Tuesday June 9 2009
Oily fish are a good source of omega-3
“Eating fish twice a week 'can help prevent eye disease',” The Daily Telegraph has reported. It said a study has found that omega-3 fatty acids, found in oily fish such as salmon and tuna, could help prevent age-related macular degeneration (AMD), the main cause of blindness in elderly people.
The study asked around 3,000 people with various stages of AMD about their diet and followed them up over time to see whether their AMD progressed. Half the participants were also given a daily supplement, containing antioxidants such as vitamins C and E and beta-carotene. The researchers found that taking the supplements together with a diet high in fatty acids “appeared to be counterproductive”, as the combination had less effect than a diet high in fatty acids but without supplements. The study also found that the risk of the disease progressing to an advanced stage might be lowered by eating foods with a low glycaemic index (GI). Low GI foods release their sugars into the blood more slowly than foods with a high GI.
The complex results from this study suggest that a diet rich in the DHA (docosahexaenoic acid) form of omega-3 may reduce the progression of early stage AMD in people not taking certain dietary supplements. Also, a low GI diet rich in omega-3 may reduce the risk of progression to advanced AMD. It should be noted that the results of this research may have been affected by factors other than the dietary ones investigated and require careful interpretation. In general, eating a healthy, balanced diet, including omega-3 fatty acids and low GI foods, may have various health benefits.
Where did the story come from?
This research was conducted by Dr C-J Chiu and colleagues from Tufts University, the University of Wisconsin School of Medicine and Public Health and the EMMES Corporation. The study was funded by the US Department of Agriculture, National Institutes of Health, Johnson & Johnson Focused Giving Program, American Health Assistance Foundation and the Ross Aging Initiative. The study was published in the peer-reviewed British Journal of Ophthalmology.
What kind of scientific study was this?
This study looked at whether the risk of developing age-related eye disease, in particular age-related macular degeneration (AMD), is affected by eating particular diets and taking certain dietary supplements. AMD is a major cause of blindness and results from deterioration of the macula, an area near the centre of the retina responsible for the central field of vision.
Researchers used data collected from people enrolled in a randomised controlled trial called the Age-Related Eye Disease Study (AREDS). The authors of this current paper reported that the AREDS trial had found that “people at risk of developing advanced AMD would benefit by taking high-dose antioxidants (vitamin C, vitamin E, beta-carotene) plus zinc oxide”.
Other studies have suggested that individuals may be protected from AMD by certain nutrients found in the diet (lutein, zeaxanthin and certain omega-3 fatty acids) and by eating a low GI diet. The GI of a food is an indicator of how fast the carbohydrates it contains will cause an increase in blood sugar levels. A high GI indicates a fast release and a low GI indicates a slow release. In the current study, the researchers wanted to look at whether supplement intake and diet might interact with each other and affect the risk of AMD progression.
In the AREDS trial, 3,640 participants were randomly assigned to receive one of four different daily supplement tablets. These were: a placebo, antioxidants (500mg vitamin C, 400IU vitamin E and 15mg beta-carotene), zinc (80mg as zinc oxide) with copper (2mg as cupric oxide), or antioxidants plus zinc.
At the start of the study, participants filled in questionnaires about their characteristics and provided information about their diets in a food frequency questionnaire. They also had a physical and eye examination, which included photographs of the macula. In particular, these photographs looked for the accumulation of deposits of material in the macula, called drusen. Although most people develop a few small drusen as they age, more and larger drusen in the macula are an early sign of AMD.
Macular photographs were repeated after two years and then every year until the end of the eight-year follow-up. Eyes were graded into five groups according to the level of signs of AMD present. Eyes were considered to have either early (groups 1 to 3) or advanced (groups 4 and 5) AMD. Over the follow-up period, the researchers noted when an eye first progressed to a higher AMD group.
For the current study, data on 2,924 participants (80% of those randomised) and 5,146 eyes was available for analysis. This excluded people with diabetes at the start of the study, those whose food questionnaires did not describe feasible energy intakes, those lost to follow-up or with missing data, and eyes with advanced AMD at the start of the study.
The researchers looked at how dietary factors affected AMD progression time. All nutrient variables were adjusted based on the individual’s total energy intake. The 25% of participants with the lowest intake of the DHA (docosahexaenoic acid) or EPA (eicosapentaenoic acid) forms of omega-3 fatty acid were compared with participants with higher intakes of these nutrients.
The researchers then carried out analyses to look at whether the supplements an individual was taking as part of the test affected these outcomes.
What were the results of the study?
Progression of early AMD
The researchers found that, overall, progression of early AMD was not significantly affected by dietary GI, consumption of beta-carotene or consumption of the DHA or EPA forms of omega-3 fatty acids.
However, the effect of dietary DHA on progression of early AMD was found to vary depending on what supplements participants were taking. Looking at the groups taking different supplements and placebo separately, the researchers found that:
- Consuming higher levels of DHA was associated with a reduced risk of progression of early AMD in people taking placebo supplements.
- There was no significant effect of DHA on progression of early AMD when participants consumed higher levels of DHA and used the supplements containing antioxidants or zinc or both.
Progression to advanced AMD
Having a low GI diet reduced the risk of progressing to advanced AMD. This protection occurred regardless of what supplements were being taken, but the level of protection varied slightly between the different supplement groups. The low GI diet and supplementation appeared to boost each other’s effects.
The researchers found that consuming the highest levels of DHA (64mg a day or more) and EPA (42.3mg a day or more) reduced the risk of progressing to advanced AMD. The quarter of participants with the highest consumption of DHA or EPA reduced their risk of progressing to advanced AMD by about 25% compared with the quarter of participants with the lowest consumption of these fatty acids (less than 26mg a day DHA or less than 12.7mg a day EPA). This reduction was not affected by the supplements the person was taking.
The researchers also found that:
- Having a low GI diet and high intake of omega-3 fatty acids (DHA or EPA) appeared to reduce the risk of progressing to advanced AMD more than any of these dietary factors alone.
- There was a trend towards increased risk of progressing to advanced AMD with higher intake of beta-carotene, but this trend did not reach statistical significance.
- There was no significant effect of dietary vitamin C, vitamin E, zinc or lutein/zeaxanthin on the risk of progressing to advanced AMD.
What interpretations did the researchers draw from these results?
The researchers concluded that there was an association between eating a diet high in DHA omega-3 fatty acid and a slower progression of early AMD.
Eating a diet with lower GI and higher intakes of DHA and EPA was associated with a reduced progression to advanced AMD.
What does the NHS Knowledge Service make of this study?
These results show that a complex interaction between diet and supplements is involved in the progression of AMD. The interaction of diet and supplements appeared to be of benefit in some cases but appeared to counteract each other’s benefits in some cases.
Developing specific diet and supplement guidance for those with AMD requires further research into this interaction. Ideally, people should aim to eat a healthy diet, including omega-3 fatty acids and low GI foods, as it is likely to bring a range of health benefits.
There are some further important points to note when interpreting this study:
- People were randomly assigned to which supplement they would receive in the AREDS trial but could not be randomly assigned to their diet. This means that when comparing groups with different diets, the analysis may be affected by factors other than those assessed and that are not balanced between the groups (confounders). These may affect the likelihood of AMD worsening.
- The study analysed each eye separately. The fact that some participants contributed more than one eye to the analyses may have affected results.
- Diet was only assessed at the start of the study and participants’ diets may have changed over the seven-year follow-up.
- Although the study reported using a food frequency questionnaire that had been tested and shown to be a valid way of measuring intake, there may still have been some inaccuracies in people’s recall of what they ate.
- The majority (97%) of participants in the test were white, which means results may not be applicable to other ethnic groups.
- The authors note that because of the risk of lung cancer with taking beta-carotene, the AREDS supplement is not recommended for smokers.
- The researchers carried out a large number of statistical tests, which can lead to finding significant results by chance. The results should be interpreted cautiously.
- The study did not report the number of people or eyes with AMD progression in each group. This makes it difficult to determine the importance of the reported changes in risk. Also, the authors did not report exactly how many people fell into each of the groups compared. If very few people fell into some of the groups, this would reduce the reliability of results.