Vitamin D is a secosteroid hormone. Vitamin D receptors are present in the majority of body tissues. The manifestations of hypovitaminosis D - linked to dysfunction of target tissues - are various, including osteoporosis, cancer, tuberculosis, hypertension, multiple sclerosis, depression, dementia, sarcopenia, propensity to fall… The serum 25-hydroxyvitamin D threshold value to avoid these adverse health events is around 30 ng/mL. Only 15% of the elderly reach this target concentration. For the remaining 85% with no supplements, the severity of hypovitaminosis D appears to be a biomarker of chronic diseases and of frailty. Conversely, the supplementation for correction of hypovitaminosis D positively impacts bone and non-bone morbidities - such as risks of falls and fractures - and reduces the mortality rate. A daily intake of at least 800-1,000 IU supplemental vitamin D(3) per day is the key.