More sun exposure allows the ultraviolet rays of the sun to reach the skin directly, allowing the 7-dehydrocholesterol under the skin to be converted into vitamin D3, which reaches the liver in the bloodstream and becomes 25-(OH)D3 by the action of 25-hydroxylase and 1,25-(OH)2D3 by the action of 1α-hydroxylase in the kidney. 1,25-(OH)2D3 promotes the synthesis of calcium-binding protein in the intestine, which facilitates the absorption of elemental calcium in the intestine. 1,25-(OH)2D3 promotes the synthesis of calcium-binding protein in the intestine, which facilitates the absorption of elemental calcium in the intestine. Therefore, more sun exposure can promote the synthesis of natural vitamin D and facilitate the absorption of elemental calcium in the gastrointestinal tract. In the sunshine, you should pay attention to the sunshine at 9-10 pm and 3-4 pm, and avoid the midday sun in summer. If the sun is not bright in winter, you should sunbathe at 10:00 or 11:00, close to the noon time may be more effective. And when sunbathing can not play umbrella, and can not apply sunscreen, these can block the role of ultraviolet radiation on the skin. If it is in the winter time, you can’t sunbathe through the glass, because the glass can filter the ultraviolet rays in the sun, although the sun is on the skin, but the ultraviolet rays are not irradiated to the skin, and there is not much effect, so you can’t sunbathe through the glass.