Challenging the present definition of "normal" vitamin D levels obtained by a single blood test. Can we develop a formula to predict vitamin D levels in the 4 seasons from a single season's measure?

Publications on the health effects of vitamin D (25(OH) D) had almost triplicate in the last 10years, not only for its known "calcemic effects" (calcium, phosphor, PTH), but for the more recent findings on its "non-calcemic effects" (all-cause and cardiovascular mortality, and relation with certain types of cancer). Part of these publications deal with the definition of what is a "normal" circulating level of 25(OH) D that may distinguish between health and disease. The literature also deals with seasonal variations of vitamin D, showing levels that rise in summer and fall in winter and with DBP phenotypes and geographical location that affect seasonality of 25(OH) D measurements. Despite the knowledge of the existence of these phenomena many studies on vitamin D fail to acknowledge the time of the year the blood sample was extracted. Thus, when we compare results from different studies without defining the season that the samples were drawn, we compare incomparable figures. Furthermore, it is quite absurd to define "normal levels" as a static measure (over or under a certain value) using a single blood test when the value measured is known to change with seasons. Knowing that people have different vitamin D levels in different seasons of the year, we should ask ourselves which of these measurements should be used to define a "real" or "normal" level? Is it the lower one? Is there a "mean measure" that should be used for this matter? If yes, how do we obtain it? Do we have to make 4 seasonal measurements in each patient? Alternatively, might there be a possibility of developing a formula to help us obtain the mean from a single season's measure or one season's prediction from another season's measurement? And knowing that DBP phenotypes and geographical location affect seasonality of 25(OH) D measurements; shouldn't we include this in the equation? In this article I will discuss the hypothetical existence of an Individual Mean Annual vitamin D level that I will call the "IMAD level" and a recovery formula "RF" that may be used to calculate this mean having one single measure (in any of the 4 seasons) and to predict any season's value from another season's measurement. IMAD levels should be obtained in the two main DBP phenotypes, taking into account the geographical location of the test.