7
ability to cure rickets, but it is currently understood that they are metabolized differently; vitamin D3 could be more than three times as effective as vitamin D2 in raising serum 25(OH)D concentrations and maintaining those levels for a longer time, and its metabolites have superior affinity for vitamin D-binding proteins in plasma. A blood calcidiol (25-hydroxy-vitamin D) level is a satisfactory way to determine the cumulative effect of sun and diet in relation to vitamin D although serum 25(OH)D levels do not indicate the amount of vitamin D stored in other body tissues; it has a fairly long circulating half-life of 15 days.[9] Circulating 1,25(OH)2D is generally not a good indicator of vitamin D status because it has a short half-life of 15 hours and serum concentrations are closely regulated by parathyroid hormone, calcium, and phosphate.[10] Levels of 1,25(OH)2D do not typically decrease until vitamin D deficiency is severe. In overweight persons increased fat mass is inversely associated with 25(OH)D levels. This association may confound the reported relationships between low vitamin D status and conditions which occur more commonly in obesity as the circulating 25(OH)D underestimates their total body stores. A concentration of over 15 ng/ml (>37.5 nmol/L) is recommended. Higher levels (>30 ng/ml or >75 nmol/L) are proposed
Measuring vitamin D status
by some as desirable for achieving optimum health but there is not enough evidence to support them. A study of highly sun exposed (tanned) heathy young skateboarders and surfers in Hawaii found levels below the proposed higher minimum of 30 ng/ml in 51% of the subjects. The highest 25(OH)D concentration was around 60 ng/ml (150nmol/L). A similar <using the same data> study in Hawaii found a range of (11–71 ng/mL) in a population with prolonged extensive skin exposure while as part of the same study Wisconsin breastfeeding mothers were given supplements. The range of circulating 25(OH)D levels in women in the supplementated group was from 12–77 ng/mL. It is noteworthy that the levels in the supplemented population in Wisconsin were higher than the sun exposed group in Hawaii (which again included surfers because