Inclusion Criteria:  medical students aged between 18-25 years; presence of presence of vitamin D deficiency or insuficiency (serum total vit D3 level less than ); signed informed consent.  Exclusion Criteria:  regular intake of probiotics, prebiotics, antibiotics, anticonvulsants, calcium and vit. D supplements for 3 months prior the inclusion; previously diagnosed allergy to gut microbiota-based therapy; gastrointestinal disorders including food allergy, gluten-sensitive enteropathy, ulcerative colitis; suffering from any psychiatric illness; presence of diabetes mellitus, chronic cardiovascular or respiratory disease, an active malignant tumor or chronic infections; participation in another clinical trial; pregnancy or lactation; any condition thought to be associated with poor compliance (e.g., alcoholism or drug addiction) or any condition or circumstance that would, in the opinion of the investigator, prevent completion of the study or interfere with analysis of study results.