The aim of the study was to compare the profiles of 25-hydroxyvitamin D serum concentration in healthy adults after 4 weeks of using vitamin D supplementation either in a liposomal or a traditional formulation.

The study will be randomized, cross-over, participants will be randomly assigned to one of the study groups: A) the group receiving vitamin D3 in traditional formulation B) the group receiving vitamin D3 in liposomal formulation. The day before the start of supplementation (T0), 2 weeks (T1) and 4 weeks (T2) after the start of supplementation, participants will have their blood drawn. Then, after a wash-out period of 2 months, the groups will be switched. Participants will again receive daily supplementation with vitamin D3 for 4 weeks. Blood will be drawn before the start of supplementation (T0), 2 weeks (T1) and 4 weeks (T2) after the start of supplementation.

Inclusion Criteria:

• Women and men, 18-60 years old.
• Signed informed consent.
• No injuries or hospitalizations within the last 3 months.
• No vitamin D supplementation for the 2 months prior to the study initiation.
• Refraining for the period of the clinical trial from consuming foods additionally fortified with vitamin D and dietary supplements containing vitamin D
• Refraining for the period of the clinical trial from the use of cosmetic therapies that use prolonged body illumination

Exclusion Criteria:

• Unwilling to give consent
• Injuries within the last 3 months
• Obesity, BMI above 30
• Bariatric surgery
• Cystic fibrosis, celiac disease, Crohn's disease
• Metabolic diseases
• Drugs reducing cholesterol absorption, thiazide diuretics, glucocorticosteroids, anticonvulsants, calcium-containing preparations,
• Planned prolonged exposure to sunlight during the study period