The cholesterol in human body becomes 7-dehydrocholesterol after oxidation in the body, and the 7-dehydrocholesterol in the skin turns into vitamin D3 precursor substance under the exposure to ultraviolet light, which is absorbed by the blood and transformed into 1,25-dihydroxyvitamin D3 under the action of hydroxylase in the liver and kidney, which belongs to active vitamin D3 and can promote the absorption and utilization of calcium, so normal people receive daily unobstructed ultraviolet Therefore, a normal person can receive at least 15 minutes of unobstructed UVB radiation daily to meet the amount of vitamin D required in the body. When the liver and kidneys are not functioning properly, the absorption and conversion of vitamin D will be affected, and the absorption and metabolism of calcium and phosphorus will be affected, and a certain concentration of cholesterol should be maintained.