The body’s highest concentration of DHA (docosahexaenoic acid), a long-chain omega-3 fatty acid found mainly in fish, is found 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 young infants, DHA affects the eye’s ability to distinguish fine spatial detail such as closely spaced lines, known as visual resolution acuity. Infants who do not obtain enough DHA during fetal development (pregnancy) have sub-optimal visual acuity and less DHA in their retinas. These differences are especially notable in infants born preterm. In term infants, differences in visual acuity are less consistent, in part because term infants have some DHA in their body fat. However, some studies have reported better visual acuity in term infants fed formula with added long-chain omega-3 fatty acids. Both breast milk and most infant formula contain DHA.
As children grow older, the reduced visual acuity when DHA is insufficient disappears. However, some believe that early deprivation may have lasting effects on visual function later in life. This idea is supported by the finding that infant monkeys deprived of omega-3 fatty acids in fetal life failed to achieve the DHA content reached by animals that received omega-3s in fetal life, even though the deprived animals received sufficient omega-3s after birth and for the first 3 years of life.
It may benefit visual development, if infants continue to receive some DHA throughout their first year. Although the maturation of visual acuity slows in the second six months of life, one-year old infants who were fed DHA-rich eggs from age 6 to 12 months had improved visual acuity compared with infants who had no dietary DHA. This finding suggests that children’s visual acuity continues to benefit from dietary DHA throughout the first year of life.
We do not know whether the early improvements in infant visual acuity associated with adequate DHA has long-lasting effects. Some have suggested it might have advantages in cognitive development and in visual function later in life. The foundation for healthy eyesight begins by ensuring that pregnant women and young infants have sufficient DHA in their diet.
It should be noted that the omega-3 fatty acid found in plants, alpha-linolenic acid, does not provide enough DHA for optimum retinal development during pregnancy because too little is converted to DHA. For optimum visual development in early life, the mother needs a dietary source of DHA. This may be DHA-rich foods, such as fatty fish, DHA-rich eggs or fish oil-fortified foods, or fish oil supplements. DHA appears to improve visual acuity in infants who eat foods containing DHA for the first year of life, though these findings need additional confirmation. Fortunately, the retina hangs on to its DHA tightly. Having adequate DHA in pregnancy and at least the first year of life lays the foundation for good life-long vision.
A fully referenced version of this article is available from the editor.