The body’s highest concentration of DHA (docosahexaenoic acid), a long-chain omega-3 fatty acid found mainly in fish, is in the retina. Concentrations of DHA may be as high as 65%. Why is this molecule so important in the retina?
DHA is a critical part of retinal structure. Its presence enhances the development of photoreceptors, specialized cells in the retina necessary for vision. High DHA concentrations are needed for rhodopsin-a pigment in the photoreceptor rod cells-to respond to light in a way that permits vision in dim light and at night. The highly unsaturated nature of DHA has unique effects on retinal cell membranes allowing them to transmit light signals very quickly.
In aging, visual function usually declines owing to changes in retina and other eye cells. Cell membranes lose some of their fluidity, cell structure changes, deposits accumulate, oxidation causes damage and cells are lost. These changes contribute to impaired vision in later life.
In age-related macular degeneration (AMD), yellowish deposits (drusen) accumulate in the macula, the central region of the retina (Illus). Cells in the macula break down and vision becomes distorted and blurry. Vision loss may develop. Drusen may develop into advanced forms of AMD, threatening the ability to see. As the most common or dry type of AMD progresses, the macular cells break down. Finally, the macular cells break down and central vision becomes severely impaired.
A second type of AMD, known as wet or neovascular AMD, is responsible for 90% of the vision loss in this condition. In wet AMD, the blood vessels behind the eye become abnormal and fragile, risking leakage and hemorrhage. Advanced AMD may affect one or both eyes and either wet or dry type can lead to blindness.
Some research suggests that AMD occurs less frequently in people with higher fish or DHA intakes, but others have found no association between AMD and omega-3 consumption. More recently, fish consumption twice a week or more was linked to significantly lower risk of AMD in a study of twins. Smoking increased the risk nearly 2-fold. Interestingly, in patients with advanced AMD, higher omega-3 PUFA consumption was associated with lower risk of the condition, but only when omega-6 or vegetable PUFA consumption was low. People with higher vegetable fat or linoleic acid (an omega-6 PUFA) intakes were more likely to develop AMD. These observations were also confirmed in patients with early or intermediate stages of the disease. People with higher fish intakes and low linoleic acid consumption had significantly slower disease progression than patients who consumed large amounts of vegetable fat, or processed baked goods.
These studies suggest that the type of fat we consume can reduce or increase the chance of developing AMD. High intakes of polyunsaturated vegetable oils may increase the chance of developing the condition and undermine the effects of fish oil omega-3s. Boosting fish and fish oil consumption may lower the chance of developing AMD and slow its progress once it has started. Whether omega-3s can prevent the condition we do not know. Because omega-3s appear promising, the National Eye Institute is conducting a large clinical trial in the U.S. to see whether fish oil omega-3s in combination with high levels of antioxidant vitamins and select minerals can retard the progress of AMD in people with intermediate or advanced stages of the condition in one eye. It will be more than 5 years, however, before the study is completed.
A referenced version of this article is available from the editor.