Vitamin D has multiple systemic effects: bone and calcium metabolism, muscle function, insulin responsiveness, body-composition regulation, cell differentiation, and the immune system. Proper status of vitamin D is found to be related to risk reduction in hypertension, cardiac and vascular diseases, autoimmune diseases, and others. Furthermore, vitamin D supplementation resulted in improved endothelial function. Limited sun exposure may lead to vitamin D deficiency, and it may be assumed that modern life styles lead to a lack of sun exposure. Long work-days may be the primary risk factor for vitamin D deficiency. The purpose of this research is to study the effect of vitamin D treatment on multi systemic functions in young healthy men with vitamin D deficiency due to working conditions.

The study has 2 stages: first stage would be survey of 400 employees, who will fill out a questionnaire to identify occupational and demographic risk factors, a nutritional questionnaire, and data from periodic checkups will be collected. Participants will sign a consent form for vitamin D level determinations, and for freezing blood samples. From this survey, the participants that have vitamin D deficiency (25(OH)D levels < 20 ng/ml) will continue to the second stage of the study - administration of vitamin D or placebo. Participants in the interventional study will sign an additional consent form. Length of follow-up: one year. All parameters will be tested at 0, 6, and 12 months.

Inclusion Criteria:

• Healthy men between 20-65 years old
• For stage 2: baseline 25(OH)D levels < 20 ng/ml

Exclusion Criteria:

• Liver dysfunction
• Kidney dysfunction
• Patients with unbalanced chronic diseases
• Regular use of medications that lower serum vitamin D levels, or which interfere with intestinal vitamin D absorption