Supplements for good vision: Antioxidant formulations provide different benefits for the eyes – meta-analysis
Researchers in Singapore conducted a meta-analysis of randomised controlled trials (RCTs), which showed that various combinations of antioxidants can have specific benefits for eye health, pointing to the possibility of developing tailored and informed approaches for eye supplements. The study was funded by the National University of Singapore.
“Overall, antioxidant supplementation was estimated to improve eye health parameters, whereas different combinations of antioxidants may also have varying effects on improving visual health from multiple perspectives,” wrote the researchers in Advances in Nutrition.
A key underlying cause of visual impairment is the accumulation of reactive oxygen species and an increase of oxidative stress in the eye, resulting in biomolecule damage. Cellular damage causes eye disorders that lead to age-related macular degeneration (AMD) and degeneration of retinal ganglion, causing glaucoma.
Although it has been well established that antioxidants play an important role in regulating oxidative stress status in the eye, the comparative effectiveness and potential additive effect between groups of antioxidants have not been systematically studied. Therefore, the researchers wanted to investigate the effect of dietary antioxidant supplements on eye health through a systematic review and network meta-analysis of RCTs.
Four databases – PubMed, Embase, CINAHL, and Cochrane – were searched, and relevant RCTs were identified. A total of 38 articles were included in the network meta-analysis. They were categorised into nine groups – one placebo and eight distinct antioxidant-supplemented groups: Placebo, L (lutein), L+Z (zeaxanthin), L+Z+FA (fatty acids), L+Z+MZ (meso-zeaxanthin), antioxidant mixture (MIX)+FA+L, MIX+FA+L+Z, MIX+L+Z, and Z.
The dosage varied, with levels of lutein or zeaxanthin ranging from 2 mg to 20 mg or even more. Most trials used omega-3 fatty acids, except for the one by Bovier et al, who used a combination of different fatty acids. Studies that used other antioxidants besides those found in lutein or zeaxanthin groups were grouped under ‘MIX’, which included supplements such as anthocyanins (another type of antioxidant), minerals, and vitamins. The duration of treatment varied from 6 to 208 weeks, and the average age of the participants ranged from 19 to 77 years old.
The researchers looked out for the effects of antioxidant supplementation on macular pigment optical density (MPOD), a measure of the density of Macular Pigment (MP) in the eye. The MP consists mainly of lutein and zeaxanthin, along with a smaller amount of another carotenoid called meso-zeaxanthin (MZ). These pigments collectively form a protective layer at the back of the eye in the macula, a critical area responsible for central vision and fine detail. MP protects the eyes by absorbing blue light, preventing damage caused by oxidative stress to the retina. So, the denser the MP the more protective it is. The presence of MP is associated with better visual performance and a lower risk of AMD and other vision problems.
They also looked out for antioxidant effects on visual acuity (ability to see details at a specific distance), photo stress recovery time (the duration it takes for the eyes to recover from exposure to bright light), and contrast sensitivity at low, normal, and high spatial frequency (the ability to distinguish between light and dark at different levels of detail or object sizes, low being the least amount of details, and high being the most detailed).
The researchers found that all groups who took the supplements significantly showed increased MPOD and contrast sensitivity at low spatial frequency. The L+Z combination led to significant improvement for photo stress recovery time, and the L+Z+FA combination was ranked best MPOD and second best for contrast sensitivity at low spatial frequency. For visual acuity, only the MIX+FA+L intervention elicited a significant improvement. The L+Z+MZ intervention was the best-ranked intervention for contrast sensitivity at low spatial frequency, whereas the L intervention was shown to be the most effective for contrast sensitivity at normal spatial frequency.
Additionally, this meta-analysis confirmed the supportive role of fatty acids in combination with macular xanthophylls (the constituents of MP) in improving MPOD.
“This review showed that various combinations of antioxidants would have distinct effects on improving visual health from multiple angles and particularly confirmed the adjunctive role of fatty acids in combination with macular xanthophylls in improving macular pigment optical density,” said the researchers, who explained that the effectiveness of antioxidants depends on their specific physiological functions, how well they are absorbed by the body, and their ability to reach and act upon the relevant biological structures, ensuring optimal protection and support for overall health.
Thus, the researchers suggest that different groups of antioxidants can work cooperatively to enhance eye protective effects.
“Although supplementation of one antioxidant may not show improved eye health, incorporating a mixture of various antioxidant groups resulted in enhanced visual outcomes as the regimen of supplements used in the Age-Related Eye Disease Studies, including vitamin C, vitamin E, beta-carotene, zinc, and omega-3 FA collectively improved visual outcomes and was associated with lowered risk of diseases progression in patients with age-related macular degeneration,” said the researchers.
“In conclusion, overall, antioxidant supplementation can improve eye health. Incorporating FAs with macular xanthophyll is more effective at improving eye MPOD, whereas macular xanthophyll combined with antioxidants enhances visual acuity, and the xanthophyll-only group enhances contrast sensitivity mostly. This suggests that different combinations of antioxidants work differently in improving visual health in multiple perspectives,” said the researchers.
However, they warned that these findings should be interpreted with caution due to low quality of evidence, primarily influenced by indirectness and potential publication bias. For example, future research should aim for more detailed categorisation, carefully considering the impacts of individual minerals and their dosages to better understand their distinct roles in eye health.
Source: Advances in Nutrition
DOI: https://doi.org/10.1016/j.advnut.2024.100216
“Effect of Antioxidant Supplementation on Macular Pigment Optical Density and Visual Functions: A Systematic Review and Network Meta-Analysis of Randomized Controlled Trials”
Authors: Weili Hu, Vernice Seah et al.
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