A new study shows that blood levels of 25-hydroxyvitamin D, or 25(OH)D, in the U.S. population are at their highest in August and at their lowest in February.

In an evaluation of data from 3.44 million blood samples of 25(OH)D, spanning 287 consecutive weeks, researchers analyzed the proportion of sera that were vitamin D–sufficient, defined as 25(OH)D ≥25 ng/mL.

In the temperate northern hemisphere, vitamin D levels show a “lag of the seasons” pattern, peaking in August and troughing in February. Although the correlation between the seasons and vitamin D has been known for some time, the new findings have added precision to the estimates of vitamin D seasonality.

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This information can help establish statistical associations between vitamin D levels and the development of a given disease.


  1. Kasahara AK et al.PLoS One. 2013;8[6]:e65785.