When 25 hydroxyvitamin D is low, you need to take regular vitamin D orally, and you can also get sunlight. When ultraviolet light from the sun hits the skin, 7-dehydrocholesterol can be converted to vitamin D3, which becomes 25-hydroxyvitamin D3 by liver hydroxylase and 1,25-dihydroxyvitamin D3 by kidney 1α-hydroxylase, thus promoting the synthesis of calcium-binding protein and facilitating the absorption of elemental calcium in the intestine. Vitamin D and elemental calcium are essential supplements for bone health, especially as ionized calcium has some difficulty in absorption in the gastrointestinal tract. Vitamin D facilitates the absorption of calcium, so it is important to take oral calcium supplements along with vitamin D. Calcium and vitamin D are essential supplements for bone health. If osteoporosis is present, calcium and vitamin D supplementation should be accompanied by anti-osteoporosis medications, such as those that promote bone formation and those that inhibit bone resorption, all of which need to be used under the guidance of a specialist.