A large scale, detailed medical study showed a positive correlation between a person's fat intake and their level of macular degeneration. Macular degeneration occurs when a part of the eye, called the macula, deteriorates. This causes many problems, such as losing the ability to distinguish fine details, night-blindness, and blurry or distorted vision. Since symptoms occur with age, it's very important to establish causes of macular degeneration such that people can exercise preventative care. It's been shown that diets rich in fat, especially certain types of fat, have a high probability of increasing one's risk of developing this type of eye condition.
Over a period of twelve years, medical researchers at Tufts University followed nearly 90,000 people 50 years of age or older. They kept track of their detailed fat intake as well as measuring their macular degeneration and published their results in 2001. In general, they found that people who got 42% or more of their calories from fat were more likely to develop macular degeneration. Overall, they recommended that people over 50 years old keep their calories from fat down to 20-25%. But the study also showed differences in the various kinds of fat intake, such as saturated versus polyunsaturated and trans fat, to cause diverse connections to macular degeneration.
This study found some fats especially "bad." Diets rich in trans fats, such as baked commercial goods, and dairy were approximately 225% more likely to cause macular degeneration. Surprisingly small portions of these foods proved dangerous. Just one serving a day of things like ice-cream, bacon, chips, whole milk, hot dogs, cookies, sausage, butter, or cake, were unhealthy. Unprocessed meat, like steak, lamb, or pork, need only be eaten once a week before the subjects' risk of macular degeneration increased by 35%. These foods are examples of polyunsaturated fats that include the omega-6 fatty acid.
However, there is no connection between macular degeneration and "good" fat intake that includes the omega-3 fatty acid, or "bad" saturated fat. Safe fat alternatives include fish, such as canned tuna or fresh salmon, and nuts. These can be eaten almost every day with no observable detriment to macular health. Researchers do not understand why saturated fat, which is dangerous to cardiovascular health under other circumstances, was not connected to macular degeneration. Of course, even though this study is rigorous, nutritionists are still learning about what makes some fats beneficial and others detrimental.