Could nitrates in leafy greens prevent glaucoma?
Glaucoma is a leading cause of blindness; even if treatment is received, 10% of people with the condition will eventually lose their sight.
Unfortunately, as it stands, there is no cure; to make matters worse, there are no early symptoms that might assist in catching the disease in its developmental stage.
This most recent investigation shows that an increase in dietary nitrate and green vegetables significantly reduces the risk of primary open-angle glaucoma (POAG).
POAG is the most common form of glaucoma and is characterized by an increase in eye pressure and a gradual loss of vision.
Researcher Jae H. Kang led the investigative team based at Brigham & Women's Hospital and Harvard Medical School in Boston, MA.
The group used data from the Nurses' Health Study and the Health Professionals Follow-up Study.
In total, the analysis investigated 63,893 women (1984-2012) and 41,094 men (1986-2012). All participants were over the age of 40 and had no history of POAG. For the duration of the follow-up, 1,483 cases of POAG occurred in the sample.
Primary open-angle glaucoma
POAG is caused by a slow clogging of the eye's drainage canals that slowly increases the pressure within the eye. "Open-angle" refers to the angle at the point where the cornea meets the iris. In POAG, this angle is normal. For angle-closure glaucoma - a rarer version of glaucoma - the angle is reduced.
An impaired autoregulation of the blood flow in the optic nerve has also been implicated in POAG. The vascular endothelium (the lining of the circulatory system) helps regulate this blood flow and is considered to play a role in the etiology of glaucoma. One of the factors that impacts the functioning of the vascular endothelium is nitrous oxide (NO).
Dietary nitrate, predominantly derived from green, leafy vegetables, is converted to NO and has been shown to be beneficial for blood circulation. Kang and her team wanted to see if an increase in nitrate might have a positive effect on glaucoma.
Nitrates reducing glaucoma
In the present study, every 2 years, the group's diets were assessed for their green vegetable intake, and their nitrate intake was calculated. The participants were split into five groups accordingly; the highest group consumed roughly 240 mg of nitrate per day and the lowest approximately 80 mg per day.
The researchers found that the group consuming the most nitrogen had a 20-30% lower risk of POAG.
The results were even more substantial for a type of glaucoma associated with a dysfunction of blood flow autoregulation, known as POAG with early paracentral visual field loss. In this case, nitrogen-rich diets lessened the chances of developing the disease by 40-50%.
The study's authors recognize that cause and effect is impossible to infer from this type of study, but they also recognize the significance of the findings:
"These results, if confirmed in observational and intervention studies, could have important public health implications."
Nitrate (NO3-) is a varied chemical with a host of seemingly contradictory roles. Millions of kilograms of nitrate are produced yearly as fertilizer; it is also used as an oxidizing agent in explosives. Another, albeit less common, use of nitrate is during the curing of meats.
For the purpose of this article, our primary interest is nitrate's potential benefit on cardiovascular health. Dietary nitrates are thought to help keep oxygen levels high in the blood and consequently reduce the thickness of the blood, lowering blood pressure and preventing potentially dangerous clotting.
Although there are numerous nitrate-rich vegetables to choose from, in the current study, only two vegetables were robustly associated with a drop in glaucoma:
"The only vegetable that was consistently inversely associated with POAG was kale or collard greens: 1 serving or more per month of kale or collard greens was significantly associated with 55% to 70% reduced odds of POAG."
If you are considering increasing your nitrate intake through vegetables, these are some of the best sources:
- Collard greens
- Green beans
The evidence for the benefits of nitrates and nitrites on human health, and particularly glaucoma, are steadily mounting. Whatever their overall effects, there are plenty of good reasons to up the number of vegetables in your diet. Medical News Today recently covered a study showing that drinking beetroot juice reduces high blood pressure.