People with retinitis pigmentosa who take daily vitamin A and eat a couple servings of fish each week have a dramatically slower decline in distance and retinal visual acuity, according to a new report.
The effect was equivalent to 18 additional years of central vision, lead author Dr Eliot L. Berson of the Massachusetts Eye and Ear Infirmary in Boston told Reuters Health.
"This is the first time visual acuity has ever been shown to be preserved in this disease," he added. The findings were published online in the Archives of Ophthalmology.
About one in 4000 people has retinitis pigmentosa, which occurs when rod and cone receptors throughout the retina degenerate and lose function.
Slow progress of disease
Patients with the condition typically develop night blindness as adolescents and lose side vision as young adults. Tunnel vision develops as the condition becomes more advanced, and many patients are nearly blind by the age of 60. There is currently no treatment.
Starting in 1984, Dr Berson and his team ran a series of three clinical trials in adults with retinitis pigmentosa comparing 15,000 IU of daily vitamin A as oral retinyl palmitate to placebo. Those trials lasted from four to six years.
The researchers found vitamin A slowed decline in retinal function in the patients in each trial. And while concerns have been raised that giving high doses of vitamin A to patients could damage the liver, Dr Berson and his colleagues saw no toxic effects.
In the current analysis, Dr Berson and his colleagues pooled data from 357 patients participating in the three trials that were receiving vitamin A, and then gauged each participant's intake of omega-3 fatty acid using food questionnaires that participants had completed at enrolment and annually thereafter.
Consume two servings of oily fish per week
The researchers compared declines in visual acuity in patients with omega-3 intakes at or above the median of 0.20 grams a day – about one can of sardines a week – with those who consumed less than the median.
The decline in Early Treatment Diabetic Retinopathy Study distance acuity among the patients with high omega-3 fatty acid intake was 0.59 letters per year, compared to 1.00 letter per year in patients with low intake (p=0.001). Snellen retinal acuity declined by 1.5% a year in patients with a high omega-3 intake, while the decline was 2.8% per year for those with a low intake (p=0.03).
The researchers did not adjust for potential confounders, so causal inference cannot be made based on the current findings. Still, Dr Berson did not shy away from making recommendations.
He stressed that in order for retinitis pigmentosa patients to reap the potential benefits of this preventive strategy, they must consume 15,000 IU of vitamin A as oral retinyl palmitate, not in any other form, as well as one or two servings of oily fish per week. The study did not look at supplements, he added, but in patients who absolutely cannot eat fish, one 250 milligram tablet of docosahexaenoic acid (DHA) per day would be equivalent to the amount of fish found to be putatively protective.
Combine vitamin A and omega-3 rich diets
"We are sometimes asked how one or two nutritional supplements can benefit patients with so many different gene defects," Dr Berson told Reuters Health in an email. He and his colleagues, as well as other researchers, have proposed the following explanation: During the day, when rods aren't required for vision, they deliver vitamin A to cones via Müller cells. But DHA must be present in order for vitamin A to be released.
Dr Berson said he often uses the analogy of a truck delivering four molecules of vitamin A and one molecule of DHA from the rods to the cones. Without DHA, he adds, the vitamin A can't get off the truck. And as the rods degenerate, they produce less and less vitamin A. "Patients are advised to take vitamin A to replace their rods, and eat oily fish to enhance delivery of vitamin A to cones," he said.
He and his colleagues conclude: "The treatment regimen of vitamin A combined with an omega-3 rich diet (0.20 g/d) should make it possible for many patients to retain both visual acuity and central visual field for most of their lives."
(Anne Harding, Reuters Health, February 2012)
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