The conversion of 7-dehydrocholestrol to cholecalciferol requires sunlight.
The complete synthesis of vitamin D in the body first begins with 7-dehydrocholestrol in the skin. With exposure to UV light, it is converted to cholecalciferol (D3). This is then converted by 25-hydroxylase in the liver to 25-hydroxyvitamin D. 25-Hydroxyvitamin D is then converted to 1,25-dihydroxyvitamin D, the active form of vitamin D, by 1-alpha-hydroxylase in the kidney. Without sunlight, active vitamin D cannot be produced. Other sources of vitamin D include ergocalciferol (D2), which can be converted to 25-hydroxyvitamin D by 25-hydroxylase in the liver.
Bordelon et al. discuss vitamin D deficiency. Symptoms include lower back pain, muscle aches, proximal muscle weakness, and bone pain. If vitamin D deficiency is detected, serum levels of 25-hydroxyvitamin D should be measured. If levels are lower than 20-30 ng/mL, then a diagnosis of vitamin D deficiency is made. Treatment should consists of bringing vitamin D levels back up to normal. Treatment includes oral 50,000 IU of vitamin D2 each week for 8 weeks, then a maintenance dose of 800-1,000 IU per day.
Ross et al. discuss the dietary sources of vitamin D. Natural sources include fatty fish and oil and egg yolk. Artificial sources of vitamin D include foods that are fortified with vitamin D such as milk which is fortified with 400 IU per quart.
Illustration A shows the synthesis of vitamin D and the locations for specific reactions. This video describes the synthesis and functions of vitamin D.
Answer 1,3-5: These forms of vitamin D do not require sunlight for their production.
Bordelon P, Ghetu MV, Langan RC. Recognition and management of vitamin D deficiency. Am Fam Physician. 2009 Oct 15;80(8):841-6. Review.
PMID:19835345 (Link to Abstract)
Institute of Medicine (US) Committee to Review Dietary Reference Intakes for Vitamin D and Calcium; Ross AC, Taylor CL, Yaktine AL, et al., editors. Dietary Reference Intakes for Calcium and Vitamin D. Washington (DC): National Academies Press (US); 2011. 3, Overview of Vitamin D. Available from: http://www.ncbi.nlm.nih.gov/books/NBK56061/