This randomized controlled superiority pilot trial will evaluate the efficacy of two vitamin D3 formulations administered over eight weeks in adults with subclinical or clinical vitamin D deficiency. The study will compare changes in serum 25-hydroxyvitamin D concentrations between formulations, as well as the proportion of participants who will achieve vitamin D sufficiency. Both formulations are expected to improve vitamin D status, with the hypothesis that one formulation will demonstrate superior bioavailability, resulting in greater and faster increases in circulating 25-hydroxyvitamin D. The findings will provide preliminary evidence to inform formulation selection and dosing strategies for correcting vitamin D deficiency and will support the design of larger, confirmatory trials.

Inclusion Criteria:

• Adults aged 18 years and over
• Serum 25(OH)D < 75 nmol/L
• BMI 18.5 - 29.9 kg/m2
• Free of clinically significant acute disorders and severe chronic diseases
• No planned travel to high-UV destinations or tanning bed use during the trial
• Willing to avoid non-study vitamin D supplements
• Able to give written informed consent and comply with study visits
• Submitted informed consent

Exclusion Criteria:

• Pregnancy of breast feeding
• Underweight or obesity
• History of any dietary supplement use within 8 weeks before screening
• Medications that materially alter vitamin D metabolism or calcium balance
• Subjects with a history of medicine or alcohol abuse
• Abnormal values for lab clinical chemistry (> 2 SD)
• Unwillingness to return for follow-up analysis
• Participation in other clinical trials within 60 days prior to screening

Data obtained through this study may be provided to qualified researchers with academic interest in vitamin D. Data will be coded, with no PHI included. Approval of the request and execution of all applicable agreements are prerequisites to the sharing of data with the requesting party.

Data requests can be submitted starting 9 months after article publication and the data will be made accessible for up to 24 months. Extensions will be considered on a case-by-case basis.

Only qualified researchers with academic interest in vitamin D research