Table 1.

Vitamin D deficiency: Risk factors and etiopathogenetic mechanisms

Pathophysiology	Etiology	Conditions
Reduced synthesis of vitamin D3 in skin	Reduced exposure to ultraviolet B (UV B) rays Increased absorption of UV B rays	Areas with low sun exposure[43] Sunscreen use[44,45]
		Increased melanin pigment in the skin
Reduced oral intake and bio-availability of vitamin D from food sources	Low vitamin D3 in the foods Reduced bio-availability due to high amount of phytates and phosphates Malabsorption syndromes	Vegetarian food sources, unfortified milk High-fiber foods Cystic fibrosis, celiac disease
Impaired metabolism of vitamin D	Decreased synthesis of 25-OH-D3	Liver disorders
	Decreased synthesis of 1,25(OH) 2D3	Chronic renal disorders
	Increased de-activation of 25-OH-D3 and 1,25(OH) 2D3 and conversion to calcitroic acid	Drug-induced HAART, glucocorticoids
Increased loss of vitamin D	Increased urinary excretion of 25-OH-D3	Nephrotic syndrome

D3 - calcitriol; HAART - Highly active antiretroviral therapy