Humans; Male; Female; Infant, Newborn; Bone Diseases, Metabolic/diagnosis; Bone Diseases, Metabolic/epidemiology; Bone Diseases, Metabolic/etiology; Bone Diseases, Metabolic/metabolism; Bone Diseases, Metabolic/physiopathology; Bone Diseases, Metabolic/prevention & control; Bone Diseases, Metabolic/therapy; Infant, Premature, Diseases/diagnosis; Infant, Premature, Diseases/epidemiology; Rickets/diagnosis; Rickets/epidemiology; Rickets/etiology; Rickets/metabolism; Rickets/physiopathology; Rickets/prevention & control; Rickets/therapy; Calcification, Physiologic/physiology; Calcium/administration & dosage; Calcium/metabolism; Phosphorus/administration & dosage; Phosphorus/metabolism; Milk, Human; Parenteral Nutrition/adverse effects; Nutrition Disorders/pathology; Infant, Low Birth Weight/growth & development; Vitamin D/administration & dosage