Vitamin D functions in the body through both an endocrine mechanism (regulation of calcium absorption) and an autocrine mechanism (facilitation of gene expression). The former acts through circulating calcitriol, whereas the latter, which accounts for more than 80% of the metabolic utilization of the vitamin each day, produces, uses, and degrades calcitriol exclusively intracellularly. In patients with end-stage kidney disease, the endocrine mechanism is effectively disabled; however, the autocrine mechanism is able to function normally so long as the patient has adequate serum levels of 25(OH)D, on which its function is absolutely dependent. For this reason, calcitriol and its analogs do not constitute adequate replacement in managing
vitamin D needs of such patients. Optimal serum 25(OH)D levels are greater than 30 ng/mL (80 nmol/L). The consequences of low 25(OH)D status include increased risk of various chronic diseases, ranging from hypertension to diabetes to cancer. The safest and most economical way to ensure adequate
vitamin D status is to use oral dosing of native vitamin D. (Both daily and intermittent regimens work well.) Serum 25(OH)D can be expected to rise by about 1 ng/mL (2.5 nmol/L) for every 100 IU of additional
vitamin D each day. Recent data indicate that cholecalciferol (vitamin D3) is substantially more potent than ergocalciferol (vitamin D2) and that the safe upper intake level for vitamin D3 is 10,000 IU/d. The prevalence of
vitamin D deficiency in Iranian adults using cut-off values of 20 ng/ml. According to the results of recent studies, the prevalence of
vitamin D deficiency in children of Iran is estimated to be 79 - 81/3%. In other studies, the prevalence of
vitamin D deficiency among adults and adolescents in Iran has been estimated to be approximately thirty percent. In Iran, the
vitamin D supplementary program has been applied by the Office of Nutrition Department Society in whole of the country among different age groups since 2014. Therefore, the status of
vitamin D intake was necessary to design new appropriate interventions. Because its results can help to identify the weaknesses and implementation problems of the program. Qualitative analysis showed that there are several problems to implement the program in the country such as distributing the supplement, the existed barriers to use the supplement by people and funding. The health professionals’ solutions including a need to intervene a multiple strategy that contains training, financing, sufficient and regular distribution of the supplement and to apply the alternative methods such as food fortification.