"What's clear is the link between oxygen and photoreceptor damage, as well as the potential of antioxidant[s]," said lead researcher Peter Campochiaro."These experiments suggest that an optimised regimen of antioxidants may help to protect patients with retinitis pigmentosa."
The researcher leading the studies suggests however that diet rather than supplements could be more effective, but the doses studied raise doubts as to whether similar effects observed in the laboratory could be reproduced in humans through dietary means.
The scientists from John Hopkins University looked at the effects of vitamin E, vitamin C, and alpha-lipoic acid administration to mice with a form of retinitis pigmentosa (RP).
In patients with RP, rod photoreceptors die from a mutation, but it has not been known why cone photoreceptors die. In earlier studies from the same researchers, it was found that exposing mice to pure oxygen resulted in high levels of oxygen in the retina, which killed both rods and cones - light-sensitive cells named for their shapes.
"This was the clue that the high oxygen levels that occur naturally in the retina after rods die was the suspect regarding cone cell death. To test this, we used antioxidants, which protect cells from oxygen damage, and since they allowed many more cones to survive, it proves that the suspect is guilty," explained Campochiaro.
The research, published in the July online edition of the Proceedings of the National Academy of Sciences (doi: 10.1073/pnas.0604056103), consisted of injecting the mice with the antioxidants alpha-tocopherol (200 mg per kg), vitamin C (250 mg per kg) and alpha-lipoic acid (100 mg per kg).
In this mouse model of RD, the rods are said to completely degenerate by day 18, and then the cones start to degenerate, with 85 per cent reported to be dead by the time the mice are 35 days old.
The researchers reported that 40 percent of the cones survived in the mice receiving vitamin E or alpha-lipoic acid. This was about double the amount measured in the vitamin C group, and the control group.
Campochiaro stressed that the benefits observed for the antioxidants do not constitute a cure, but may merely slow the development of blindness. "That alone would be an enormous help," he said.
Such doses as observed in this animal study are not recommended for humans - the vitamin E dose would equate to an intake of 14000 mg for a 70kg human which exceeds the safe upper limit of 900 mg - Campochiaro told NutraIngredients.com : "It is possible that a diet rich in antioxidants could help, but the clinical trial to prove it would be long and difficult and I doubt that anyone would want to take it on."A growing body of science is showing that antioxidant carotenoids such as lutein and astaxanthin have significant effects on improving eye health, particularly in relation to age-related macular degeneration (AMD), the leading cause of blindness in people over 50.
Campochiaro told NutraIngredients.com that, although his group was not looking at these carotenoids for RP they are beginning to look at "a number of other agents and we have some interesting results that we can't yet discuss."
"Much more work needs to be done to determine if what we did in mice will work in humans," said Campochiaro.