Non-pharmacological measures should serve as the first-line treatment in individuals with high normal blood pressure (BP) levels or grade 1 hypertension and low-moderate cardiovascular disease risk. Salt intake reduction and the dietary patterns of the Dietary Approaches to Stop Hypertension (DASH) diet and the Mediterranean diet (MedDiet) have been recognized as effective dietary measures for BP reduction. To the best of our knowledge, no clinical trials were designed to compare the effects of these dietary strategies. The purpose of the present trial was to compare the effects of salt restriction, the DASH, and the MedDiet combined with the salt restriction on BP levels and cardiometabolic risk factors in adults with high normal BP or grade 1 hypertension over 3 months.

This was a 3-month, single-center, single-blind, randomized, controlled, clinical trial, with 4 parallel groups. Before their randomization, the potential participants visiting the Hypertension Unit of the Hippocration General Hospital of Athens for the first time were assessed for eligibility during two screen visits. Anthropometric measurements, demographic characteristics, dietary intake, physical activity level, smoking history, and alcohol consumption were recorded. In addition, office and ambulatory BP measurements, as well as fasting blood samples and 24-hour urine samples were obtained. Patients were eligible for inclusion if they had high normal BP (office systolic BP: 130-139 mmHg and/or office diastolic BP: 85-89 mmHg) or grade 1 hypertension (office systolic BP: 140-159 mmHg and/or office diastolic BP: 90-99 mmHg), based on the average of the means of the last two out of three measurements made during the two screen visits and were free of antihypertensive drug treatment and major medical conditions. Enrolled patients were blindly randomized to one of the four study groups, i.e., control group (CG), salt restriction group (SRG), DASH diet combined with salt restriction group (DDG), or MedDiet combined with salt restriction group (MDG). After randomization, all patients were followed monthly for 3 months in individual sessions, coordinated by the clinical dietician. For the patients of all four study groups, the goal was to maintain the initial body weight stable. Patients in the CG received the usual advice about salt restriction given to the patients by the clinical dietician at the Hypertension Unit. They were followed at the same frequency as the patients in the other study groups to obtain the study's intended measurements, without any other intervention. For the patients in the three intervention groups, the goal was to limit sodium intake to 2,000 mg/ day. Patients in the SRG were given a detailed booklet containing information about table salt and during each 45-min individual session, they received intensive counselling and training to increase adherence to salt restriction. Patients in the DDG and the MDG received a more extensive booklet, which also contained information about the assigned dietary pattern and an individualized eating plan, while during the 45-min individual sessions, patients in these two intervention groups were intensively counselled and trained to increase adherence to salt restriction and the assigned dietary pattern. Adherence to the assigned diet and/or salt restriction was established through subjective and objective measures, i.e., 7-day food records, adherence scores, and 24-hour urine tests. Anthropometric indices, dietary intake, physical activity level, and office BP were measured at baseline, and during each follow-up visit. Ambulatory BP measurements and collection of fasting blood samples and 24-hour urine samples were conducted at baseline and the end of the 3-month intervention.

Inclusion Criteria:

1. High normal BP (office systolic BP: 130-139 mmHg and/or office diastolic BP: 85-89 mmHg) or grade 1 hypertension (office systolic BP: 140-159 mmHg and/or office diastolic BP: 90-99 mmHg) and low-moderate 10-year cardiovascular disease risk.
2. Willingness to participate in a dietary intervention to control BP and to attend individual sessions.
3. Signed informed consent form for participation.

Exclusion Criteria:

1. Current use of anti-hypertensive medications.
2. Current use of any other medications or agents affecting BP levels (e.g., nonsteroidal anti-inflammatory drugs).
3. Use of food supplements accompanied by a refusal to discontinue them.
4. Participation in a clinical study involving a drug or device within 3 months of screening.
5. Active weight loss or participation in a weight loss treatment program within 3 months of screening.
6. Secondary hypertension.
7. Atomic history of cardiovascular disease (e.g. acute myocardial infarction, stroke, heart failure).
8. Diabetes Mellitus (Type 1 & 2).
9. Chronic kidney disease, defined as an estimated Glomerular Filtration Rate (eGFR) <60 mL/min/1.73m2.
10. Liver disease (e.g., hepatitis, cirrhosis).
11. Lung disease (e.g., chronic obstructive pulmonary disease).
12. Inflammatory bowel disease (e.g., Crohn's disease, ulcerative colitis).
13. Celiac disease.
14. Active malignancy or cancer therapy.
15. Current major psychiatric disorder or current drug abuse.
16. Body Mass Index >40 kg/m2.
17. Current alcohol consumption >14 drinks per week.
18. Current or planned pregnancy before the end of the study, or breast-feeding.
19. Any concomitant conditions that in the opinion of the investigator interfere with the diet or other situations requiring specific nutritional management.

Due to the nature of this study, which was a dietary intervention evaluating the effect of certain dietary strategies on blood pressure levels and cardiometabolic risk factors, masking of the trial participants and the clinical dietician (sub-investigator), who provided the dietary education and was solely responsible for the assignment of patients to the study groups was not possible. However, patients were completely unaware of the other groups of the study and were treated according to strict criteria, preventing deviations from the intended intervention. Moreover, the principal investigator, care providers, and all members of the medical team, who conducted the office and the ambulatory BP measurements (outcomes assessors) were blinded to patients' allocations.