Eating fish may fight AMD sight loss

“Tuna sandwiches 'could help prevent sight loss in old age,'” the Daily Mail has reported. The newspaper says that new research into the omega 3 fatty acids found in some types of fish show that they may cut the risk of age-related macular degeneration (AMD), which is the most common cause of sight loss in those over 50.

This study followed about 38,000 female health professionals in the US for an average of 10 years. It looked at whether their intake of fish and omega 3 fatty acids affected their risk of developing AMD. About 0.6% of the women developed AMD during the study. Those who consumed the highest levels of two particular forms of omega 3 were around 38% less likely to develop the condition than those who consumed the least. Women who ate oily fish (such as canned tuna, or mackerel) at least once a week were 44% less likely to develop AMD than those who ate these foods less than once a month.

The study has a number of strengths, including its size and its exclusion of women with AMD at the start of the study. Its limitations include the fact that food intakes were assessed at the start of the study only (and may therefore have changed over time). The study also relied on women to report their AMD diagnosis, meaning some cases may have been missed. The authors of the study have called for a randomised controlled trial to assess whether some forms of omega 3 can prevent AMD. This seems justified, and would help to confirm their effects.

Where did the story come from?

The study was carried out by researchers from Harvard School of Public Health and Harvard Medical School. It was funded by the US National Institutes of Health. The pills and packaging used in this study were provided by Bayer Healthcare and the Natural Source Vitamin E Association. The study was published in the peer-reviewed medical journal Archives of Opthalmology.

The Daily Telegraph and Daily Mail have provided balanced interpretations of this study.

What kind of research was this?

This was an analysis looking at whether intake of omega 3 fatty acids or fish affected risk of developing age-related macular degeneration (AMD) in a cohort of women. AMD is a common cause of blindness in the elderly. It is caused by a gradual deterioration of cells in the macula region of the retina, the light-sensitive layer lining the back of the eyeball. The macula is responsible for central vision. Omega 3 fatty acids are reported to naturally be found at high levels in the retina, and there is a theory that higher levels of these fatty acids in the diet may reduce risk of AMD.

This type of analysis is the best way to look at whether an environmental exposure (such as diet) is related to a particular outcome (AMD in this case). However, there are some limitations: when looking at the effects of specific nutrients, there are inherent difficulties in accurately assessing how much a person consumes without measuring everything they eat. This study used accepted methods to assess food intake and estimate levels of omega 3 fatty acids in this diet; however, there may still be some inaccuracies. In addition, it can be difficult to isolate the effects of one nutrient from our complex diet, which contains a wide variety of different nutrients in different combinations.

When a specific dietary component, such as omega 3 fatty acids, is thought to have health benefits, this can be tested in a randomised controlled trial. The researchers report that there is currently an ongoing trial intended to assess whether omega 3 fatty acids can prevent progression to advanced AMD.

What did the research involve?

This study included women who were taking part in the Women’s Health Study, a randomised controlled trial which looked at low-dose aspirin and vitamin E as potential means for preventing cardiovascular disease and cancer in women who had not previously had these conditions. The women were all health professionals, and their average age at the start of the study was 54.6 years.

This study assessed omega fatty acid intake in the women’s diets at the start of the study using data from food frequency questionnaires. The researchers used this information to estimate how much of the omega 3 and omega 6 fatty acids the women consumed. The women were followed-up for an average of 10 years, with researchers identifying any women who developed AMD. The analyses included 38,022 women who did not have AMD at the start of the study that had completed the food questionnaires.

The food frequency questionnaires asked how often the women consumed specific amounts of 131 different food items over the past year. This included canned tuna fish; dark-meat fish such as mackerel, salmon, sardines, bluefish and swordfish; other fish; and shrimp, lobster or scallops as a main dish. The researchers used data on how much omega 3 and omega 6 fatty acids were in these foods to estimate the women’s daily intakes of these compounds. The omega 3 fatty acids assessed were docosahexaenoic acid (DHA), eicosapentaenoic acid (EPA), docosapentaenoic acid, and α-linolenic acid. The omega 6 fatty acids assessed were linolenic acid and arachidonic acid.

In addition, at the start of the study the women were asked about potential risk factors for AMD and whether they had ever been diagnosed with AMD. Only those women without AMD were included in this study. The women were sent a questionnaire every year, asking whether they had been diagnosed with macular degeneration in either eye. Self-reported diagnoses were checked against medical records, and the woman’s ophthalmologist or optometrist was contacted to provide details. The researchers wanted to identify those women who had AMD that significantly affected their vision. They defined this as having vision of 20/30 or worse (20/20 vision is considered to be normal vision; as the denominator gets larger this indicates worsening vision).

The researchers then looked at the relationship between omega 3 fatty acid intake and risk of AMD. To do this, they split the women into three groups based on their intake (the lowest third, middle third and highest third of intakes). They compared the rate in each of the higher groups with the rate in the lowest intake group. They also looked at how AMD risk related to omega 6 fatty acid intake, the ratio of omega 6 to omega 3 fatty acid intake, and intake of fish and seafood.

The analyses took into account factors that could affect results, including smoking, alcohol use, body mass index, multivitamin use, history of an eye examination in the two years before the start of the study, and which treatment the woman received in the original randomised controlled trial they were taking part in.

What were the basic results?

During follow-up, 235 of the 38,022 women (0.6%) were confirmed to have developed visually-significant AMD.

Women who had consumed the highest amounts of the DHA omega 3 fatty acid were 38% less likely to develop AMD than women who consumed the lowest amounts (relative risk [RR] 0.62, 95% confidence interval [CI] 0.44 to 0.87). Women who had consumed the highest amounts of the EPA omega 3 fatty acid were 34% less likely to develop AMD than those who consumed the lowest amounts (RR 0.66, 95% CI 0.48 to 0.92).

Consumption of other omega 3 fatty acids (docosapentaenoic acid or α-linolenic acid), or omega 6 fatty acids was not linked with AMD risk.

When the researchers looked at fish and seafood consumption overall, they found that women who ate one-or-more servings per week were 42% less likely to develop AMD than those who consumed less than one serving per month (RR 0.58, 95% CI 0.38 to 0.87). When the analysis was broken down by type of fish or seafood eaten, a reduction in AMD risk was seen with higher dark-meat fish and canned tuna fish consumption, but not other fish or seafood (shrimp/lobster/scallops).

How did the researchers interpret the results?

The researchers conclude that their findings suggest that “regular consumption of docosahexaenoic acid and eicosapentaenoic acid and fish was associated with a significantly decreased risk of incident AMD and may be of benefit in primary prevention of AMD”. They say that their results need to be confirmed in randomised controlled trials.

Conclusion

This large study suggests that eating more oily fish may reduce a woman’s risk of developing age-related macular degeneration (AMD), and that this may be related to the long chain omega 3 fatty acids in these fish. The study’s strengths include its size, collection of data in a prospective fashion, exclusion of women with AMD at the start of the study from the analyses, and confirmation of AMD diagnoses by contacting the women’s eye specialists. There are some limitations:

• Although accepted methods were used to assess women’s food intakes in the past year at the start of the study, there may still be some inaccuracies in how they recalled their intakes. In addition, food intake may have changed during follow-up.
• The identification of women with AMD relied on them visiting their eye specialist for check-ups and reporting any diagnoses to the researchers. Some women with the condition may not have been diagnosed if they did not go to their eye specialist for assessment.
• Although the study took into account some factors that could affect results, there may still be unknown or unmeasured factors that are having an effect.
• The researchers note that although other observational studies have supported a link between increased consumption of omega 3 fatty acids and a reduced risk of advanced AMD, the evidence regarding early-stage AMD (as assessed in the current study) is less clear cut.
• The study was in women only, and they were all health professionals. The results may not be representative of other groups.

The researchers are correct in calling for a randomised controlled trial to confirm whether the long chain omega 3 fatty acids can reduce the risk of developing AMD. Such a trial could avoid the limitations of an observational study.