Hypovitaminosis D - Diagnosis

Diagnosis

The serum concentration of 25-hydroxy-vitamin D is typically used to determine vitamin D status. It reflects vitamin D produced in the skin as well as that acquired from the diet, and has a fairly long circulating half-life of 15 days. It does not, however, reveal the amount of vitamin D stored in other body tissues. The level of serum 1,25-dihydroxy-vitamin D is not usually used to determine vitamin D status because it has a short half-life of 15 hours and is tightly regulated by parathyroid hormone, calcium, and phosphate, such that it does not decrease significantly until vitamin D deficiency is already well advanced.

One study found that vitamin D₃ raised 25-hydroxy-vitamin D blood levels more than did vitamin D2, but this difference has been adequately disproved to allow reasonable assumption that D₂ and D₃ are equal for maintaining 25-hydroxy-vitamin D status.

There has been variability in results of laboratory analyses of the level of 25-hydroxy-vitamin D. Falsely low or high values have been obtained depending on the particular test or laboratory used. Beginning in July 2009 a standard reference material became available which should allow laboratories to standardise their procedures.

There is some disagreement concerning the exact levels of 25-hydroxy-vitamin D needed for good health. A level lower than 10 ng/mL (25 nmol/L) is associated with the most severe deficiency diseases: rickets in infants and children, and osteomalacia in adults. A concentration above 15 ng/ml (37.5 nmol/L) is generally considered adequate for those in good health. Levels above 30 ng/ml (75 nmol/L) are proposed by some as desirable for achieving optimum health, but there is not yet enough evidence to support this.

Levels of 25-hydroxy-vitamin D that are consistently above 200 ng/mL (500 nmol/L) are thought to be potentially toxic, although data from humans are sparse. In animal studies levels up to 400 ng/mL (1,000 nmol/L) were not associated with toxicity. Vitamin D toxicity usually results from taking supplements in excess. Hypercalcemia is typically the cause of symptoms, and levels of 25-hydroxy-vitamin D above 150 ng/mL (375 nmol/L) are usually found, although in some cases 25-hydroxy-vitamin D levels may appear to be normal. It is recommended to periodically measure serum calcium in individuals receiving large doses of vitamin D.

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. However, as vitamin D is fat-soluble, excess amounts can be stored in fat tissue and used during winter months, when sun exposure is limited.

A study of highly sun-exposed (tanned) healthy 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 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 it was the same data set).

Another study of African Americans found that blood levels of 25(OH)D decreased linearly with increasing African ancestry, the decrease being 2.5-2.75 nmol/L per 10% increase in African ancestry. Sunlight and diet were 46% less effective in raising these levels among subjects with high African ancestry than among those with low/medium African ancestry. It could be possible that vitamin-D metabolism differs by ethnicity.