The usual difference between vitamin D₂ and D₃ is the difference in source, with vitamin D₂ being an ossiferol and vitamin D₃ being a cholecalciferol. However, the effects on the body and the principles of action of both are usually the same. Vitamin D₂ is mainly derived from plant foods and is usually obtained by the reaction of ergosterol under ultraviolet light. Vitamin D₃ is mainly synthesized by the body, but can also be derived from animal foods, for example, mainly from cod liver oil, which is usually obtained by extraction. Vitamins D₂ and D₃ are inactive, and are only active when converted to 25-hydroxyvitamin D₃, which promotes the absorption of calcium in the body, usually by the liver and kidneys. Vitamin D deficiency in the body usually leads to rickets in children and osteomalacia in adults, and requires supplementation with vitamin D₂ and D₃ preparations, which can also be obtained through sun exposure. It is recommended that patients with vitamin deficiencies need to consult a hospital and be treated under the supervision of a doctor.