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Researchers determine if nitrate-rich vegetables lower high blood pressure in participants with prehypertension or untreated grade 1 hypertension.

High blood pressure, also called hypertension, affects millions of people worldwide. In the United States alone, hypertension affects 75 million people. Globally, there is an importance placed on sustaining a healthy blood pressure to prevent hypertension. This is because hypertension is the leading cause of heart attack and stroke.

Inorganic nitrate and nitric oxide are important to heart health. Inorganic nitrate forms nitric oxide, and nitric oxide widens blood cells. An increased intake of nitrate can allow the rebuilding of nitric oxide within the body.

In a recent study published by the American Journal of Clinical Nutrition, researchers in Australia examined 30 participants to see if ingesting nitrate-rich vegetables would lower high blood pressure. This study included men and women between the ages of 21 and 75 years old.

The study was a randomized, controlled crossover trial where participants underwent three four-week treatment periods. Each four-week treatment period was followed by a four-week resting phase, resulting in a total study length of 24 weeks. One treatment period required a consumption of high-nitrate vegetables, another required low-nitrate vegetables, and a third (control treatment) did not require any increase in vegetables.

Before and after treatment, researchers gathered measurements regarding the participants’ height, weight, waist circumference, and hip circumference. They conducted a variety of tests such as an electrocardiogram, blood pressure tests, fasting blood test, fasting saliva test, fasting urine test, a medical examination, and a food-frequency questionnaire. They also assessed arterial stiffness before and after treatment. Participants also provided researchers with any medications or supplements they were taking at the time, as well as a detailed medical background.

Participants experienced minor changes to their diets. During the high-nitrate and low-nitrate periods, they consumed mixed vegetable juices with breakfast and dinner. The high-nitrate treatment required participants to consume a daily amount of 150mg of nitrate, while the low-nitrate treatment required 22mg of nitrate.

During the control period, they drank water blended with one-quarter of an orange and eight grams of maltodextrin with breakfast and dinner. For the control group, participants took 6mg of nitrate. With the use of food diaries, researchers saw that participants were following the treatment periods correctly. Digestion of nitrate and nitrite was observed through blood, saliva, and urine samples.

Although overall nitrate, nitrite, and carotenoid levels increased within participants, the findings show that increasing nitrate-rich vegetables did not lower blood pressure, nor did it pose a change in arterial stiffness.

The authors mention that there are many trials that have tested this hypothesis, but varying results continue to show. While some studies have discovered lower blood pressure, others have found no changes whatsoever. In the studies that have shown a lower blood pressure, there are possible factors that could have influenced these outcomes. These factors include the amount of nitrate consumed by participants, pre-nitrate levels, pre-vegetable consumption, as well as participants’ age and overall wellbeing.

For this study, the authors note potential factors as to why blood pressure did not lower. If participants were already receiving a sufficient amount of nitrate prior to partaking, the extra nitrate might not have been metabolised properly. Also, the 12-hour fast participants did prior to providing blood, saliva, and urine samples could have affected salivary and urinary nitrate concentrations. Although these factors are kept in mind, this study had a long time period, and no participants were on antihypertensive medications, making it a reliable and unique trial.

Written by Laura Laroche, HBASc, Medical Writer

References:

(1) Blekkenhorst C, Lauren, et al. “Nitrate-rich vegetables do not lower blood pressure in individuals with mildly elevated blood pressure: a 4-wk randomized controlled crossover trial.” ASN. 2018. 107: 894-908. Online.
(2) Centers for Disease Control and Prevention: Blood Pressure. 2018, https://www.cdc.gov/bloodpressure/index.htm, assessed 7 June. 2018.

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