25 hydroxyvitamin D, which is vitamin D, is an item measured by 25 vitamin D in the serum, which can be directly converted into vitamin D under the effect of light and sunlight, thus promoting the absorption of calcium, if the lack of 25 hydroxyvitamin D is also a lack of vitamin D, which can cause some calcium deficiency diseases, and in serious cases patients will appear rickets, when children lack vitamin D, you can often sun exposure and intake of vitamin D to supplement.