Eating green leafy vegetables daily may decrease the risk of glaucoma, a serious eye disease, by 20 percent or more over many years, a new study suggests.
"We found those consuming the most green leafy vegetables had a 20 to 30 percent lower risk of glaucoma," said study leader Jae Kang.
Kang is an assistant professor of medicine at Brigham and Women's Hospital and Harvard Medical School in Boston.
Glaucoma is an eye condition that usually develops when fluid increases in the front part of the eye and causes pressure, damaging the optic nerve.
It can lead to loss of vision, according to the U.S. National Eye Institute.
Kang's team followed nearly 64,000 participants in the Nurses' Health Study from 1984 through 2012, and more than 41,000 participants in the Health Professionals Follow-up Study from 1986 through 2014.
The men and women were all 40 or older. None had glaucoma at the start of the study, and they had eye exams every two years.
Over the 25-year follow up, almost 1,500 people developed glaucoma. The researchers looked at the consumption of green leafy vegetables among the participants.
The investigators divided the participants into five groups, from the highest level of leafy green vegetable consumption to the lowest. Those who ate the most averaged about 1.5 servings a day, or about one and a half cups a day, Kang said.
Those in the group eating the least leafy greens ate about a serving every three days, according to Kang.
What is it about leafy greens that may help eye health?
"In glaucoma, we think there is an impairment of blood flow to the optic nerve," Kang said. "And an important factor that regulates blood flow to the eye is a substance called nitric oxide." Green leafy vegetables contain nitrates, which are precursors to nitric oxide, the researchers said.
"When you consume a higher amount of green leafy vegetables, you have greater levels of nitric oxide in your body," Kang said.
Findings from the study were published online in the journal JAMA Ophthalmology.
The findings make sense, said Dr Rahul Pandit, an ophthalmologist at Houston Methodist Hospital, who reviewed the new research.
Read: Glaucoma in a nutshell
This study, he said, is the first study to look at a large population and show that higher consumption of green leafy vegetables appears to decrease glaucoma risk.
"We do have some data that people with glaucoma have impaired nitric oxide production in the eye," added Pandit, who is also an associate professor of ophthalmology at Weill Cornell Medical College in New York City.
The findings suggest that "maybe this is something we can apply clinically", Pandit said. The advice to eat more green leafy vegetables seems low risk, Pandit said. He suggested people ask their doctor whether eating and increasing green leafy vegetables is a good idea for them.