FISH, OILS, AND VISION
Martha Neuringer, PhD, assistant professor, and William E. Connor, MD, professor at Oregon Health Sciences University, have researched and written about the good effects on vision of certain fatty acids, particularly DHA,1 found in fish. The fatty acids consist of long chains of carbon atoms.2 Scientists often speak of omega-3 fatty acids as n-3 fatty acids and omega-6 fatty acids as n-6 fatty acids. Here are some fatty acids from the two series and some of their food sources:
eicosapentaenoic acid (20:5) "EPA"
docosahexaenoic acid (22:6) "DHA"
gamma-linolenic acid (18:3)
arachidonic acid (20:4)
The first number in each parenthesis refers to the number of carbon atoms in the chain; the second number refers to the number of unsaturated centers. (Note the confusing similarities among several of these names: "Many fatty acids were given trivial names before their structures were fully known, and these are so fully embedded in the literature that their use can hardly be avoided"3)
The point for us to remember about these two series of fatty acids is that no animal, including humans, can synthesize any fatty acid with a double bond at either the n-3 or the n-6 position; consequently, we are completely dependent on dietary sources for these fatty acids. The shortest-chain fatty acid in each series, linoleic acid (18-2, n-6) and alpha-linolenic acid (18:3, n-3), is present is plants. Aquatic plants are also able to form some of the longer-chain n-3 fatty acids, and fish, at the apex of that food chain, are an excellent source of both EPA and DHA. Most animals, including humans, have enzymes for making the longer, more unsaturated fatty acids in each series. Arachidonic acid (20:4, n-6) is a major component of most membrane lipids. DHA (22:6, n-3) is a major component of the lipids in the retina, cerebral cortex, testis, and sperm.
There is evidence that the enzymes for producing these long-chain fatty acids are not yet active in our newborn infants. Professors Neuringer and Connor point out that human milk contains DHA, whereas our present infant formulas contain none, and that infants raised entirely on formula do not increase the DHA in the gray matter of the brains, while those receiving human milk are increasing their DHA.
Professors Neuringer and Connor studied the depletion of n-3 fatty acids in rhesus monkeys whose only dietary fat was safflower oil, which has a "n-6/n-3 ratio of 255:1." The monkeys’ mothers had also received this deficient diet throughout gestation. The control group received soybean oil, relatively rich in alpha-linolenic acid "(n-6/n-3 ratio of 7:1)"4 The levels of DHA in the retinas of the n-3 depleted monkeys fell to half the levels in controls at birth and to 20 percent at two years, with compensating increases in 22-carbon chain n-6 fatty acids. The researchers demonstrated vision abnormalities in the n-3 fatty acid deficient monkeys, which adapted more slowly than the control monkeys after a flash of light. Also, their visual acuity was down by 25 percent at 4 weeks and by 50 percent at 8 and 12 weeks. "This study was the first direct demonstration of visual impairment in n-3 fatty-acid deficient animals, and provided strong evidence that these fatty acids are important for normal visual development in a primate species with a retina and visual system very similar to their human counterparts. The deficit in visual acuity may have been due entirely to effects on the retina, or may also have been mediated by changes in the central visual system"5 in the brain.
The researchers demonstrated the reversibility of the depletion of DHA in the cerebral cortex of four young rhesus monkeys by replacing most of the safflower oil in their diet with a fish oil rich in EPA and DHA. Within 12 weeks the amount of DHA had attained the levels in the control animals, which suggests that "when a dietary source becomes available, DHA is avidly incorporated. Therefore, it is possible to alter brain fatty acid composition by dietary means, even in animals well past the period of rapid brain development."6 However, when the researchers tested the retinas of these monkeys, the abnormalities produced by the earlier deficiency had not disappeared.
Fatty acids need to make up only 1 or 2 percent of our total diet. However, the ratio between n-6 and n-3 fatty acids is very important because the two series compete for the same enzyme system to lengthen and desaturate the carbon chains. Oils such as safflower, sunflower, and peanut, which are extremely high in n-6 fatty acids, have been used one at a time to deplete animals of DHA. Professors Neuringer and Connor conclude, "In view of the findings on retinal function, it seems prudent to recommend that the human diet provide an adequate amount of n-3 fatty acids and an n-6/n-3 ratio of 4:1 to 10:1, particularly during pregnancy, lactation, and infancy."7 In general, then, it would be wise to use a variety of oils and to eat plenty of ocean fish.
1 Neuringer, Martha, and William E. Connor,
"n-3 Fatty Acids in the Brain and Retina: Evidence for their
Essentiality," Nutrition Reviews, Vol. 44, No. 9, September 1986,