Effectiveness of dietary inorganic nitrate for lowering blood pressure in hypertensive adults: a systematic review.

OBJECTIVE: The objective of this review was to evaluate the effectiveness of inorganic nitrate on blood pressure in hypertensive adults.

INTRODUCTION: Hypertension is associated with increased risk of morbidity and mortality in adults. Inorganic nitrate could be beneficial for lowering blood pressure and reducing cardiovascular disease risks. Evidence related to the treatment of hypertension through sources of inorganic nitrate has been presented.

INCLUSION CRITERIA: The review considered studies on adults aged 18 years and over, with blood pressure greater than 120/80 mmHg, undergoing interventions focusing on the effects of inorganic nitrate on blood pressure. Studies that included inorganic nitrate intake via dietary modification, in the form of a dietary supplement, and/or by the consumption of beetroot juice were considered. The comparator was no intervention of inorganic nitrate; different dosage, frequency, duration of inorganic nitrate; and other interventions that are administered to reduce and manage blood pressure. The primary outcomes were systolic and diastolic blood pressure effects. Experimental, quasi-experimental, analytical observational and pilot study designs were considered for inclusion.

METHODS: Databases were searched for published and unpublished studies, available in English, from January 2013 to January 2018. Critical appraisal was conducted using standardized instruments from the Joanna Briggs Institute (JBI) and the methodological quality of included studies was considered to be moderate. Data were extracted using the JBI data extraction instrument. Data were presented in a narrative form due to the heterogeneity of included studies.

RESULTS: Twelve papers were included in the systematic review with a total of 321 participants. Ten were randomized controlled trials and two were quasi-experimental studies. All participants had baseline blood pressures greater than 120/80 mmHg. Some studies included participants with comorbidities such as diabetes or heart failure with preserved ejection fraction. Inorganic nitrate was administered multiple ways throughout the studies including the following: beetroot juice, beetroot gel, nitric oxide lozenge, high nitrate diet, and raw and cooked beet juice. Doses and treatment intervals varied. Some studies included exercise as part of the intervention protocol.Meta-analysis was not conducted due to heterogeneity that existed within the studies. Blood pressure was measured in multiple settings by manual, digital or ambulatory means. The noted outcome patterns were as follows: no change in systolic blood pressure and diastolic blood pressure, decrease in systolic blood pressure and diastolic blood pressure, or decrease in systolic blood pressure with no change in diastolic blood pressure. Possible reasons for the diverse findings include the following: age, comorbidities, use of antihypertensives by participants; source and dose of nitrate; and intervention and follow-up time frames.

CONCLUSIONS: There is insufficient evidence to support or refute the use of inorganic nitrate for any effect on blood pressure at this time. Therefore, there is no concrete base for the development of practice guidelines until stronger evidence becomes available. The gaps in the literature along with the study limitations identified necessitate the need for more research on inorganic nitrate and how it relates to blood pressure.