Data from previous DASH interventions (51, 52) was used to estimate a sample size of 185 participants per group required to detect at least a 5.3 mmHg difference in mean systolic BP at 8 weeks, and a 5.7 mmHg difference in the 12-week analysis excluding the 25 extended support pilot study participants. Power of 80%, alpha error rate of 0.05, and a standard deviation of 18 mmHg based on previous 8- or 9-week trials were assumed.

A total of 370 participants will be recruited (approximately 120 from each site) with equal representation of men and women. At each clinic, separate lists of men and women in the target age range with physician-diagnosed HTN and at least two systolic readings ≥130 mmHg will be created. Each list will be randomly sequenced by the study biostatistician. Starting at the top of the lists, the research team will work with clinic staff to review medical charts for additional eligibility criteria. For patients who meet eligibility criteria, study staff will contact the primary care provider to secure approval for participation. Patients will be contacted by telephone and/or in-person at their next clinic visit to ascertain interest about the study and confirm eligibility. Eligible individuals will be scheduled for an initial baseline visit, ideally within 2 weeks of the in-person contact. Final determination of eligibility will occur at the second baseline visit based on an additional BP measure and compliance with study protocols (described below).

Participants must: (1) be at least 18 years old; (2) have had a clinical diagnosis of HTN for at least 1 year; (3) be on a stable regimen of antihypertensive medications for a minimum of 2 months OR not currently medicated, without anticipated changes for the duration of the study; (4) have average systolic BP ≥ 130 mmHg at the past two clinic visits and at the in-person screening visit; (5) have regular medical care and permission from their primary care provider to participate; (6) be physically and cognitively able to use the home BP monitoring device; and (7) be willing and able to follow all other study procedures. People will be ineligible if they: (1) experienced incident CVD or stroke within the previous 6 months; (2) have average diastolic BP ≥ 100 mmHg during the run-in period; (3) have a known diagnosis of secondary HTN (e.g., renal artery stenosis); (4) have diagnosed Stage 4 or 5 kidney disease; (5) have a recent history or high blood potassium due to taking certain medications that can raise potassium levels; (6) are participating in another health research study; (7) are currently or planning to become pregnant during the course of the study; or (8) are receiving treatment for cancer or another serious or terminal medical condition.

The in-person screening will take place at the first baseline visit, which will occur within 2 weeks of initial screening for eligibility and interest. A research assistant will describe the study and risks of participation. The site study coordinator will also review the study and measure BP. For people whose average BP is within eligibility limits, the site study coordinator will administer the full informed consent protocol, explaining study goals, procedures, and potential risks, in addition to requesting Health Insurance Portability and Accountability Act (HIPAA) authorization to allow ongoing access to their medical records (HIPAA is meant to protect a patient from having their information disclosed without the individual’s consent or knowledge). Clinic staff will enter information pertinent to the patient’s care in the medical record notes as it is deemed appropriate and will document provider approval to participate in the study. These notes will be left to the discretion of clinic staff and will not be shared with the study team. Participants will provide written informed consent and HIPAA authorization.

For the extended support pilot study, at the 8-week appointment, while intervention group participants are completing their follow up data collection, the research assistant will access the next assignment on a randomized list created by the study biostatistician. If intervention group participants are interested in participating in the pilot study, they will undergo an additional informed consent process and be further randomized to one of the following two groups: (1) pilot study control, or (2) extended support (pilot study intervention). Potential participants will be clearly informed that their choice whether to participate in the pilot study will not affect their participation in the 12-week follow-up for the main trial.

Following randomization to the intervention group, participants will be scheduled for an initial appointment with an RD employed by the clinic and trained on all study procedures. This 55-min counseling visit will include: purpose of study visit (5 min.), energy needs calculation (5 min.), overview of the DASH diet based on calorie needs (15 min.), basic education on health risks of high BP and interpretation of current BP (4 min.), elicitation of patient perspective on which eating behaviors are affecting BP and ranking of readiness for behavior change (8 min.), co-creation of a customized eating plan (10 min.), and discussion of other lifestyle factors that may be affecting BP (e.g., tobacco use) (8 min.). Following the visit, the RD will assist participants with completing their first grocery order (10 min.). Participants will be provided with a daily eating goal handout for each DASH food group based on their estimated calorie needs. This handout was developed by a study co-investigator and RD (MSW), which was then reviewed and approved by RD community research staff. The handout includes references to hand images for each food group to estimate portion size (e.g., fist, palm, thumb) in addition to standard household measures (e.g., cup, Tablespoon). Participants will also receive a 56-page DASH diet booklet published by the NHLBI (53). During each weekly telephone session with the RD, specific pages of the booklet are referenced to facilitate participants’ gradual review of its content.

NOSH was developed to provide follow up medical nutrition therapy via telenutrition for the management of HTN. Typically delivered in person over multiple sessions, medical nutrition therapy is an evidence-based approach used by RDs to address chronic conditions, including HTN (54). However, multiple barriers, such as transportation and costs, preclude patients from accessing in-person healthcare, including medical nutrition therapy (55–57). The NOSH telenutrition curriculum will be delivered either over the phone or via password-protected secure Zoom platform, depending on participant preference, across eight weekly sessions by an RD or trained health educator under the direct supervision of an RD. Each session will begin with a check-in discussion about current BP monitoring at home with participant reporting of at-home readings (5 min.), interactive discussion about a particular eating goal within the DASH diet using a scripted, culturally-tailored telenutrition curriculum (10 min.), and placement of a $30 unrestricted grocery order (5 min.). Weekly topics will include: (1) seasoning without sodium; (2) fruits instead of processed sweets; (3) vegetables; (4) nuts, seeds, and beans; (5) whole grains; (6) healthy fats; (7) high-calcium dairy alternatives and low-fat dairy; and (8) lean meat, poultry, and fish. Telenutrition curriculum topics will relate to items that comprise the DASH diet score and include 2–3 learning objectives per session. Each session will incorporate motivational interviewing (58, 59) and provide time for the participant to reflect upon current eating behaviors, rate self-confidence in making needed eating behavior change, and develop a weekly action plan to improve dietary adherence within that session’s topic. This curriculum script for weekly telephone sessions was developed by a study co-investigator and RD (MSW), which was then reviewed, culturally tailored, and approved by RD community research staff. These staff members had various levels of experience that ranged from 7 years working with AI/AN adults to more than 20 years of experience working with AI/AN adults in their specific communities. At the end of each session, the RD will assist participants in placement of an unrestricted grocery order. A weekly list of suggested DASH foods specific to that week’s telenutrition topic was developed by the academic-community nutrition team for RD reference. Weekly food lists emphasized a variety of affordable perishable and non-perishable foods that could be accessed at the local grocery store.

RDs will be trained in patient-centered care and communications skills in three 2-h webinars. Sessions will be conducted by a master trainer who was involved in the DASH trials. Topics will include HTN, how to work with people with limited health literacy/numeracy skills, and how to address barriers to DASH and medication adherence. Webinars also will include a detailed review of the DASH diet and strategies to assess diet, set goals, provide advice, arrange follow-up, and monitor progress. Finally, a didactic session with role-playing will be completed to ensure training comprehension. Fidelity will be maintained through booster sessions conducted every 6 months.

At the first baseline visit and at every subsequent clinic visit, systolic and diastolic BP will be measured on-site by auscultation using a mercury sphygmomanometer. A total of three BP measurements, separated by 30 s, will be obtained at each visit on the right arm of participants, using a cuff of appropriate size, after they rest quietly in the seated position for at least 5 min. If the participant indicates that there is a medical or post-surgical reason for not having the BP measured on the right arm (or if the right arm is missing), then BP will be measured with the cuff on the left arm.

Height and weight will be measured at the second baseline visit to calculate BMI as a continuous variable (kg/m²) and categorized to conventional thresholds (< 25 kg/m² = normal; 25–29 kg/m² = overweight; ≥ 30 kg/m² = obese). Weight will be measured and at each following clinic visit to calculate BMI. Participants will be asked to remove shoes, heavier outerwear, and to empty heavy items from their pockets before weighing. If a participant is unable to remove shoes or heavier outerwear, two pounds will be removed from weight.

Fingerstick blood samples will be acquired at the second baseline visit and at each follow-up clinic visit to assess blood lipids. Blood lipids will be measured as total cholesterol, HDL cholesterol, LDL cholesterol, and triglycerides (all mg/dL), using the Cholestech portable analyzer (61). Hyperlipidemia will be defined as LDL cholesterol >130 mg/dL.

Two methods will be used to evaluate dietary intake at the second baseline visit and at each of the follow-up clinic visits. First, at the baseline, 8-, and 12-week assessments, the site study coordinator will assist participants with completing a food recall using the Automated Self-Administered 24-Hour (ASA24®) Dietary Assessment Tool developed by the National Cancer Institute (62). This website guides participants through a 24-h food recall and provides an animated guide and audio and visual cues that enhance use in low-literacy populations. Respondents report frequency of eating occasions and times of consumption. Detailed prompts assess food preparation, portion size, food source, and additions to food items. Dietary intake estimated through the ASA24 will be used the primary method for estimating changes in dietary intake for various components of the DASH diet across study timepoints and between intervention groups. Second, at baseline, self-reported consumption of fats, fruits, and vegetables will be assessed using the validated Block Food Frequency Questionnaire, which reflects dietary patterns during the past year (63). To reduce respondent burden at 8- and 12-week assessments, participants will complete the Fruit and Vegetable Screener, a validated, self-report measure of daily fruit and vegetable intake (64) and the Fat Screener, a questionnaire which assesses percent of energy intake from fat (65). Respondents are asked to indicate how often during the prior month they ate a variety of fruits and vegetables and a variety of food products that are high in fat, respectively. The (65) These food frequency questionnaires will be used to descriptively characterize any dietary changes indicated by the ASA24.

A 24-h urine collection and a spot urine collection will be used to measure electrolytes at the second baseline visit and each follow-up clinic visit by ion-selective potentiometric methods and urine creatinine by a standardized enzymatic assay on the Vitros 5.1 platform (66). Collections with total volume < 450 mL/d or creatinine <10 mg/kg/d will be considered incomplete (67). Participants will be instructed in proper technique and provided with two 3 L wide-mouthed, labelled, screw-top plastic urine jugs along with a collection device (urinal or hat). Twenty-four-hour urine collections will start following first-morning void on the day prior to the study visit and end with inclusion of first morning void on the visit day, and the spot urine will be collected in-person at the visit. Urine volume will be measured using a graduated cylinder, with aliquots frozen at -80C prior to assay.

For participants who have not experienced previous CHD or stroke events, the 10-year risk of incident CHD (68) or stroke will be estimated using prediction algorithms specific to AI/AN adults. Both risk scores were developed by the Strong Heart Study, a large prospective cohort study of CVD in AI/AN adults from three distinct geographic populations (including Oklahoma) (68). The scores are based on data collected during baseline and at each clinic visit (age, HTN medications, systolic BP, blood lipids, diabetes, smoking status, and albuminuria).

Participants will be instructed to bring all current medications (prescription, over the counter, and herbal or vitamin supplements) to each clinic visit. The site study coordinator will review and record each participant’s medications and will query the patient’s medical records. Current antihypertensive medications will be documented and coded using a protocol from an ongoing study of cerebrovascular disease among American Indians (69).

Tobacco and alcohol use at each clinic visit will be assessed with questions previously used in rural AI/AN populations (70). Smoking questions will distinguish current, past, and never smokers, including number of cigarettes smoked per day and age at initiation or cessation. Use of smokeless tobacco will not be assessed for this study. Alcohol consumption will be assessed with questions asking about frequency and type of current and past consumption of alcoholic beverages, after a standard definition and reference images for the equivalent of one alcoholic beverage is provided.

We will document prevalent diabetes, stroke, or CVD (congestive heart failure, myocardial infarction, or CHD) via patient self-report at the first baseline visit and medical chart abstraction. Urine micro- and macro-albuminuria will be categorized based on lab values where a participant’s albumin: creatine ratio is 30–299 mg/g or ≥ 300 mg/g, respectively (71).

Sex, age in years, completed education, current employment, and marital status will be collected at the first baseline visit. Documented administrative variables will include clinic site, study arm (control, intervention), and primary care provider (anonymous ID number, used only for statistical analysis).