The efficiency of different vitamin D formulations in treating suboptimal vitamin D status in adult population will be determined. It will provide insights into the influence of formulation on vitamin D absorption.

A randomised controlled intervention trial will be conducted on at least 72 subjects (18-65 years) with suboptimal Vitamin D status. Subjects will be selected from a pool of subjects in Part 1 of the study, after completion of winter sampling.

Study subjects will be randomised into four study groups (at least 18 subjects per study group). Three different treatments will be compared with a control group, which will not receive any supplemental Vitamin D. The three tested Vitamin D preparations in the treatment groups will be (A) oil-based VALENS vitamin D; (B) water-based VALENS vitamin D; and (C) Vitamin D capsules with starch-adsorbed vitamin D (powder). All these products are commercially available. Prior to study, actual level of vitamin D will be determined using appropriate analytical method. The treatment will be with 1000 IU Vitamin D per day, for two months, in all three groups. Second serum vitamin D level will be determined on last day of the treatment, and compared with pre-treatment vitamin D level.

Inclusion Criteria:

• vitamin D suboptimal status
• Signed Informed consent form (ICF),
• Caucasian race
• Age between 18 and 65 years at the time of the signature of Informed consent form (ICF),
• Willingness to avoid a consumption of any food supplements containing Vitamin D during the study, except the ones prescribed by the study researchers, and to avoid use of solarium or other artificial UVB sources
• Willingness to follow all study procedures

Exclusion Criteria:

• Pregnancy or breastfeeding,
• Known or suspected allergy to any ingredient of the tested products in Part 2,
• Pronounced avoidance of sunshine (eg reporting of allergy to the sun)
• Use of food supplements containing Vitamin D or fish oil or omega 3 fatty acids in last three months prior to inclusion,
• Special dietary habits used by a small part of the population (veganism, low-carb high-fat (LCHF) diet, caloric restriction diet, note: vegetarians are not excluded)
• Diets prescribed by the medical profession
• Disorders of the kidneys, thyroid, digestive tract, osteoporosis and other bone diseases, skin diseases
• Other diseases and conditions that affect the absorption and synthesis of vitamin D
• Exposure to stronger sunlight in the last three months prior to engaging in a survey (travel to countries with stronger sunshine, use of solarium),
• The consumption of margarine or plant substitutes for milk / milk products several times a day, as these foods are most often enriched with vitamin D,
• Visiting the solarium in the last three months before joining the survey,
• Mental incapacity that precludes adequate understanding or cooperation.