The randomized comparative study was performed on 99 outpatients with below 30 ng/ml of vitamin D. Participants were randomized into three groups to receive vitamin D3 supplementation in three different forms - sublingual spray, oral oil droplets, and capsules. Each participant received vitamin D3 4000 IU daily for 30 days and the vitamin D levels were measured before and after the intervention.

This randomized comparative study was performed in Liepaja, Latvia, on 99 outpatients, with vitamin D levels below 30 ng/ml. Participants were randomized into three groups to receive vitamin D3 supplementation in three different forms - sublingual spray, oral oil droplets, and capsules. Before intervention blood samples were taken to determine vitamin D3 level, creatinine, and GFR, each participant received vitamin D3 4000 IU daily for 30 days. After that blood test was performed again, to measure changes in vitamin D3 levels in the blood. Participants answered the questionnaire to collect information about things that might affect vitamin D3 levels, such as dietary habits, physical activities, usage of sunscreen, compliance, etc.

Inclusion Criteria:

• Vitamin D3 level in the blood below 30 ng/ml.

Exclusion Criteria:

• Celiac disease, Crohn's disease, ulcerative colitis, autoimmune gastritis.
• Patients with mental health problems.
• Vitamin D level ≥ 30 ng/ml.
• Pregnant women.
• Any other illness or condition that the researcher deemed may interfere with the results.
• Patients who refuse the studies.