This study will assess the impact of Vitamin D supplementation on hypertensive patients with Vitamin D deficiency, it will also investigate the relationship between serum Vitamin D, irisin levels, blood pressure, and heart rate variability (HRV) in hypertensive and normotensive individuals.

It is hypothesized that a decrease in blood pressure and an increase in HRV will be noted in the hypertensive treatment group (subjects on supplemental vitamin D) in comparison to the hypertensive control group (placebo) upon vitamin D supplementation.

In a prospective study conducted on 160 participants, 80 hypertensive and 80 normotensive individuals, assessing correlations between Vitamin D, irisin, blood pressure, and HRV, hypertensive vitamin D deficient participants will be recruited in a randomized controlled trial. In this clinical trial, 60 hypertensive participants with Vitamin D deficiency will receive either Vitamin D supplementation (25,000 IU weekly) or a placebo for 8 weeks (30 participants in each group).

Outcome measures include systolic and diastolic blood pressure, HRV, serum Vitamin D, and serum irisin levels before and after supplementation.

The aim is to explore the potential role of Vitamin D and irisin in regulating blood pressure and improving HRV, potentially offering preventive measures for hypertension management.

Inclusion Criteria:

Hypertensive: Adult individual diagnosed with hypertension by a specialist (at least two systolic blood pressure readings above 140 mmHg or diastolic blood pressure readings above 90 mmHg, measured by a qualified professional following American Heart Association guidelines) managing hypertension through medication adherence, and regularly attending heart clinic appointments, with no other systemic diseases.

Control: Healthy adult individuals without hypertension or a history of other systemic diseases who volunteer and participate in the study, which will be recruited from relatives, friends, colleagues or patient accompanies. - (*Note: The healthy participants were included in the first part of the study before starting the clinical trial)

Exclusion Criteria:

• 1. Hypertensive individuals who have heart failure and other cardiac or systemic diseases or with recent cardiovascular events.

  2. Secondary hypertension 3. Hypertensive patients who require medication change during the course of the study.

  4. Individuals on Lipid-lowering agents (e.g. simvastatin). 5. The use of other medications affecting blood pressure is not part of the study protocol.

  6. Any subjects on regular supplemental vitamin D or receiving multi-vitamins regularly with vitamin D content of more than 880 IU, one month before recruitment.

  7. Individuals with a BMI over 35 or less than 18 kg/m2. 8. Subjects with a significant change in lifestyle, for example, exercise, weight change, or on a specific dietary regimen during or in the previous four weeks of the study.

  9. Subject diagnosed with neoplasm, or inflammatory or autoimmune diseases 10. Pregnant or lactating female