25 hydroxy vitamin D3 refers to activated vitamin D3. The vitamin D contained in cod liver oil is inactive and must be activated by the kidneys and liver in order to function as a catalyst for calcium absorption. It is not easily destroyed by osteoclasts. Most of the vitamin D3 used in clinical practice is directly activated vitamin D3, so the effect of promoting calcium absorption is better than that of traditional cod liver oil and inactive vitamin D. Especially for elderly people with poor liver and kidney function, it is better to take 25 hydroxy vitamin D3, which can be used without increasing the burden on the liver and kidneys.