Vitamin D is a group of liposoluble pro-hormones consisting of 5 different vitamins: vitamins D1, D2, D3, D4 and D5.
Vitamin D3 (cholecalciferol) promotes renal calcium reabsorption, intestinal phosphorous and calcium absorption and bone mineralization processes and also differentiation of some cell lines and some neuromuscular functions
Cholecalciferol comes in 3 different biological forms:
- The true colecalciferol, which is the inactive and non-hydroxylated form of vitamin D3;
- Calcidiol, which is the form whose values are measured in the blood;
- Calcitriol, the biologically active form of vitamin D3.
A systematic shortage
In Italy, 80% of the population is lacking in Vitamin D : Vitamin D deficiency affects about half of young Italians in the winter months. The chronic condition increases with aging as almost all of the elderly population in Italy does not have enough vitamin D. 
An Italian study in 2003  shows that over 700 postmenopausal women 76% of them had absolutely insufficient vitamin D levels. These pieces of evidence solidify the belief, even among doctors, that there is a need for vitamin D supplementation in the country to ensure adequate levels for all ages. 
Vitamin D3 Deficiency causes:
- Decreasing serum levels of calcium and phosphorus
- Secondary hyperparathyroidism and increased alkaline phosphatase concentration.
- Subsequently, there are alterations of mineralization processes with rickets (in the child) and osteomalacia (in the adult) and muscle weakness, bone deformation and pain
- SIOMMMS, Vitamina D: l'80% degli italiani è carente, siommms.it.
- S. Adami, E. Romagnoli, V. Carnevale, A. Scillitani, A. Giusti, M. Rossini, D. Gatti, R. Nuti, S. Minisola "Linee guida su prevenzione e trattamento dell'ipovitaminosi D con colecalciferolo" Reumatismo, 2011.
- Isaia G, Giorgino R., Rini G.b., Bevilacqua M., Maugeri D., Adami S. "Prevalence of hypovitaminosis D in elderly women in Italy: clinical consequences and risk factors." Osteoporos Int 2003.
- L.Tafaro, R. Benvenuto, A. Martocchia, I. Indiano, P.Frugoni, E. De Marinis, M. Stefanelli, S.Cola, S.Pascali, A. Devito, P.Falaschi "LINEE GUIDA NELLA SUPPLEMENTAZIONE DI VITAMINA D”
- Charles Pierrot-Deseilligny, Clinical implications of a possible role of vitamin D in multiple sclerosis, in Journal of Neurology, vol. 256, nº 9, 1º settembre 2009