Question

What are the main sources of vitamin D?

Our most important natural source of vitamin D is innate production through solar ultraviolet B radiation which penetrates the skin and converts 7-dehydrocholesterol to 25(OH)D (calcidiol), which is then rapidly converted to 1,25 (OH)2D (calcitriol). During summer months, 15 minutes of whole body exposure to midday sunlight provides approximately 10,000 IU vitamin D3 in fair-skinned individuals.1

However, for four to six months of the year, a large part of Europe receives insufficient UV-B radiation from sunlight for the production of vitamin D to occur.2

Few foods naturally contain or are fortified with vitamin D, and the typical diet in Western Europe provides less than 10% of the daily vitamin D requirement.3

Notes 

Exposure to sunshine should always be moderated to avoid sun damage. 

References 

1. Cannell JJ et al. Diagnosis and treatment of vitamin D deficiency. Expert Opinion. Pharmacotherapy, 2008; 9(1):1-12.
2. Webb et al. Influence of season and latitude on the cutaneous synthesis of vitamin D3: Exposure to winter sunlight in Boston and Edmonton will not promote vitamin D3 synthesis in human skin. J Clin Endocrin Metab 1988; 67(2):373-78.
3. Wagner et al. Breastfeeding Med. 2008; 3(4):239-46.