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Essential Fatty Acids & Vision: The Science
The Influence of Fat and Vision
Essential fatty acids (EFAs) are polyunsaturated fatty acids (PUFAs) that are necessary for optimal health and play a crucial role in maintaining visual function. The two main EFAs are omega-3 (n-3) and omega-6 (n-6) PUFAs. While both are important, it is docosahexaenoic (DHA) made from alpha-linolenic acid (ALA) an omega-3 that is critical for optimal retinal function. The modern Western diet tends to be higher in n-6 PUFAs and lower in n-3 PUFAs, which it is speculated can lead to insufficient levels of retinal DHA.
In this article, we will discuss the influence of essential fatty acids on vision and how they can affect the onset and progression of retinal diseases.
The Effects of Dietary n-3 FA Deficiency are Those on Visual Function
The photoreceptor cells in the retina, the innermost layer of the eye, (see figure 1) are responsible for capturing light and converting it into neural signals that are sent to the brain for interpretation. DHA an omega 3, a product of ALA metabolism, is abundant in the retina and is essential for its proper function.
For optimal functioning it has been established that the retina requires 19% DHA. This level of DHA supports efficient conformational changes to occur in rhodopsin (the light receptor) during transduction (Jastrzebska, Debinski, Filipek, & Palczewski, 2011).
When compared to other tissues, both the brain and the retina are enriched with DHA but have limited capacity for DHA synthesis. There are two sources of DHA, either the body makes it from ALA in the liver or DHA is consumed in the diet, primarily from fatty fish.
Studies have shown that omega-3 PUFAs play a crucial role in maintaining visual function. (Querques, Forte, & Souied, 2011). A deficiency of these fatty acids has been linked to several visual impairments, including decreased visual acuity, impaired color vision, and reduced contrast sensitivity
Omega-3 PUFA Could Have a Protective Role in Reducing the Onset and Progression of Retinal Diseases
Retinal diseases, such as age-related macular degeneration (AMD) and diabetic retinopathy, are leading causes of blindness worldwide. While the exact mechanisms underlying these diseases are not fully understood, research has shown that omega-3 PUFAs may have a protective role in reducing their onset and progression. (Chua & Flood, 2006)
Several studies have investigated the role of omega-3 PUFAs in AMD, a disease that affects the macula, the central part of the retina responsible for fine, detailed vision (see figure 2). Epidemiological studies have suggested that omega-3 may have preventive effects.
An earlier randomized prospective study, known as NAT2 (Souied, et al., 2013), found that individuals who attained high levels of eicosapentaenoic acid/docosahexaenoic acid (EPA/DHA) in their red blood cell membranes were significantly protected against AMD, as compared to those with consistently low levels of EPA/DHA.
In diabetic retinopathy, a complication of diabetes that affects the blood vessels in the retina, studies have shown that omega-3 PUFAs may help reduce inflammation and oxidative stress, both of which contribute to the development and progression of the disease.
The Importance of ALA for Retinal DHA Function
A recent study (Sinclair, Guo, & Abedin, 2022) used a dose-response approach to evaluate the level of ALA content of the diet required to achieve optimal retinal DHA levels. Hempseed oil is a rich source of ALA an omega-3 fatty acid.
Two factors illustrate the uniqueness of this study. Firstly, its focus on ALA as the sole source of omega 3, and secondly, understanding the minimum amount of dietary ALA to reach a retinal DHA value within the optimal retinal function range (19% DHA). While other studies have explored ALA as a source of omega 3, many have relied on fish sources containing preformed DHA/EPA. This study, however, utilizes only ALA, making it particularly relevant for vegans and vegetarians.
Prolonged dietary deficiency of ALA can reduce retinal DHA by >80%, which leads to compromised retinal function and suboptimal responses of the retina to light. This has been observed in various animal species, including primates, rats, cats, and guinea pigs (Jeffrey, Weisinger, Neuringer, & Mitchell, 2001). When ALA is absent from the diet, linoleic acid (LA) is converted to 22:5n-6,docosapentaenoic acid, an omega 6 indicating a deficiency of ALA and DHA. Therefore, measuring tissue levels of docosapentaenoic acid can serve as a biochemical indicator of ALA and DHA deficiency.
Conclusion
Essential fatty acids play a critical role in maintaining visual function and protecting against retinal diseases. Omega-3 PUFAs are particularly important for proper retinal function, and may offer protective benefits against retinal diseases. The exact ratio of omega-6 to omega-3 PUFAs that is optimal for visual health is still under debate, and according to Professor Walter Willett of Harvard, the belief that a higher ratio of omega-6 to omega-3 PUFAs is best has yet to be proven through studies.
Nevertheless, it is important to maintain an appropriate level of both in the diet in order to promote optimal visual health. By understanding the influence of essential fatty acids on vision, we can take steps to create a healthy diet and reduce the risk of visual impairment and retinal diseases.
References
Chua, B., & Flood, V. (2006). Dietary fatty acids and the 5-year incidence of age-related maculopathy. Ophthalmology, 981-986.
Jastrzebska, B., Debinski, A., Filipek, S., & Palczewski, K. (2011). Role of membrane integrity on G protein-coupled receptors: Rhodopsin stability and function.
Jeffrey, B., Weisinger, H., Neuringer, M., & Mitchell, D. (2001). The role of docosahexaenoic acid in retinal function. Lipids, 859–871.
Querques, G., Forte, R., & Souied, E. (2011). Retina and Omega-3. Journal of Nutrition and Metabolism.
Sinclair, A., Guo, X.-F., & Abedin, L. (2022). Dietary Alpha-Linolenic Acid Supports High Retinal DHA Levels. Nutrients, 301.
Souied, E., Delcourt, C., Querques, G., Merle, B., Smith, T., & Benlian, P. ( 2013). NAT2 Study: Omega-3 Levels in red blood cell membranes correlates the preventive effect. Investigative Ophthalmology & Visual Science, 54.
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