Recommendations on recommended 25(OH)D serum levels vary across authorities, and vary based on factors like age. US labs generally report 25(OH)D levels in ng/mL. Other countries often use nmol/L. Oneng/mL is approximately equal to 2.5nmol/L.
A 2014 review concluded that the most advantageous serum levels for 25(OH)D for all outcomes appeared to be close to 30ng/mL (75nmol/L). The optimal vitamin D levels are still controversial and another review concluded that ranges from 30 to 40ng/mL (75 to 100nmol/L) were to be recommended for athletes. Part of the controversy is because numerous studies have found differences in serum levels of 25(OH)D between ethnic groups; studies point to genetic as well as environmental reasons behind these variations. Supplementation to achieve these standard levels could cause harmful vascular calcification.
A 2012 meta-analysis showed that the risk of cardiovascular diseases increases when blood levels of vitamin D are lowest in a range of 8 to 24ng/mL (20 to 60nmol/L), although results among the studies analyzed were inconsistent.
In 2011 an IOM committee concluded a serum 25(OH)D level of 20ng/mL (50nmol/L) is needed for bone and overall health. The dietary reference intakes for vitamin D are chosen with a margin of safety and overshoot the targeted serum value to ensure the specified levels of intake achieve the desired serum 25(OH)D levels in almost all persons. No contributions to serum 25(OH)D level are assumed from sun exposure and the recommendations are fully applicable to people with dark skin or negligible exposure to sunlight. The Institute found serum 25(OH)D concentrations above 30ng/mL (75nmol/L) are "not consistently associated with increased benefit". Serum 25(OH)D levels above 50ng/mL (125nmol/L) may be cause for concern. However, some people with serum 25(OH)D between 30 and 50ng/mL (75nmol/L-125nmol/L) will also have inadequate vitamin D.