Age-related macular degeneration (AMD) refers to the slow deterioration of the cells in the macula, a tiny yellowish area near the centre of the retina, which contains light-sensitive cells that send visual signals to the brain. Sharp, clear, ‘straight ahead’ or central vision - used mostly for reading, writing, driving and identifying faces - is processed by the macula. In the most severe forms of AMD, straight lines become crooked and wavy, distinct shapes are blurry and there is a fog in the centre of your vision. Your peripheral vision, however, is not affected.
Of all the illnesses of the ageing eye - including glaucoma and cataracts - AMD is the only one that is sharply on the rise. In the UK, registrations for AMD have burgeoned by 30-40 per cent. Worldwide, some 30 million people have the condition - a figure that is expected to treble over the next 25 years (Bull World Health Org, 1995; 73: 115-21) - and six million Americans have vision loss because of AMD, with another 13-15 million suffering from early signs of it.
Given this new epidemic, new treatments for AMD are always being explored, including retinal cell transplants, drugs that will prevent or slow the progress of the disease, laser treatment, radiation therapy, gene therapy and even a computer chip implanted in the retina that may help simulate vision.
But what medicine has seldom explored is the role of diet in the development of this epidemic or, indeed, its parallels with heart disease. New evidence places the blame squarely at the door of processed food, particularly processed fats.
The role of fats
Recently, a group of researchers from Harvard Medical School and the Harvard School of Public Health set out to determine whether diet had any affect on the development of AMD. They selected 261 participants, aged 60 or older, with early or intermediate AMD and visual acuity of 20/200 in at least one eye. Over the next four and a half years, the researchers studied the participants’ dietary intake and compared it with the progression of their disease. Specifically, they looked at the amount and type of fat the patients were consuming in their daily diets (Arch Ophthalmol, 2003; 121: 1728-37).
What they found was quite extraordinary. Those consuming high-fat diets were three times as likely to progress to advanced forms of AMD compared with those whose intake of fat was lowest.
But the risks relating to the kinds of fats consumed confounded the usual expectations. Although intake of any animal fat was associated with a doubling of risk of the disease, higher levels of animal-fat intake did not increase the risk any further. In other words, you increase your risk of developing AMD by eating flesh foods, but your risk doesn’t increase with the quantity of meat that you eat.
The real risk for AMD was associated with vegetable-fat intake. Consuming high levels of these types of fats nearly quadrupled the risk of the disease progressing. These fats included the monounsaturated, polyunsaturated and trans unsaturated fats. And in this case, quantity did matter. The more of these you ate, the greater your risk.
The researchers also made another connection that is most unusual in these types of studies. They noted a doubling of risk with intake of processed foods, which are usually laden with these types of processed vegetable fats.
Other kinds of fats proved protective. Fish and nuts, both rich in omega-3 fatty acids, slowed progression of the disease - so long as your intake of the usual omega-6 fatty acids was also low.