https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7006052/
Serum Concentration of Vitamin D
Ideal values of VD serum levels are not precisely defined. In 2011, the Institute of Medicine, established that serum 25(OH)D3 concentrations of at least 20 ng/mL, corresponding to a recommended VD3 dietary allowance of 600 IU/day for ages 1 to 70 years and 800 IU/day for ages 71 years and older, could be considered adequate (21). This is because lower levels have been found to be associated with an increased risk of bone metabolism alterations, falls, and myopathy (22–25). However, several scientific societies have established an ideal value of 30 ng/mL, and a number of experts have suggested that, in order to assure prevention of certain skeletal effects, serum 25(OH)D3 concentrations must be maintained between 40 and 60 ng/mL. Some authors define values between 20 and 30 ng/mL as insufficiency and values lower than 20 ng/mL as deficiency (26). Others consider deficiency a value ≤10 mg/mL and insufficiency a value between 11 and 20 ng/mL (27). Another poorly defined quantity is the maximum tolerable level, with some evidence seeming to indicate that concentrations higher than 100 ng/mL can be maintained without clinical problems (28), while others indicate that serum 25(OH)D3 concentrations higher than 60 ng/mL in adults could be associated with an increased risk of death (29).
