Seasonal variations in vitamin D status will be determined on a cohort of Slovenian adults using longitudinal study design, and association between vitamin D status and different Caucasian phototypes, skin colour and melanin index investigated .

Inclusion Criteria:

• 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 hihg-fat 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.

General Slovenian adult population, men and women, skin phototypes I-IV.