Summer 2018

Eating for Healthy Eyes

Why complex carbs are the clear choice for preserving vision.

By David Levin

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Illustration: Peter Crowther

Age-related macular degeneration (AMD)—the leading cause of vision loss after age 50—can leave a person feeling powerless. Over months or years, AMD patients slowly lose their sight, moving ever closer to blindness. In most cases, there’s no cure, but a team at Tufts has found signs that arresting the disease may not require creating new drugs, but simply tweaking patients’ diets.

Sheldon Rowan, assistant professor of ophthalmology at the School of Medicine and a scientist in the Laboratory for Nutrition and Vision Research at the Human Nutrition Research Center on Aging at Tufts, said there are plenty of indications that the types of carbohydrates we eat play a role in the development of AMD. People who eat lots of simple carbohydrates, like those in white bread and sweetened beverages, are more likely to get the disease. This could be because simple carbs break down rapidly during digestion, creating a spike in blood sugar that can lead to widespread inflammation, a condition linked to AMD. Complex carbohydrates found in whole grains, however, break down more slowly, resulting in lower blood glucose. If blood glucose stays low over a long period, Rowan said, it can lower incidence of AMD.

To understand why, Rowan tested the two diets on laboratory mice. Over a year, he fed one group of mice “high-glycemic” foods—ones with lots of simple starches. A second group got a “low-glycemic” diet, rich in complex carbs, but otherwise identical in calories and nutrients. In a third group, Rowan switched the mice’s diet from high- to low-glycemic foods halfway through the study. Sure enough, mice with the low-glycemic diet did not develop AMD, while mice fed the high-glycemic diet almost all came down with the disease, a result in keeping with previous research. In the mice that switched diets, though, Rowan saw something completely unexpected: Not only did they avoid AMD, but the existing damage to their retinas was reversed. “No one had ever seen that before,” Rowan said of the findings, which were reported in Proceedings of the National Academy of Sciences. “This suggests that just changing to a healthier eating pattern could have a huge impact.”

Rowan found that mice with high-glycemic diets also had high levels of molecules called “advanced glycation end products” (AGEs) in their bodies. “AGEs are toxic end products of sugars,” Rowan said. “They can damage the proteins and lipids that a cell needs to function.” In the retina, these damaged proteins slowly accumulate in a sort of cellular garbage pile, forming yellow deposits called drusen, which destroy retinal cells.

Rowan also identified certain chemicals, including serotonin, in the mice’s blood and urine that acted as markers for AMD. Because these chemicals have been linked to bacteria in the gut, Rowan wondered how microbes might be involved in AMD development. He reasoned that because simple carbohydrates are easy to digest, they are fully metabolized before entering the intestines and effectively “starve” microbes. This could discourage the growth of protective bacteria, while allowing species that create inflammation and other stresses to thrive. After testing the levels of various bacteria living in each mouse’s gut, Rowan found some early evidence that may confirm this idea.

“There could be ‘good’ bacteria in the gut that are neuroprotective, and there could also be ‘bad’ bacteria that are proinflammatory,” he said. “From this study, we can’t parse out good versus bad, but it does show us that molecules associated with gut bacteria are playing a role in AMD.”

Identifying those molecules could one day lead to new drug treatments. But until then, AMD patients may be able to improve their vision just by switching up their diet. “This gives us a tremendous opportunity,” said Tufts biochemist Allen Taylor, director of the Laboratory for Nutrition and Vision Research. “In humans, this is the equivalent of switching out four or five slices of white bread each day for whole grains. It’s a minor alteration that will pay great benefits.”

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