People with age-related macular degeneration (AMD) should eat oily fish at least twice a week to keep their eye disease at bay, say scientists.
Omega-3 fatty acids found in abundance in fish like mackerel and salmon appear to slow or even halt the progress of both early and late stage disease.
The researchers base their findings on almost 3,000 people taking part in a trial of vitamins and supplements.
The findings are published in the British Journal of Ophthalmology.
An estimated 500,000 people in the UK suffer from AMD, which destroys central vision.
Protective
Experts have already suggested omega-3 may cut the risk of getting AMD by a third, and now this latest work suggests these fats also benefit patients who already have the disease.
Progression to both dry and wet forms of advanced AMD disease was 25% less likely among those eating a diet rich in omega-3 fatty acids.
People with advanced AMD who also consumed a low-GI diet, eating of foods that release their sugar more slowly, and who took supplemental antioxidant vitamins and minerals like vitamin C and zinc appeared to reduce their risk of disease progression by even more - by up to 50%.
Substituting five slices of wholegrain bread for white bread every day out of a total intake of 250g of carbohydrate might cut out almost 8% of advanced age related macular degeneration over five years, say the authors.
Surprisingly, however, the supplements were counterproductive for those with early AMD, negating the benefits of omega-3 fats, and even appeared to increase the risk of disease progression.
Those who took all the antioxidant vitamins plus zinc, and who a high daily intake of beta carotene - found in yellow and green vegetables - were 50% more likely to progress to advanced disease.
The researchers at Tufts University, Boston, believe omega-3 fatty acids offer protection against AMD by altering fat levels in the blood after a meal that can be damaging to the body.
'Moderation'
But they say it is not clear whether patients should also consider taking supplements as well as omega-3 because of their mixed findings.
They suggest that eating two to three servings of fatty fish, such as salmon, tuna, mackerel, shellfish, and herring every week, would achieve the recommended daily intake (650mg) of omega-3, substantially cutting the risk of both early and late stage AMD.
The UK's Food Standards Agency says people should eat at least two portions of fish a week including one of oily fish.
But they caution that too much oily fish is bad because it can contain low levels of pollutants that can build up in the body.
Most people can safely eat up to four portions a week, but girls and women who might have a baby and those who are pregnant or breastfeeding should limit their intake to two portions a week.
A spokeswoman from RNIB said good nutrition was very important for both general and eye health.
"These findings appear to be consistent with previous research that has shown that eating omega-3 poly-unsaturated fats as part of a balanced diet may help prevent the development of age-related macular degeneration, the main cause of severe sight loss in the UK.
"RNIB hopes that this will further highlight why looking after your eyes should be a key motivation in maintaining a healthy lifestyle," she said.
