Objectives To examine knowledge and beliefs about the biological roles of dietary inorganic nitrate in UK-based nutrition professionals, and to explore potential differences by participants’ education level.
Setting An online questionnaire was administered to UK-based nutrition professionals, exploring knowledge and/or beliefs across five areas: (1) health and performance effects of nitrate; (2) current and recommended intake values for nitrate; (3) dietary sources of nitrate; (4) methods of evaluating nitrate intake and (5) nitrate metabolism.
Participants One hundred and twenty-five nutrition professionals.
Primary outcome Knowledge and beliefs about inorganic nitrate.
Results Most nutrition professionals taking part in the survey had previously heard of inorganic nitrate (71%) and perceived it to be primarily beneficial (51%). The majority believed that nitrate consumption can improve sports performance (59%) and reduce blood pressure (54%), but were unsure about effects on cognitive function (71%), kidney function (80%) and cancer risk (70%). Knowledge of dietary sources of nitrate and factors affecting its content in food were generally good (41%–79% of participants providing correct answers). However, most participants were unsure of the average population intake (65%) and the acceptable daily intake (64%) of nitrate. Most participants (65%) recognised at least one compound (ie, nitric oxide or nitrosamines) that is derived from dietary nitrate in the body. Knowledge of nitrate, quantified by a 23-point index created by summing correct responses, was greater in individuals with a PhD (p=0.01; median (IQR)=13 (9–17)) and tended to be better in respondents with a masters degree (p=0.054; 13 (8–15)) compared with undergraduate-level qualifications (10 (2–14)).
Conclusions UK-based nutrition professionals demonstrated mixed knowledge about the physiology of dietary nitrate, which was better in participants with higher education. More efficient dissemination of current knowledge about inorganic nitrate and its effects on health to nutrition professionals will support them to make more informed recommendations about consumption of this compound.
- nitric oxide
- nutritional education
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Strengths and limitations of this study
This is the first study to explore knowledge and beliefs around dietary inorganic nitrate in UK-based nutrition professionals.
We provide a new questionnaire that may be used in future investigations to evaluate knowledge and beliefs of dietary inorganic nitrate in groups of professionals or countries.
The sample size for this study is relatively modest.
It is unknown how well the self-selected participants in this study reflect the wider community of nutrition professionals.
Inorganic nitrate is a polyatomic ion naturally found in a range of foods such as green leafy vegetables and beetroot, and is also added in the form of nitrate salts (eg, sodium nitrate or potassium nitrate) as a preservative to processed meat products such as ham and bacon.1 2 For many years, consumption of this compound, alongside its reduction product nitrite, was believed to increase the risk of certain forms of cancer and methaemoglobinaemia.3 As a consequence, acceptable daily intake (ADI) values of 0–3.7 mg/kg/day nitrate were established by WHO,4 and the concentration of nitrate in drinking water was restricted to 50 mg/L in the European Union and 44 mg/L in the USA.5 With emerging evidence, however, the negative health effects of nitrate, and the guidelines restricting consumption of this compound, have been questioned.1 3 6 Indeed, in 2010, WHO declared that there is ‘inadequate evidence in humans for the carcinogenicity of nitrate in food’.7 Likewise, several investigations have demonstrated that nitrate in doses normally consumed in healthy diets does not cause methaemoglobinaemia.8–10 Consequently, there has been a transition from viewing nitrate as a potentially harmful to a potentially beneficial dietary constituent, with many researchers now exploring the possible health effects of dietary inorganic nitrate. A key catalyst for this change was the discovery that nitrate is a substrate for production of the multifunctional gasotransmitter nitric oxide11 12 and, so, may influence a range of nitric oxide-mediated physiological processes with potentially beneficial effects. For example, several investigations have demonstrated that dietary supplementation with inorganic nitrate, typically in doses between 4 and 12 mmol/day (~250–750 mg/day), can reduce blood pressure (BP),13–17 improve endothelial function14 17 18 and, at least in recreationally active and moderately trained individuals, enhance exercise performance.19–24 In addition, several recent observational studies have linked increased nitrate intake with reduced risk of cardiovascular disease mortality,25 atherosclerotic vascular disease26 and ischaemic cerebrovascular disease.27
Despite the growth in nitrate-related research in recent years, it is unclear how much this new information has ‘trickled down’ to the general field of nutrition professionals; no studies have evaluated knowledge of and beliefs about inorganic nitrate among nutrition professionals (or other cohorts) to date. For example, it is unclear whether nutrition professionals consider nitrate to be primarily beneficial or harmful, whether they are aware of the key dietary sources of inorganic nitrate, understand potential ways to evaluate nitrate intake and are aware of/agree with current guidelines for nitrate consumption. Gaining a greater insight into the knowledge of, and beliefs around, inorganic nitrate among nutrition professionals could provide detailed information on the practice of these individuals and identify if there may be a need for targeted educational initiatives around inorganic nitrate physiology and its effects on health. For example, for practitioners such as dietitians and nutritionists, knowledge of, and beliefs about, inorganic nitrate and health could influence their dietary recommendations around the overall daily intake of inorganic nitrate and consumption of nitrate-rich food sources.28 Likewise, for university academics, knowledge of and beliefs about, inorganic nitrate and health could inform teaching practices and curricula and have a positive influence on student’s learning.29 Nutrition professionals also advise public and commercial entities about food safety and health, requiring efficient dissemination of new knowledge to ensure guidelines are updated when appropriate.4 5 7 Therefore, in this study, we aimed to establish the current status of knowledge related to inorganic nitrate among nutrition professionals. We also explored whether knowledge of inorganic nitrate differed depending on the education level of participants.
The present study recruited nutrition professionals as participants. Nutrition professionals were defined as individuals working in nutrition or nutrition-related fields, students studying nutrition or nutrition-related courses, and unemployed individuals qualified or with a professional interest in nutrition.
To evaluate knowledge of and beliefs about dietary inorganic nitrate among nutrition professionals, we developed a custom questionnaire—the Knowledge of Inorganic Nitrate Dietary Survey (KINDS; online supplementary data 1). The steps involved in questionnaire development are outlined below and in figure 1.
The initial questions for the KINDS were devised by OMS, GG, AB and MS during several in-person group discussions. Questions were subsequently developed and refined for clarity in consultation with JCM and KB. There were 12 key questions, which focused on participant knowledge or beliefs across five areas: (1) potential health and performance effects of dietary inorganic nitrate (questions 1–3); (2) current and recommended intake values for dietary inorganic nitrate consumption (questions 4–6); (3) dietary sources of inorganic nitrate and factors which influence the food content of this inorganic anion (questions 7–9); (4) methods of evaluating inorganic nitrate intake (question 10) and (5) nitrate metabolism (questions 11 and 12). Additional questions were included to determine participant demographics (ie, age, gender, education and employment status). The questionnaire was built using an online survey tool (Online Surveys, Bristol, UK).
Following initial development of the KINDS, a pilot version was circulated to staff in the Human Nutrition Research Centre at Newcastle University, UK. Underneath each question, a feedback box was presented such that participants could provide comments and suggestions. Further development of the questionnaire was conducted following pilot testing. This included the reordering and rewording of several questions to improve flow and clarity.
A final version of the questionnaire was approved by the research team following pilot testing (online supplementary data file 1). Subsequently, a brief description of the study aims and a link to the questionnaire was circulated to potential participants through UK-based nutrition societies (see Acknowledgements) via email, websites and/or social media (online supplementary data file 2). Additional participants were obtained by contacting UK-based universities currently offering an undergraduate or postgraduate degree in nutrition or nutrition-related course (ie, Nutrition, Dietetics, Food Sciences, Sport and Exercise Science), who were asked to circulate a link to the questionnaire to relevant staff and students. Data collection ran from 13 April to 31 December 2018.
Calculation of a Nitrate Knowledge Index
In addition to examining responses to individual questions on the questionnaire, we derived a 23-point index of nitrate knowledge. We identified by group consensus questions where there was clear evidence for a correct answer, and awarded one point for correct responses and zero points for incorrect responses. Questions where current evidence is ambiguous or where there is no correct response (ie, the answer reflects a belief or opinion on inorganic nitrate) were not included in the Index. Recently published systematic reviews and meta-analyses were used to inform these decisions.17 24 30 31
All statistical analyses were conducted using SPSS V.24, and figures were produced using GraphPad PrismV.8. Statistical significance was defined as p<0.05. We used the X2 test to compare potential differences in questionnaire response by participant highest level of education (undergraduate degree or below, Masters degree or equivalent, PhD). Additionally, the Kruskall-Wallis test was used to compare scores between different education levels for the Nitrate Knowledge Index.
Patient and public involvement
There was no involvement of patients or the public in establishing the research questions of this study or defining the outcome measures. Likewise, patients/the public were not involved in the design, recruitment to, or conduct of this study. Patients or the public were not consulted regarding the interpretation or writing of the results. We do not have plans to disseminate the results of this study directly to participants. However, the data will be included in presentations given by the authors to a wide range of audiences.
A total of 125 individuals completed the KINDS online questionnaire (table 1). The majority of questionnaire respondents were female (73%), aged ≤30 years (42%) and possessed a masters-level qualification (48%).
Overall nitrate knowledge
A summary of the results from this study is presented in table 2. Overall, 71% of participants who took part in this survey had heard about inorganic nitrate, and 51% of participants believed that this polyatomic ion is primarily beneficial. Most participants were aware that inorganic nitrate can improve sports performance (59%) and reduce BP (54%), but were unsure about other physiological effects potentially associated with nitrate consumption including glucose levels (78%), lung function (73%), cancer risk (70%), cognitive function (71%) and kidney function (80%). Knowledge of nitrate intake and the ADI for this compound was generally poor. Indeed, 65% of participants were unsure of the average population intake of nitrate and 64% of participants were unsure of the nitrate ADI. Likewise, 80% of participants were unsure whether the ADI for nitrate required revision. Knowledge of dietary sources of inorganic nitrate and factors influencing the food content of nitrate was generally good, with 70%, 69%, 42% and 52% of participants correctly identifying spinach, beetroot, lettuce and radish as high in nitrate, and 46%, 51%, 43% and 42% of participants correctly identifying sausage, tomato, chocolate and bacon as low in nitrate. The majority of participants were aware that the nitrate content of food is influenced by cooking (59%), season (58%), soil conditions (79%), use of fertiliser (71%) and storage conditions (47%), all of which are consistent with current literature. However, most participants were unsure about the effect of pickling on food nitrate content (45%) and the nitrate content of drinking water (56%). There was no clear consensus on the best biomarker to monitor nitrate intake. Most participants (65%) were aware of at least one compound (ie, nitric oxide or nitrosamines) that is derived from dietary nitrate in the body, but mixed responses were given for the mechanism involved in nitrate to nitrite conversion in the mouth. The median (IQR) score for the Nitrate Knowledge Index was 127–16 out of 23 potential points.
Education-related differences in nitrate knowledge
Knowledge of inorganic nitrate was notably different between individuals with different education levels, with greater knowledge typically observed in those possessing a Masters degree or PhD compared with an undergraduate degree. Specifically, participants with a Masters degree or PhD were more likely to have heard of inorganic nitrate (undergraduate: 41%; Masters: 78%; PhD: 86%; p=0.001) and to perceive this compound as primarily beneficial (undergraduate: 28%; Masters: 63%; PhD: 53%; p=0.002), compared with those possessing an undergraduate degree or lower. More highly educated individuals were also more likely to agree that inorganic nitrate improves sports performance (undergraduate: 35%; masters: 71%; PhD: 64%; p=0.017) and reduces BP (undergraduate: 41%; masters: 53%; PhD: 69%; p=0.016). In addition, individuals with a PhD were more likely to estimate correctly the population mean nitrate intake of 50–200 mg/day (undergraduate: 3%; masters: 14%; PhD: 28%; p=0.017), while individuals with either a masters or PhD were more likely to correctly identify beetroot as a high nitrate food (undergraduate: 45%; masters: 76%; PhD: 81%; p=0.020). Participants with a PhD showed greatest knowledge of nitrate metabolism in the body and were more likely to identify correctly both compounds into which nitrate is converted in the body (undergraduate: 0%; masters: 7%; PhD: 31%; p<0.001). Conversely, none of the participants with an undergraduate-level degree identified both compounds which nitrate is converted into in the body. Likewise, individuals with a PhD were more likely to identify bacterial reductases as responsible for oral nitrate conversion into nitrite (undergraduate: 21%; masters: 31%; PhD: 58%; p=0.012). The median (IQR) scores for the Nitrate Knowledge Index for undergraduate, masters and PhD-level participants were 10,2–14 13,8–15 13,9–17 and were significantly different between groups (p=0.01). Post hoc analysis with Bonferroni adjustment revealed significantly greater nitrate knowledge in participants with a PhD versus undergraduate degree or below (p=0.01), and a trend towards greater nitrate knowledge in participants with a Masters degree or equivalent versus undergraduate degree or below (p=0.054; figure 2).
This study aimed to evaluate knowledge of and beliefs about dietary inorganic nitrate among nutrition professionals, and to compare knowledge and beliefs between individuals with different education levels. The key findings were that most individuals who responded to the survey had heard of inorganic nitrate, and perceived this compound to be primarily beneficial. Overall, participants showed good awareness of the dietary sources of inorganic nitrate and of the factors that may influence the food content of this anion. Conversely, participants showed poor knowledge of the concentration of nitrate in drinking water. Over half of our participants were unsure of the mean population intake of nitrate, the ADI for this compound and whether the ADI requires revision. Nitrate knowledge was greater in individuals with a PhD, and tended to be greater in individuals with a masters-level qualification, compared with those possessing an undergraduate degree.
The physiological effects of inorganic nitrate consumption have received considerable research interest in recent years. Strong evidence now exists showing the potential for supplemental nitrate to reduce BP13–17 and to enhance exercise performance19–24 in healthy individuals, although evidence in clinical populations is less clear.32 This growing evidence was reflected in participant responses, with over half of participants stating that these were physiological effects of inorganic nitrate. Interestingly, in some areas, the nutrition professionals surveyed were more optimistic about beneficial effects of nitrate than justified by the present state of the knowledge. Over a quarter of participants believed that inorganic nitrate improves cognitive function, despite the lack of consistent evidence to support this notion,33–38 and a recent meta-analysis reporting no overall effect of nitrate on cognition or cerebral blood flow.30 Similarly, almost a quarter of participants claimed that nitrate improved lung function, despite little evidence existing to support this notion.39 40 This illustrates the possible risk of ‘overselling’ the physiological benefits of any ‘new’ bioactives like nitrate, long before sufficient evidence is available to accurately assess the magnitudes of potential benefits for relevant population groups of short-term or long-term increases in intake.
The current ADI for nitrate (0–3.7 mg/kg/day) has been questioned by several researchers in recent years1 3 6 so we explored participants views of this issue. We found that knowledge in this area was generally poor, with the majority of individuals surveyed unsure about the population mean intake of inorganic nitrate (65%) and the current ADI for this compound (64%). Moreover, most nutrition professionals were unsure about whether the ADI for nitrate requires revision (80%). Increasing knowledge in these areas could be valuable for two key reasons. First, it would allow more individuals to make informed contributions to the debate around nitrate consumption, and help derive consensus on whether the nitrate ADI requires revision. Second, it would help nutrition professionals (particularly those working as practitioners) make more informed recommendations around nitrate intake—something which is likely to be increasingly important in the coming years given the rising interest in nitrate among researchers and the public. Interestingly, most nutrition professionals showed good knowledge of dietary sources of nitrate and factors that influence the food content of this compound. This suggests that most nutrition professionals would be able to make recommendations as to how to increase or decrease intake of dietary nitrate. However, as discussed above, until recently such advice focused exclusively on reduction of what was considered a contamination risk. So, nutrition professionals are now faced with the much more challenging task to assess in which cases increased nitrate intake might be recommended and if so by how much.
Overall knowledge of inorganic nitrate, as reflected by the Nitrate Knowledge Index, was significantly better in individuals with a PhD and tended to be better in those possessing a Masters degree compared with those possessing an undergraduate-level qualification. Less than half of all individuals with an undergraduate-level qualification had heard of inorganic nitrate, and these participants were generally unsure about the physiological effects, habitual consumption, sources and metabolism of nitrate. This finding is broadly consistent with previous studies which show greater knowledge of single dietary compounds such as sodium in individuals with a higher education level.41 Greater coverage of nitrate in undergraduate-level nutrition courses could be of particular value. Given the breadth of nitrate research including investigation of cellular mechanisms using in vivo and in vitro models, whole body physiology in clinical trials, epidemiology and public health, nitrate could serve as an excellent exemplar for teaching about nutrition research methodology. Increased awareness of dietary nitrate in nutrition professionals could also have implications for improving cardiovascular health in the general population given over 25% of adults in the UK possess elevated BP,42 and this polyatomic ion could represent a potential therapeutic intervention to target these ‘at-risk’ individuals.
Strengths and limitations
This study provides novel information on nitrate knowledge and beliefs among nutrition professionals, using a new questionnaire which could be used to evaluate nitrate knowledge and beliefs in other groups of professionals or other countries. Nevertheless, our study has certain limitations. The sample size in this investigation was relatively modest and self-selected, making it possible that our results may not be fully representative of the community of nutrition professionals. We attempted to reach as wide an audience as possible by circulating the questionnaire through several nutrition societies and universities, but it is possible that we did not reach all relevant groups of nutrition professionals. Importantly, those who responded to the questionnaire may have a greater interest in nitrate than non-respondents, potentially skewing our results to suggest greater nitrate knowledge than is present in the whole field of nutrition professionals.43 We provide new information on differences in nitrate knowledge based on education level, which could be used to inform curriculum development on nutrition-related courses including Continued Professional Development. However, we were unable to compare knowledge of nitrate between individuals employed in different fields of nutrition because many participants identified simultaneously practising across a range of different nutrition areas. Therefore, potential differences in nitrate knowledge between nutrition professionals with different academic and non-academic roles remains unclear.
This study provides novel information on knowledge of and beliefs about inorganic nitrate among nutrition professionals. Our findings suggest that while many nutrition professionals have good awareness of dietary sources of this inorganic anion and of the factors affecting the food content of nitrate, knowledge of health-related effects of nitrate are more variable, which may reflect the rapidly evolving state of the knowledge in this nascent research area. Knowledge of current and recommended values for nitrate intake was generally poor, and knowledge was overall much lower in those with an undergraduate versus masters or PhD-level qualification. Increasing education about inorganic nitrate and its impact on health, with an emphasis on recent developments in the scientific consensus, particularly at undergraduate level, but also as among graduates, may be advantageous to empower nutrition professionals to make more informed recommendations about this compound and adapt appropriately to new developments.
The authors thank the following nutrition societies for circulating the questionnaire to their members: the Association for Nutrition (AfN), the British Association for Parenteral and Enteral Nutrition (BAPEN), the British Dietetics Association (BDA), the Need for Nutrition Education Program (NEdPRO) and the Sport and Exercise Nutrition Register (SENr).
Correction notice This article has been corrected since it was published.
Contributors MS conceived the study and designed the study alongside OMS, GG, AB, AM, JCM and KB. OMS conducted the statistical analysis, with guidance from MS. OMS and MS drafted the manuscript. All the authors participated in the interpretation of the results and critical revision of the manuscript and approved the final version. The corresponding author attests that all listed authors meet authorship criteria and that no others meeting the criteria have been omitted.
Funding The authors have not declared a specific grant for this research from any funding agency in the public, commercial or not-for-profit sectors.
Competing interests All authors have completed the ICMJE uniform disclosure form at www.icmje.org/coi_disclosure.pdf. MS and JCM report grants from Alzheimer’s Research UK during the conduct of the study. All other authors report no conflict of interest. All authors report no financial relationships with any organisations that might have an interest in the submitted work in the previous three years and no other relationships or activities that could appear to have influenced the submitted work.
Patient consent for publication Not required.
Ethics approval The study was approved by the Newcastle University Ethics Committee (4961/2018).
Provenance and peer review Not commissioned; externally peer reviewed.
Data availability statement No data are available.
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