“Oily fish ‘cuts eye disease risk’” reported BBC News yesterday, saying that eating food rich in omega-3 (such as oily fish) “could help some people avoid one of the most common causes of vision loss”.
The stories are based on a systematic review and meta-analysis of studies that looked at the effects of dietary fatty acids on age-related macular degeneration (ARMD). There may be an inappropriate emphasis in some news reports, which imply that “eating oily fish twice a week can significantly reduce the risk of developing a debilitating eye disease”. The underlying evidence used in the meta-analysis is weak, and the researchers themselves say that although their review suggests that there is an association between fatty acids and reduced risk of ARMD, it is too soon to draw definitive conclusions from the results.
Where did the story come from?
Dr Elaine Chong and colleagues from the University of Melbourne, the National University of Singapore and the Cancer Council of Victoria in Australia carried out this research. The study was funded in part by the National Health and Medical Research Council in the form of a public health scholarship to one author. It was published in the peer-reviewed medical journal Archives of Ophthalmology .
What kind of scientific study was this?
The study is a systematic review and meta-analysis of studies, which have assessed the role of dietary fatty acid and fish intake in preventing ARMD. The researchers searched seven databases of clinical studies for any that might be relevant. They looked for different study designs (i.e. randomised controlled trials, case-control studies, cohort studies and cross-sectional studies). As with all systematic reviews, the researchers assessed all the studies they found for relevance to their question. They only included those that had a clear definition of the fatty acid intake (i.e. omega-3 fatty acids or fish oils), had a clear definition of ARMD, used appropriate analysis and adjusted for confounders, and only those that provided their results in a format which could be easily combined in a meta-analysis. The three randomised controlled trials they found either did not address prevention or did not look at ARMD as an outcome, and so could not be included.
The researchers extracted the results from all the studies and combined them using meta-analysis to get a single result, which would show what effect omega-3 fatty acids or fish oils have on early or late ARMD. There were four sub-questions:
- The effect of omega-3 fatty acids on early ARMD.
- The effect of omega-3 fatty acids on late ARMD.
- Fish intake and early ARMD.
- Fish intake and late ARMD.
What were the results of the study?
Of 2,754 articles found, 50 seemed relevant, though after further assessment the researchers decided that it was appropriate to combine only nine of them to answer their question. There were three prospective cohort studies, three case-control studies, and three cross-sectional studies included in their meta-analysis. This amounted to 88,974 participants (including 3,203 cases of ARMD – 1,847 cases of early ARMD and 1,356 cases of late ARMD).
The researchers compared the effects of intake in the highest intake category with the effects of the lowest intake category. They did not pool studies assessing the effect of omega-3 fatty acids on early ARMD, but found that dietary omega-3 fatty acids reduced the odds of late ARMD by 38% (95% confidence interval [CI] 0.48 to 0.82). For fish intake, they found that the odds of early ARMD were reduced by 24% (95% CI 0.64 to 0.90), and the odds for late ARMD were reduced by 23% (95% CI 0.53 to 0.85).
What interpretations did the researchers draw from these results?
The researchers conclude that consumption of fish twice or more per week and foods rich in omega-3 fatty acids was associated with a reduction in risk of both early and late ARMD. Importantly, they warn that the results about late ARMD should be “interpreted cautiously” because the underlying evidence is particularly weak.
What does the NHS Knowledge Service make of this study?
- Most studies did not differentiate between long- and short-chain omega-3 fatty acids. The researchers recommend that future studies should analyse the specific effects of these two different types. Results from this systematic review and meta-analysis are consistent with another recent systematic review that found the same direction of effect but concluded that the evidence was insufficient to draw definitive conclusions. Meta-analyses of the types of studies included here (case-control, cohort, cross-sectional) are inherently less reliable than meta-analyses of randomised controlled trials.
- A better study design to definitively answer this question is the randomised controlled trial. Without strong evidence from such studies (even small ones that have been pooled), the application of these results for people wishing to use fish oils to prevent ARMD is unclear. In the observational studies that were used, other dietary or lifestyle factors may be responsible for the differences in risk for ARMD. Those with a high intake of fatty acids may be systematically different from those with low intake and this difference (high intake of antioxidants for example) could be having a protective effect on disease risk.
It is too soon to state that fish oils can prevent ARMD, as some news sources have. The evidence around the question is too weak for such a definitive statement, and more research is needed. There is no apparent reason why a randomised trial looking at prevention could not be designed.
Sir Muir Gray adds...
There is some good evidence to support oily fish already. Further research on this claim is needed, and will be done because of the importance of the disease.