How much fat you eat - and what kind - may affect your risk of age-related vision loss, new research shows.

The researchers studied the role of dietary fat in age-related macular degeneration (AMD), the leading cause of blindness in the US and the third leading cause worldwide. AMD is caused by abnormal blood vessel growth behind the retina or breakdown of certain cells within the retina itself.

Cigarette smoking and heart disease both influence AMD risk, but the role of diet is less clear, Dr. Niyati Parekh of New York University in New York City and her colleagues with the CAREDS Research Study Group note. The CAREDS study - which stands for Carotenoids in Age-Related Eye Disease Study -is an offshoot of the Women’s Health Initiative, a major 15-year study of postmenopausal women’s health.

Few studies investigating fat intake and AMD have looked at the earlier stages of the disease, or evaluated diet before AMD diagnosis. The CAREDS data allowed the researchers to do both.

They looked at 1,787 women who were 50 to 79 years old when they entered the study, in 1994. All had reported their fat intake for 1994 through 1998, and all were tested for AMD between 2001 and 2004.

Overall, total fat intake had no influence on AMD risk. But when the researchers looked at women based on age, they found that for women younger than 75, those in the top fifth based on their fat intake were at 70 percent greater risk of intermediate AMD than those in the bottom fifth. The reverse was true in women 75 and older, with those who consumed the most fat at 50 percent lower AMD risk than those who consumed the least.

When the researchers looked at saturated fat, they found that higher intake boosted AMD risk in women younger than 75, but not in older women.

Intake of omega-6 fatty acids, chiefly found in vegetable oils, was closely linked to omega-3 intake, with high consumption of both fatty acid types roughly doubling AMD risk. But higher monounsaturated fat intake was associated with lower AMD risk.

The effects of high fat diets could be due in part to the fact that these diets are likely to be deficient in certain nutrients that could be protective against AMD, the researchers note. Also, they add, omega-6 fatty acids can increase inflammation. The fact that omega-3s, which have been shown in other studies to help protect against AMD, weren’t protective in the current study may have been because they were consumed in association with too much omega-6, the researchers suggest.

“Despite the results of the present study, the larger body of epidemiological evidence suggests that the intake of long chain omega-3 PUFAs and/or fish is related to a lower risk of AMD,” they write.

Despite the complexity of the current findings, the researchers say, the results point to two conclusions: high total fat intake can affect the risk of AMD, and high omega-6 intake may be particularly harmful, to the extent that it can “mask” any positive effects of omega-3 intake.

SOURCE: Archives of Ophthalmology, November 2009.