The study was carried out by researchers from Zhejiang University and the APCNS Center of Nutrition and Food Safety in China, and was funded by the National Natural Science Foundation of China, the Ministry of Education of China, and the National Basic Research Program of China.
It was published in the peer-reviewed British Medical Journal.
The Mail Online covered this story appropriately, with quotes to highlight the limitations of the research.
What kind of research was this?
This was a systematic review and meta-analysis that pooled existing studies looking at whether women's consumption of fish and the fatty acids found in fish was related to their risk of breast cancer.
Many studies have assessed the link between dietary fatty acids and breast cancer risk in humans. The researchers say that the dietary fatty acids found in oily fish (marine n-3 PUFAs) have shown the most potential for reducing cancer risk when tested in laboratory and animal studies. These studies were the ones the researchers were most interested in looking at.
However, there have been inconsistent results in human studies. A systematic review is the best way of summarising the best available evidence on a given research question. Pooling these results can give a more robust result than the individual studies, as long as they are sufficiently similar.
When analysing the link between diet and health outcomes such as cancer, it is not practical to carry out a randomised controlled trial (RCT). This is because people are unlikely to agree to follow a very specific diet for many years so researchers could assess the diet's effect on risk.
The best type of study design for this is a prospective study, where people's diets are assessed and they are followed up to see if they develop cancer. These were the type of studies the review focused on.
However, these type of studies are limited. Because people are not randomly assigned to different diets, they may differ in other ways as well – for example, people who eat more oily fish may have more healthy diets generally, or may do more exercise.
These differences may contribute to any differences seen in the health of fish eaters and non-fish eaters, making it difficult to identify exactly what effect the fish itself is having.
This problem is called confounding. Studies can take this into account, but it is difficult to know if its effect has been completely removed. The review's results are influenced by the quality of the studies being pooled.
What did the research involve?
The researchers searched two databases of the published scientific literature to identify prospective studies assessing the link between consumption of fish as a whole, the fatty acids found in oily fish (n-3 PUFAs), and breast cancer. They statistically pooled the results of these studies to calculate the strength and size of any effect.
Two researchers independently identified the relevant studies and extracted the data. Having two people do this increases the reliability of the results. If there were any disagreements, they were resolved by discussion with a third researcher.
Only prospective studies (prospective cohort, nested case-control, and case-cohort studies) were looked at and the researchers assessed their quality with a standard scale.
The researchers looked at studies assessing either intake of fish or a calculated marine intake of n-3 PUFA based on reported diet. They could measure intake either just based on women's reports of their diet or on measurements of fatty acids in their bloodstream.
When pooling the results from the studies, the researchers used the results that compared women with the highest intakes of n-3 PUFA with women with the lowest intakes. As studies usually present results in different ways, the researchers selected the results that took into account the largest number of possible confounding factors for pooling.
The researchers used standard methods to pool the studies and look at whether the results indicated differences between the studies being pooled.
They also looked at whether factors such as the country the study was carried out in affected the results.
What were the basic results?
The researchers identified 21 studies (described in 26 articles) that met their inclusion criteria:
- 11 articles assessed fish intake
- 17 articles assessed intake of the n-3 PUFAs coming from oily fish (marine n-3 PUFAs)
- 12 articles assessed intake of one specific type of n-3 PUFA called linolenic acid, which comes from plant sources
- 10 articles assessed intake of the n-3 PUFAs coming from any source (total n-3 PUFAs)
The studies included 883,585 people and 20,905 cases of breast cancer, and were all of moderate to high quality.
The researchers' analyses found no link between overall intake of fish, linolenic acid or total n-3 PUFA intake (not just from oily fish) and risk of breast cancer.
However, when they looked at intake of n-3 PUFAs specifically from oily fish, they found that women with the highest intake of marine n-3 PUFAs had a 14% reduction in their risk of developing breast cancer compared with the lowest intake (relative risk [RR] 0.86, 95% confidence interval [CI] 0.78 to 0.94).
The results were similar regardless of whether they measured intake based on the women's reports of what they consumed or on the more objective measurements of fatty acids in their bloodstream. For every extra 100mg of marine n-3 PUFAs consumed per day, there was a 5% relative reduction in breast cancer risk.
The researchers found that the effect of marine n-3 PUFAs was greater in studies that had not taken into account women's body mass index (BMI) and total energy intake in their diet. In the studies that had taken into account BMI or total energy intake, the relationship became non-significant.
How did the researchers interpret the results?
The researchers concluded that, "higher consumption of dietary marine n-3 PUFA is associated with a lower risk of breast cancer".
They say this could have implications for the prevention of breast cancer through dietary and lifestyle interventions.
This large review has pooled the results of the available studies assessing the link between one type of polyunsaturated fatty acids (n-3 PUFAs) found in oily fish and some plant sources. It found that intake of n-3 PUFAs from fish are associated with a reduced risk of breast cancer. The study's strengths include the large amount of data pooled and the fact that all the studies included collected data prospectively.
The fact that similar results were obtained even if marine n-3 PUFAs were measured in different ways (self-reporting or blood tests) is reassuring, as is the fact that greater doses seemed to be associated with a greater reduction in risk.
As with all studies, there are some limitations. The main problem is that although some of the studies took steps to reduce confounding, factors other than marine n-3 PUFA intake may be having an effect.
This means that it is difficult to say for certain that the intake of marine n-3 PUFAs directly reduces the risk of breast cancer. It seems that BMI and total energy consumption also have a degree of influence on the link seen, given that the relationship was non-significant when the two factors were taken into account.
Ideally, researchers would perform randomised controlled trial testing to see what happens if women are given marine n-3 PUFA supplements. In the interim, this review provides an up-to-date summary of the current state of knowledge. Oily fish are already recommended as part of a balanced diet.
Analysis by Bazian. Edited by NHS Choices. Follow Behind the Headlines on Twitter.