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 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.
By 2012, there were a total of 1,483 cases of open-angle glaucoma
diagnosed.
Risk differences based on nitrate consumption were very similar to
those based on leafy-vegetable consumption.
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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.”
SOURCE: http://bit.ly/1SpG32n JAMA Ophthalmology, online January 14,
2016.
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