The goal of this clinical trial is to understand the effects of oral vitamin D3 supplementation on various cardiovascular risk factors in generally healthy, young adult black women who are vitamin D deficient or insufficient at baseline.

The main questions it aims to answer are:

• Does 8 weeks of oral vitamin D3 supplementation (5,000 IU per day) improve 24 hour blood pressure metrics in generally healthy, young adult black women who are vitamin D deficient or insufficient at baseline?
• Does 8 weeks of oral vitamin D3 supplementation (5,000 IU per day) improve subjective and objectively estimated sleep health metrics in generally healthy, young adult black women who are vitamin D deficient or insufficient at baseline?
• Does 8 weeks of oral vitamin D3 supplementation (5,000 IU per day) improve various measures of blood vessel structure and function in generally healthy, young adult black women who are vitamin D deficient or insufficient at baseline?
• Does 8 weeks of oral vitamin D3 supplementation (5,000 IU per day) improve various measures of laboratory blood pressure regulation and autonomic function?

All participants will undergo baseline testing, which includes 2 continuous weeks of objective sleep monitoring using a sleep watch, one 24-hour period of ambulatory blood pressure monitoring, and one blood vessel function testing visit. Following baseline testing, vitamin D insufficient and deficient participants will be prescribed take 5,000 IU of vitamin D3 daily for 8 continuous weeks. Participants will undergo 2-weeks of sleep monitoring again during weeks 3-4 of the supplementation period and during weeks 7-8 of the supplementation period. Additionally, 24-hour blood pressure monitoring will be performed during week 4 and week 8, and blood vessel function testing will take place at the end of week 4 and again at the end of week 8.

Researchers will assess the effect of the vitamin D3 supplementation intervention by comparing all values between baseline, week 4, and week 8 to see if there is any effect of vitamin D3 supplementation on 24-hour blood pressure, sleep duration and regularity, and blood vessel structure and function following 4 and 8 weeks of supplementation.

Inclusion Criteria:

• Female
• Self-identified race is Black
• 18-30 years old
• Serum 25-hydroxyvitamin D concentration between 8-29.9 ng/ml determined at screening visit

Exclusion Criteria:

• Unwilling or unable to give consent
• Unwilling or unable to undergo a venous blood draw
• Diagnosed with any chronic diseases or conditions including cardiometabolic diseases, cardiorespiratory diseases, chronic mental or psychological illness, musculoskeletal diseases/conditions, autoimmune diseases, cancer, gastrointestinal/malabsorption disorders, hyper-/hypocalcemia, hyper-/hypoparathyroidism, hyper-/hypothyroidism, kidney disease, or a history of kidney stones
• Taking medication that may influence blood pressure or blood vessel function
• Diagnosed with a sleep disorder (e.g., insomnia, restless leg syndrome, sleep apnea), or are at high risk for a sleep disorder according to the ISI (score >14) or STOP-bang (score ≥3) questionnaires
• Currently taking medications or supplements that affect sleep (e.g., Ambien, sedatives, melatonin, etc.)
• Currently working night-shift work
• Resting blood pressure >130 or >80 mmHg
• BMI >30 kg/m2
• Currently pregnant, breast feeding, peri-menopausal, or post-menopausal
• Currently use tobacco (≥1 cigarette in the last month)
• Had COVID-19 in the past 60 days
• Received the COVID-19 vaccine or booster within in the past 14 days

Women assigned female at birth who currently identify as female