In adults with low 25-hydroxyvitamin D, it is important to consider the low intake of vitamin D from food and the reduced production of vitamin D from sun exposure to the skin, as sun exposure to the skin is the main source of vitamin D production by the body itself. Vitamin D is converted to 25-hydroxyvitamin D by the liver after it reaches the liver, so if there is liver disease or abnormal liver function, it can also lead to low metabolism of 25-hydroxyvitamin D, resulting in low concentrations. Therefore, adults with low 25-hydroxyvitamin D should increase their nutritional intake, food intake and medication, increase sunlight exposure, and promptly treat liver diseases and liver abnormalities if they have them.