Based on long-term data for more than 100,000 U.S. adults, those who consumed the most nitrate - mostly from green vegetables like kale and spinach - were 21 percent less likely than those who ate the least nitrate to develop open-angle glaucoma by the time they were in their 60s and 70s.
Open-angle glaucoma, which affects about 1 percent of the U.S. population, usually starts with loss of vision at the periphery due to fluid build-up and optic nerve damage.
Impaired blood flow is implicated in the condition, the study team points out in JAMA Ophthalmology, and nitrates can be converted in the body to nitric oxide, which improves blood flow.
"Nitric oxide signaling is important for maintaining optimal blood flow, and some evidence suggests that it may also be important for keeping eye pressure low," said lead author Jae H. Kang of Brigham and Women's Hospital and Harvard Medical School in Boston.
The researchers used data on more than 63,000 women followed from 1984 to 2012 in the Nurses' Health Study, and more than 41,000 men in the Health Professionals Follow-up Study from 1986 to 2012.
The participants were over age 40 at the beginning of the study period, had no open-angle glaucoma to start with, reported regular eye exams and completed dietary questionnaires including how often they ate green leafy vegetables like iceberg and romaine lettuces, kale, mustard greens, chard or spinach.
Based on these questionnaires, researchers calculated intakes of nitrate and of various groups of foods. They found that dark leafy greens were the biggest source of the nutrient, contributing 57 percent of the nitrate in the participants' diets.
The men and women were divided into five groups by their intake of greens and of nitrate, ranging from an average of one-third serving of leafy greens (80 milligrams nitrate) per day for the lowest-level consumers to an average one and a half servings of greens (240 milligrams nitrate) per day on the high end.
Risk differences based on nitrate consumption were very similar to those based on leafy-vegetable consumption.
People who ate the most leafy greens were 18 percent less likely than those who consumed the least greens to develop any form of open-angle glaucoma, and 48 percent less likely to develop the so-called paracentral form of the disease, which is particularly associated with blood flow, the authors note.
In the study, people who consumed more nitrate also had higher consumption of other nutrients, exercised more, smoked less and were leaner, but the authors adjusted for these factors when calculating the risk differences linked to nitrate and greens.
"The pro of increasing one's dietary nitrate intake is that, by far, vegetables are the biggest source of dietary nitrate, and vegetables are part of a healthy diet," Kang told Reuters Health by email. "Higher dietary nitrate intake has been linked to lower blood pressure, better blood circulation and better athletic performance."
But some people, like those with kidney stones or those taking warfarin to prevent blood clots, need to avoid foods like spinach, kale and collard greens, Kang cautioned.
Glaucoma is a "silent" disease and usually does not cause symptoms or visual complaints until late in its development. African Americans and the elderly are at increased risk of glaucoma.
"This is the first study to evaluate dietary nitrate in relation to glaucoma, so this study does not establish cause and effect relations," Kang said. "However, for overall health, increased consumption of vegetables, including green leafy vegetables, is important - only about 20 to 30 percent of adults meet the daily recommendation put forth in the U.S. dietary guidelines of 2 to 3.5 cups of vegetables per day."