Drinking more than four cans of diet soda per week may cause a type of diabetic eye disease that leads to blindness, according to a study.

The risk of developing diabetic retinopathy is “twofold,” Eva Fenwick, Ph. D, an author of the study, told Medscape Medical News.

The study was published in Clinical and Experimental Ophthalmology. Fenwick is a clinical research fellow at the Singapore Eye Research Institute and an assistant professor at the Duke-NUS Medical School, Singapore.

People in the study who drank regular soft drinks with sugar did not experience the same result.

The study included 609 adults. Seventy three of them had type 1 diabetes, 510 had type 2 diabetes and 26 had an unknown type of diabetes.

Diabetic retinopathy is a condition that damages the retina. Small blood vessels keep the retina healthy but if blood glucose levels are high for too long it can block those blood vessels.

That leads to scar tissue forming and impaired eyesight, according to WebMd.com.