Tuesday July 27 2010
Oily fish are a good source of omega-3 fatty acids
“A weekly dose of oily fish may help prevent the most common cause of blindness in old age,” said the Daily Express. The newspaper said that a US study found that people who ate at least one portion of oily fish a week cut their risk of advanced age-related macular degeneration (AMD) by 60% compared with people who ate fewer portions.
This study compared the diet of more than 2,000 older adults and recorded whether or not they had AMD. As the study assessed both of these factors at only a single point in time it is not possible to say whether people’s diets directly affected their development of AMD. The small number of people with advanced AMD also reduces confidence in the results from this study.
Previous studies, including more reliable cohort studies, have already suggested a link between consuming higher levels of omega-3 fatty acids and reduced AMD risk. As such, this new study does not add much to our knowledge. The best way to determine whether omega-3 supplementation can reduce the risk of AMD would be to conduct a randomised controlled trial directly testing omega-3 against a placebo.
Where did the story come from?
The study was carried out by researchers from Johns Hopkins University in Chicago and was funded by the US National Institute on Aging. It was published in the peer-reviewed medical journal Opthalmology.
The Daily Express has reported this research accurately, but did not mention any of its limitations.
What kind of research was this?
This was a cross-sectional study called the Salisbury Eye Evaluation (SEE) study. It looked at the relationship between consumption of oily fish and age-related macular degeneration (AMD), a progressive eye condition in which either breakdown of a layer covering the retina or abnormal blood vessels on the back of the eye causes vision to deteriorate. AMD (also known as ARMD) is a common cause of blindness that becomes more common as people get older. It has been suggested that eating a diet high in omega-3 fatty acids (found in oily fish and some other foods) may reduce the risk of developing the condition.
Cross-sectional studies look at different factors (in this case diet and sight) at only a single point in time. This means that it is not possible to say for sure which factor came first and, therefore, whether one factor could be causing the other.
A better design for looking at the relationship between dietary intake of oily fish and risk of AMD in the population would be a prospective cohort study. which takes a sample of people without AMD, assesses their diets and follows them over time to see who develops AMD.
However, the results of both cross-sectional and cohort studies are susceptible to being influenced by factors other than the one of interest. Therefore, to answer the question of whether taking omega-3 supplements reduces risk of AMD the best design would be a randomised controlled trial.
What did the research involve?
The researchers enrolled a random sample of people aged 65 to 84 living in Salisbury, Maryland, in the US. The participants filled in detailed questionnaires about their normal eating habits and also had an eye examination to determine whether they had AMD. The researchers then looked at whether there was a higher proportion of people with AMD among the group who ate more fish (both fish in general and fish high in omega-3 fatty acids) or among the group who ate less of these foods.
The food frequency questionnaire asked about how often participants ate certain foods over the previous year, and how large the serving sizes were. There were six fish and shellfish categories that were adapted to suit commonly eaten local dishes:
- fried fish: fried fish or fish sandwich
- oysters: oyster fritters or fried oysters
- tuna: tuna fish, tuna salad or tuna casserole
- shellfish: shrimp or lobster
- crab: crab, crab cakes or crab salad
- other fish: other fish baked or fried
The researchers calculated the amount of omega-3 fatty acids in each type of fish and shellfish using nationally accepted reference figures. Crab, other fish, oysters and fried fish were considered to have high omega-3 fatty acid content (more than 0.4g per 100g serving). The researchers calculated how much of each food category was eaten each week on average by each participant. Those eating one or more servings of fish and shellfish overall per week or eating more fish and shellfish high in omega-3 were compared with those eating less than one serving of these foods per week.
A standard test was used to test the participants for AMD, which involved having a photograph taken of the back of the eye. Two independent assessors, who did not know anything about the participants’ diets, examined the photographs for the characteristic signs of AMD. Based on their findings, people with AMD were grouped by how advanced their condition was:
- AMD 3: eyes with new blood vessels (neovascular or geographic atrophy) on the photo
- AMD 2: eyes with pigment abnormalities, but not classified as AMD 3
- AMD 1: eyes with the large yellow or white accumulations (drusen) that are diagnostic of AMD, but not classified as AMD 3 or AMD 2
The diets of those in each group were compared to the diets of those in a control group without AMD.
Participants also provided information on their other characteristics, including their gender, age, race, smoking status and education. Their body mass index was also calculated. The researchers then took these characteristics, as well as overall calorie consumption, into account in their analyses. A total of 2,391 participants (94.9% of those enrolled) provided sufficient data to be included in the final set of analyses.
What were the basic results?
Eye tests showed that:
- about three-quarters of the participants (77.1%, 1,943 people) did not have AMD
- 9.0% (227 people) had the earliest stages of AMD (AMD 1)
- 6.1% (153 people) were in the intermediate stages (AMD 2)
- 2.7% (68 people) had advanced AMD (AMD 3)
The researchers found that people who had any of the three stages of AMD did not differ from those who did not have AMD in the amount of fish and shellfish they ate each week. All participants ate about 1.1 servings of fish and shellfish a week on average.
However, people who had advanced AMD (AMD 3) were 60% less likely to eat one or more servings of fish or shellfish high in omega-3 fatty acids than people without AMD (odds ratio 0.4, 95% confidence interval 0.2 to 0.8).
There was no difference between those with early or intermediate AMD (AMD 1 or 2) and those without AMD in terms of consumption of fish or shellfish high in omega-3 fatty acids.
How did the researchers interpret the results?
The researchers conclude that their findings “support a protective effect of fish/shellfish intake against advanced AMD”. They say that future studies are needed to clarify further “the association between the consumption of fish, shellfish, zinc, and omega-3 fatty acids and the risk of AMD”.
This study showed an association between level of consumption of fish and shellfish high in omega-3 fatty acids and advanced AMD. However, it has a number of limitations that need to be taken into account:
- This study assessed diet and the presence of AMD at the same point in time, and cannot show what the participants’ fish eating habits were before the development of AMD. Therefore, it is not possible to say whether these eating habits may have directly influenced risk of developing AMD.
- The only significant association found was between advanced AMD 3 and lower consumption of fish and shellfish high in omega-3 fatty acids. However, the number of participants who had advanced AMD was small (only 68 people). Therefore, the results of this analysis may not be very robust. It is not clear why the researchers chose to divide AMD into three types, as previous studies have relied on simpler early- and late-AMD categories.
- The researchers carried out multiple statistical analyses. This increases the likelihood that some significant differences may be found by chance.
- As with all studies of this type, other factors that differ between groups with higher and lower fish consumption may be influencing the results. The researchers did take some of these into account, but there may be unknown or unmeasured factors still having an effect.
- People may find it difficult to recall their food intake accurately over an extended period, and therefore there may be inaccuracies.
- A number of previous studies have suggested that oily fish may slow or prevent AMD. It may be the case that these preliminary findings, which have received a great deal of media and internet attention, could influence the fish consumption of people with AMD and distort any potential relationships that may or may not exist between diet and AMD. However, the researchers thought this was unlikely to affect the current study, as it was carried out between 1993 and 1995, which they say was before published associations between fish consumption and AMD were first made.
Studies have already suggested a link between eating higher levels of omega-3 fatty acids and reduced risk of AMD, and the current study does not add much to our knowledge. The best way to determine whether omega-3 supplementation could be used to reduce risk of AMD would be to carry out a randomised controlled trial.