Hypovitaminosis D is a deficiency of vitamin D. It can result from inadequate nutritional intake of vitamin D and/or inadequate sunlight exposure (in particular sunlight with adequate ultraviolet B rays), disorders limiting vitamin D absorption, and conditions impairing vitamin D conversion into active metabolites—including certain liver, kidney, and hereditary disorders. Deficiency impairs bone mineralization, leading to bone softening diseases as rickets in children and osteomalacia and osteoporosis in adults. Emerging evidence suggests vitamin D plays a role in the pathogenesis of non-alcoholic fatty liver disease (NAFLD).