Tuesday April 2 2013
Oily fish is a good dietary source of omega-3 fatty acids
"Eating fish in old age 'can extend life'," The Daily Telegraph proclaims, among several mainstream papers covering the story. But before you head out to buy some MSC-certified sustainably sourced mackerel, it's worth having a look at whether this really is such good news for you.
The headlines only really apply to over-65s, and no fish were involved in the research. The news is actually based on the results of a large long-term study looking at whether blood levels of omega-3 fatty acids are associated with cardiovascular disease and mortality among older adults. These omega-3 fatty acids are found in fatty fish and seafood, as well as nuts and other dietary sources.
This research found that higher levels of omega-3 in the blood were associated with a 27% reduction in risk of death from any cause, and a 35% reduction in risk of death from heart disease. People with the highest omega-3 levels lived 2.2 years longer on average than those with lower levels.
While this study has several limitations, it is one of the few studies that have objectively measured blood levels of omega-3 fatty acids. This eliminates problems with previous research based on people merely recording what they ate.
It is worth doing more research to find which omega-3 fatty acids may help prevent cardiovascular disease and if they can reduce the number of deaths from this disease.
Where did the story come from?
The study was carried out by researchers from Harvard Medical School and the Harvard School of Public Health, the University of New Mexico, and the University of Washington. It was funded by the US National Institutes of Health.
It was published in the peer-reviewed journal Annals of Internal Medicine.
Much of the news concentrated on the benefits of eating fish. Although omega-3 fatty acids are found in fatty fish and seafood, this study did not look directly at fish consumption. Instead, it looked at levels of omega-3 fatty acids in the blood.
However, the media has been led astray by a press release put out by the Harvard School of Public Health. In it, the study's lead author suggests that their findings mean that people should eat a modest amount of fatty fish. The Independent, the Daily Express, the Daily Mail and The Daily Telegraph have all largely cut-and-paste their text, including quotes, from this press release.
What kind of research was this?
This was a cohort study looking at three fatty acids:
- docosahexaenoic acid (DHA)
- eicosapentaenoic acid (EPA)
- docosapentaenoic acid (DPA)
It looked at whether there is an association between the levels of these three fatty acids and total levels of omega-3 fatty acids in the blood, and the total number of deaths and the number of cause-specific deaths in healthy older adults who did not take fish oil supplements.
Although this is the ideal design for this sort of study, cohort studies cannot exclude the possibility that other factors (confounders) are responsible for any associations seen. This study would also have been stronger if levels of omega-3 fatty acids in the blood had been measured at multiple time points, so that the possibility that they changed over time could be excluded.
What did the research involve?
The researchers recruited 2,692 healthy adults aged 65 years or older (average age 74 years) into the Cardiovascular Health Study.
At the start of the study (in 1992), the researchers measured the participants' levels of fatty acids in the blood, and also assessed their cardiovascular risk factors. The participants were then followed-up for 16 years (until 2008) to see if they had died, and if so, from what causes.
The researchers analysed the levels of omega-3 fatty acids – including the three specific ones (DHA, EPA, or DPA) – in participants' blood at the start of the study. They investigated whether they were associated with cardiovascular disease or the risk of death from any cause.
What were the basic results?
During the study there were:
- 1,625 deaths
- 359 fatal and 371 non-fatal cardiovascular disease events
- 130 fatal and 276 non-fatal strokes
After adjusting for demographic, cardiovascular, lifestyle and dietary factors, the researchers found that the three omega-3 fatty acids were associated with a significantly lower risk of mortality. This was true for the three omega-3 fatty acids individually and when the results for all three were combined.
Having blood levels of omega-3 fatty acids in the top 20% was associated with a 27% reduced risk of death from any cause when compared with having blood levels of omega-3 fatty acids in the lowest 20%. For the specific fatty acids analysed:
- blood levels of EPA in the top 20% were associated with a 17% reduced risk of death from any cause compared with blood levels of EPA in the lowest 20%
- blood levels of DPA in the top 20% were associated with a 23% reduced risk of death from any cause compared with blood levels of DPA in the lowest 20%
- blood levels of DHA in the top 20% were associated with a 20% reduced risk of death from any cause compared with blood levels of DHA in the lowest 20%
The reduction in risk of death from any cause was mainly due to a reduction in risk of cardiovascular death. None of these fatty acids were strongly related to other non-cardiovascular causes of death.
People with omega-3 levels in the top 20% lived an average of 2.22 more years after the age of 65 than those with omega-3 levels in the lowest 20%.
How did the researchers interpret the results?
The researchers conclude that higher circulating individual and total omega-3 polyunsaturated fatty acid levels are associated with lower total mortality – especially deaths caused by cardiovascular disease – in older adults.
This study has found that higher levels of omega-3 in blood at the start of the study were associated with a 27% reduction in risk of death from any cause, and a 35% reduction in risk of death from heart disease in healthy older adults (aged 65 years or older) who were not taking fish oil supplements. The older adults with the highest levels of omega-3 lived 2.2 years longer on average than those with lower levels.
This research has several limitations. Omega-3 fatty acid levels were only measured at the start of the study and could have changed over time. Deaths also could have been misclassified, and the possibility that there are other factors that may be responsible for the association seen (confounders) cannot be excluded.
However, this study is one of the few to objectively measure blood levels of markers for omega-3 fatty acids. Further research is warranted to find out whether certain omega-3 fatty acids can help prevent cardiovascular disease and reduce mortality.
It should be noted that although omega-3 fatty acids are found in fatty fish and seafood, this study did not look directly at fish consumption. Instead, it looked at the levels of omega-3 fatty acids in the blood. For this reason, it is wise to take the newspaper headlines with a pinch of salt for now.
Analysis by Bazian. Edited by NHS Choices. Follow Behind the Headlines on Twitter.