Bad news for mayonnaise lovers. A pair of new studies from a Boston research team links this condiment, as well as certain vegetable oils, to an elevated risk of age-related cataracts. One of those studies, however, also suggests that oily fish, the type with dark flesh, hold cataracts at bay.
Age-related cataracts are the leading cause of blindness. Indeed, cataract surgery is the most frequent surgical procedure among U.S. Medicare recipients, collectively costing the federal government and private insurers more than $3 billion annually.
In their first study of women participating in a nearly-30-year-old diet-and-health study, the researchers link the primary fat in corn oil, linoleic acid, to incipient cataracts, called opacities, in the lenses of the women's eyes. In this study, this single fat accounted, on average, for roughly 5 percent of calories consumed each day, and more than 85 percent of polyunsaturated-fat intake.
Lens opacities are "a sensitive marker for the development of cataracts" but don't initially cause vision loss, notes Paul F. Jacques, director of nutritional epidemiology for the Agriculture Department's Jean Mayer Human Nutrition Research Center on Aging at Tufts University.
The second study, also by Jacques' team, included women who had undergone surgery for removal of cataracts. Compared with women their age who never had such a procedure, these women were much less likely to have eaten omega-3 fatty acids, the type found in many fish oils (see Omega-3's May Hit Food Labels). In this study, eating vegetable oils didn't appear to be linked with cataracts, but eating mayonnaise and salad dressings did.
The studies appear slightly contradictory, acknowledges Jacques. However, he adds, it might be that one type of fat is more likely to affect early stages of cataract susceptibility and the other has its most notable impact in the latter stages of cataract development. In any case, the findings are consistent with earlier work suggesting that fat consumption can affect a person's vulnerability to cataracts.
That makes sense because fatty substances affect the flexibility and permeability of cell membranes throughout the body. Cataracts are associated with changes in lens-membrane structure and function and, thus, could be affected by dietary fats.
For their initial study, the Boston researchers invited local women, ages 53 to 73, to have their eyes examined. All were participants in the ongoing Nurses Health Study based at Brigham and Women's Hospital in Boston and begun in 1976.
None of the 440 volunteers had a previous diagnosis of cataracts or of a disease such as diabetes that would have put them at high risk of cataracts. Roughly 45 percent of the women had clear lenses. Of the remainder, most had the first signs of cataracts in multiple types of lens tissue. Sixty percent of the women with opacities were developing a type that affects the lens' core, or nuclear region.
The researchers then evaluated the typical intake of various fats by these women, based on dietary questionnaires they had filled out at regular intervals since 1980. The women described how often they ate each of some 130 foods, and how much. Previous tests of this survey indicated that it yields accurate data on people' consumption of more than 90 percent of 70 key nutrients, including many fats.
In the April American Journal of Clinical Nutrition, the scientists report that women whose diets were richest in polyunsaturated fats were significantly more likely to have incipient nuclear cataracts, and those whose diets were lowest in this fat were least likely. A higher risk of early nuclear cataracts was also associated with diets high in vegetable oils and trans fats. This was a surprise, Jacques told Science News Online. Earlier studies in rodents had suggested that diets high in corn oil cut an animal's risk of cataracts.
No increased cataract risk was associated with intake of animal fats, which include saturated fats. Neither of the two other classes of incipient cataracts—opacities affecting the edges or back of the lens—showed a link to dietary fats of any type.
When the researchers focused on the women's consumption of various types of polyunsaturated fats, cataract risk appeared to be most associated with two fatty acids: linoleic and linolenic.
Linoleic acid is the major constituent of safflower, sunflower, corn, soybean, and cottonseed oils. Although linolenic acid is found in many plant oils, its highest concentrations tend to be in flax, soy, walnuts, and canola. Most dietary trans fats have been engineered by chemically altering such polyunsaturates to make them solid or semisolid at room temperature—ideal for margarines and shortenings.
The researchers urge caution before indicting linolenic acid for causing cataracts. First, two previous studies by other researchers found almost no linolenic in the eye's lens. Also, oils rich in this fat are smaller contributors to the U.S. diet than is linoleic-dominated corn oil. Moreover, linolenic acid occurs in many of the same sources as linoleic acid does, just in smaller quantities, making its cataract contribution hard to distinguish. Finally, the body tends to convert some of its linolenic into two long-chain omega-3 fats abundant in many fish oils: docosahexaenoic and eicosapentaenoic acids, better known as DHA and EPA, respectively.
In this study, the only major foods that actually stood out as apparent risk factors were mayonnaise and creamy salad dressings. These condiments tend to be rich in linoleic acid.
The second study by Jacques and his team also mined data from the Nurses Health Study, but in this case, dietary information supplied by some 71,000 women between 1984 and 2000. To be included for this analysis, a woman could not have had a cataract, cancer, or diabetes when the study began in 1976. After eliminating individuals with certain other potentially confounding conditions, the researchers found that 4,196 women in the group had cataracts diagnosed after age 45 and then had surgery. Roughly 60 percent of these cases involved nuclear cataracts.
As in the first study, Jacques' group assessed possible dietary contributions to cataracts by comparing the participants' typical intake of various types of fats.
One finding stood out, Jacques notes: Women who developed cataracts were far less likely to be big fish eaters.
Indeed, women who consumed dark-meat fish, such as salmon or tuna, at least once a week had a 15 percent lower risk of cataracts than did women eating fish less frequently. Dark-meat fish are the ones that tend to be especially rich in DHA and EPA.
Among foods associated with an increased risk of cataracts were mayonnaise, creamy salad dressings, cheese, beef, pork, and lamb.
Jacques says his team wants to see if it can verify its findings in a different population. It also wants to go a step beyond analyzing data on what women say they eat to actually measuring fatty acids in people's blood to see whether their abundance there correlates with a risk of cataracts.
Nutritional Epidemiology Program
Jean Mayer USDA Human Nutrition Research Center on Aging at Tufts University
711 Washington Street
Boston, MA 02111
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