Vitamin D3 is synthesized in skin exposed to ultraviolet-B-irradiation of sunlight. However, in Northern Europe, including Finland, exposure to sunlight is limited or non-existent in winter. Between November and March, dietary or supplementary vitamin D intake is essential to maintain adequate serum 25-hydroxyvitamin D (S-25(OH)D) concentrations. There are only a few natural dietary sources of vitamin D. Fish and egg yolks contain significant amounts of vitamin D3 (cholecalciferol), and some wild mushrooms are a source of vitamin D2 (ergocalciferol). Thus, vitamin D fortification of certain foods has been accepted as one strategy to improve vitamin D status of these populations. Vitamin D3 is currently mostly used in fortified products in Europe, although the production of plant-derived vitamin D2 would be more cost-effective. Data on the impact of food-derived vitamin D2 on S-25(OH)D3 are limited. An important consideration is whether food-based sources of vitamin D2 are effective in raising S-25(OH)D or whether they have a negative impact on S-25(OH)D3. The investigators investigated the bioavailability of vitamin D2 from UVB-radiated baking yeast in bread in terms of efficacy of raising total S-25(OH)D) as well its possible effects on S-25(OH)D2, S-25(OH)D3, serum parathyroid hormone (S-PTH) and serum calcium concentrations compared with D2 and D3 supplementation in a randomized placebo-controlled trial in young healthy females during winter in Finland.

Inclusion Criteria:

• good general health (no continuous medication),
• Caucasian origin,
• body mass index (BMI) <30 kg/m2
• age 20-40 years

Exclusion Criteria:

• diagnosed celiac disease or cereal allergy
• a vacation in a sunny place during the study or within the preceding 2 months
• regular use of vitamin D supplements >15 µg/d
• pregnancy or breastfeeding during the study